Track mixed paper and viewport dimension references

docs/viewport-dimension-association.svg

@@ -0,0 +1,19 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="900" height="350" viewBox="0 0 900 350">
+<style>text{font-family:Arial,sans-serif;fill:#263238} .title{font-size:22px;font-weight:600} .label{font-size:16px} .small{font-size:13px;fill:#54616a} .model{stroke:#1479b8;stroke-width:4;fill:none} .paper{stroke:#a94610;stroke-width:2;stroke-dasharray:9 5} .dim{stroke:#263238;stroke-width:1.5;fill:none}</style>
+<defs><marker id="arrow" viewBox="0 0 8 8" refX="4" refY="4" markerWidth="6" markerHeight="6" orient="auto-start-reverse"><path d="M8 0L0 4L8 8" fill="#263238"/></marker></defs>
+<rect width="900" height="350" rx="10" fill="white"/>
+<text class="title" x="30" y="38">The dimension follows its paper-space centerline</text>
+<rect x="30" y="65" width="400" height="220" fill="#f5f8fa" stroke="#c3ced5"/>
+<rect x="470" y="65" width="400" height="220" fill="#f5f8fa" stroke="#c3ced5"/>
+<text class="label" x="48" y="93">Before</text><text class="label" x="488" y="93">Centerline moved 1 paper unit right</text>
+<path class="model" d="M85 160H385M525 160H825"/>
+<circle cx="385" cy="160" r="5" fill="#1479b8"/><circle cx="825" cy="160" r="5" fill="#1479b8"/>
+<path class="paper" d="M235 113V257M705 113V257"/>
+<path class="dim" d="M385 171V244M825 171V244"/>
+<path class="dim" d="M235 232H385" marker-start="url(#arrow)" marker-end="url(#arrow)"/>
+<path class="dim" d="M705 232H825" marker-start="url(#arrow)" marker-end="url(#arrow)"/>
+<rect x="291" y="217" width="38" height="28" fill="#f5f8fa"/><rect x="746" y="217" width="38" height="28" fill="#f5f8fa"/>
+<text class="label" x="310" y="237" text-anchor="middle">50</text><text class="label" x="765" y="237" text-anchor="middle">40</text>
+<text class="label" x="30" y="316">The model endpoint stays fixed. The dimension updates from 50 to 40.</text>
+<text class="small" x="30" y="340">Association schematic; viewport 1:10, dimension style unit factor = 1, DIMASSOC = 2.</text>
+</svg>

docs/viewport-dimension-creation.svg

@@ -0,0 +1,22 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="900" height="350" viewBox="0 0 900 350">
+<style>text{font-family:Arial,sans-serif;fill:#263238} .title{font-size:22px;font-weight:600} .label{font-size:16px} .small{font-size:13px;fill:#54616a} .model{stroke:#1479b8;stroke-width:4;fill:none} .paper{stroke:#a94610;stroke-width:2;stroke-dasharray:9 5} .dim{stroke:#263238;stroke-width:1.5;fill:none}</style>
+<defs><marker id="arrow" viewBox="0 0 8 8" refX="4" refY="4" markerWidth="6" markerHeight="6" orient="auto-start-reverse"><path d="M8 0L0 4L8 8" fill="#263238"/></marker></defs>
+<rect width="900" height="350" rx="10" fill="white"/>
+<text class="title" x="30" y="38">Dimension from a paper centerline to a model endpoint</text>
+<rect x="30" y="65" width="430" height="245" fill="#f5f8fa" stroke="#c3ced5"/>
+<text class="small" x="45" y="89">Viewport 1:10</text>
+<path class="model" d="M100 160H400"/>
+<circle cx="400" cy="160" r="5" fill="#1479b8"/>
+<path class="paper" d="M250 110V260"/>
+<text class="small" x="100" y="144">Model geometry</text>
+<path class="dim" d="M400 171V244"/>
+<path class="dim" d="M250 232H400" marker-start="url(#arrow)" marker-end="url(#arrow)"/>
+<rect x="306" y="217" width="38" height="28" fill="#f5f8fa"/>
+<text class="label" x="325" y="237" text-anchor="middle">50</text>
+<text class="small" x="45" y="290">Brown dashed line: paper-space centerline</text>
+<text class="label" x="500" y="116">5 paper units between the picked points</text>
+<text class="label" x="500" y="155">5 × 10 = 50 model units</text>
+<text class="label" x="500" y="206">The viewport supplies the length scale.</text>
+<text class="label" x="500" y="240">Either pick order gives the same value.</text>
+<text class="small" x="30" y="336">Measurement schematic; dimension style unit factor = 1.</text>
+</svg>

docs/viewport-dimensions.md

@@ -1,100 +1,74 @@
-# Dimensions through paper-space viewports
+# Dimensions through layout viewports
 
-Paper-space point commands can snap to displayed model geometry through a planar,
+In paper space, snap to model geometry displayed through a planar, orthographic
-orthographic layout viewport. The acquired result retains the original model
+viewport. `DIMLINEAR`, `DIMALIGNED`, `DIMANGULAR`, `DIMRADIUS`, and `DIMDIAMETER`
-coordinate, its paper coordinate, the viewport frame, and feature identity.
+also support their usual object-pick modes through these viewports.
-Ordinary selection continues to operate on the active drawing space.
 
-`DIMLINEAR`, `DIMALIGNED`, `DIMANGULAR`, `DIMRADIUS`, and `DIMDIAMETER` use this
+## Measurement
-acquisition context. The supported object-pick modes also query viewport geometry.
-Snapping and object picking share viewport draw order, visibility, and clipping
-rules. Object picking uses the resident interaction index and applies block
-instance transforms. Nonuniformly transformed circles and circular arcs are
-declined as radial object picks because their displayed geometry is elliptical.
-Array inserts without an addressable instance index are also declined; resolved
-block geometry must remain within the original pick aperture.
 
-## Input and measurement rules
+- Model geometry supplies the viewport scale. A 100-unit model length displayed
+  at 1:10 measures 100, although it occupies 10 paper units.
+- A paper-space centerline or free sheet point can be combined with model
+  geometry through one viewport, in either pick order. The paper-space span is
+  measured using that viewport's scale.
+- Sheet-only dimensions measure paper units, even inside a viewport rectangle.
+- Model picks from a second viewport are rejected without advancing the command.
+  Dimension-line and text placement do not change the measurement scale.
+- The current dimension style's unit conversion is preserved. Angular dimensions
+  do not use length compensation. `DIMASSOC=0` explodes the compensated dimension.
 
-- Free and typed sheet points measure paper units, including points inside a
+Snapping and object picking respect visibility, clipping, and viewport draw order.
-  viewport rectangle.
+Paper geometry has priority in object picks. Ordinary selection uses the active
-- Measuring points must all belong to the same space and, when applicable, the
+drawing space. Object picks reuse the resident interaction index and resolve
-  same viewport. An incompatible pick leaves the command at its current step.
+nested block transforms.
-- Dimension-line and text placement do not change the measurement's space.
-- Paper geometry takes priority over viewport geometry at an object pick.
-- The final dimension style is applied before computing measurement scaling.
-  Length dimensions keep paper-space definition points and a negative `DIMLFAC`
-  override containing the effective user factor multiplied by `1 / viewport scale`.
-  Angular measurements do not use this length factor.
-- `DIMASSOC=0` uses the same compensated value before exploding the annotation.
 
-## Persistent measurement data
+![Measurement schematic: a five-unit sheet span measures 50 through a 1:10 viewport.](viewport-dimension-creation.svg)
 
-`OCS_VIEWPORT_MEASUREMENT` XData stores three values: version `1` (integer16),
+## Saved scale
-the positive user factor (real), and viewport compensation (real). This keeps the
-user's unit conversion separate from viewport scale. The ordinary `DIMLFAC`
-override remains sufficient for displaying the saved measurement in other CAD
-readers. The data survives DXF/DWG saves and undo/redo.
 
-With `DIMASSOC=2`, acquired features attach a dimension to its viewport, each
+Length dimensions store paper-space definition points and a negative `DIMLFAC`
-block instance, and the source entity. Intersections retain both source chains.
+override: user unit factor multiplied by `1 / viewport scale`.
-Source edits and viewport changes update the definition points and recompute
+`OCS_VIEWPORT_MEASUREMENT` XData stores version `1` (integer16), the positive user
-compensation while preserving explicit user-positioned sheet text and dimension
+factor (real), and viewport compensation (real). Keeping the factors separate
-style overrides. A successful update invalidates the saved picture block so the
+allows compensation to change without losing the user's unit conversion.
-new geometry and number are rendered and exported. Undo restores both the
+Both the override and XData survive DXF/DWG saves and undo/redo.
-reference data and the previous picture.
 
-Properties reports whether the dimension is associated, partially associated,
+## Associations
-unresolved, broken, or nonassociative. A broken or unsupported required reference
-preserves the entire dimension's last valid appearance and its original records.
-Creating another dimension does not announce geometry changes to its sources.
 
-## Imported references and support limits
+With `DIMASSOC=2`, acquired features retain their viewport, block-instance path,
+source entity, and snap feature. Intersections retain both source chains.
+Source edits and viewport changes update the dimension and its compensation,
+preserving style overrides and user-positioned text on the sheet.
 
-The resolver distinguishes the application's vertex/parameter convention from
+A sheet centerline stays on the sheet; a model endpoint follows its viewport.
-imported edge references. Imported start/end and midpoint modes use their edge
+Moving the viewport therefore changes the distance between them. Undo/redo and
-marker and parameter together. The original regression drawing's first-edge
+DXF/DWG saves preserve the references.
-start/end pair now reproduces its `0.128` measurement. An imported polyline
-endpoint beyond the first edge requires a usable stored point to validate the
-marker; zero/sentinel data cannot establish that identity. Ambiguous imports
-remain unresolved. Imported viewport scale is updated only when existing OCS
-metadata or `ACAD_DIMASSOC_CALC_DIMLFAC` separates compensation from user scaling.
 
-Unsupported cases retain their saved data and appearance:
+Properties shows association status. If a required reference is broken or cannot
+be resolved, the entire dimension keeps its last valid geometry and reference
+records. A successful refresh regenerates its displayed and exported geometry.
 
-- Perspective/oblique viewport projections and malformed/missing reference chains.
+![Association schematic: moving the sheet centerline changes the dimension from 50 to 40.](viewport-dimension-association.svg)
-- Ambiguous intersections within one batched block; they are not attached to a
-  guessed pair of children. Intersections between separate identified chains work.
-- Array inserts without an addressable instance index, nesting beyond eight levels,
-  and radial measurements of nonuniformly transformed circular geometry.
-- Imported quadrant references whose moved geometry no longer validates the stored
-  point; newly acquired circle/arc quadrants keep their source-local angle.
-- Tangency with no verified solution, two dependent perpendicular/tangent references
-  requiring a coupled solve, unsupported snap modes, and imported measurement data that does not identify the old viewport compensation.
 
-The osnap codes and reference fields follow the
+## Limits
-[Autodesk DIMASSOC reference](https://help.autodesk.com/cloudhelp/2024/ENU/AutoCAD-DXF/files/GUID-C0B96256-A911-4B4D-85E6-EB4AF2C91E27.htm).
-The restricted imported-marker interpretation above is validated by fixtures;
-it is not a claim that every exporter uses an interchangeable marker scheme.
 
-## Validation
+Unsupported cases preserve saved dimensions:
 
-`src/app/viewport_dimension_tests.rs` exercises snap-engine acquisition, command
+- Perspective or oblique views; missing or malformed reference chains.
-commits for all five commands, object picks, input retries, active positive and
+- Array instances without an addressable instance index, nesting beyond eight
-negative style factors, exploded dimensions, undo/redo, DXF/DWG persistence,
+  levels, and radial measurements of nonuniformly scaled circles or arcs.
-clipping, overlapping viewports, pan, scale, and twist. These are headless tests;
+- Intersections without two identifiable source chains inside a batched block.
-they do not establish visual acceptance in the desktop UI.
+- Ambiguous imported endpoint, midpoint, or quadrant markers. A stored point can
+  validate a feature; it cannot establish an otherwise unknown identity.
+- Tangency without a verified solution, or two perpendicular/tangent references
+  requiring a coupled solve.
+- Imported viewport scaling without OCS scale metadata or
+  `ACAD_DIMASSOC_CALC_DIMLFAC` identifying the saved compensation.
 
-Association tests also exercise regeneration, nearest and quadrant parameters,
+## Tests
-angular arc/polyline object picks, spline tangency,
-perpendicularity under nonuniform block scale, nested paths, intersection source
-edits, preserved overrides, erased sources, copy detachment, saved picture
-invalidation, and history. The checked-in DXF fixture is exercised directly.
-The original private drawing can be checked separately:
 
-```powershell
+Run `cargo test --locked --lib viewport_dimension`. Tests cover the five commands,
-$env:OPENCAD_VIEWPORT_REGRESSION_DXF = 'C:\path\to\115448.dxf'
+mixed sheet/model input, styles, clipping, object picks, source/viewport edits,
-cargo test --locked --offline --lib viewport_dimension_original -- --ignored --nocapture
+snap features, nested paths, intersections, preserved overrides, undo/redo, and
-```
+DXF/DWG persistence. The synthetic association fixture is in
-
+`tests/fixtures/dim_assoc/`.
-Do not treat passing headless checks as completed desktop visual acceptance.

src/app/command_driver.rs

@@ -98,7 +98,7 @@ impl OpenCADStudio {
         self.tabs[i].snap_result = None;
         self.last_point = None;
         // Points collected in the space being left are meaningless in the new
-        // one (PR1).
+        // one.
         self.clear_accepted_snaps();
         self.pending_click_snap = None;
         self.snapper.from_point = None;
@@ -386,7 +386,7 @@ impl OpenCADStudio {
             }
             // Typed / dynamic-input / headless points carry no snap, but the
             // accepted-snap list must stay index-parallel with the points the
-            // command collects (PR1). Interactive picks record themselves in
+            // command collects. Interactive picks record themselves in
             // the click handler and never reach here.
             if !self.record_accepted_snap(i, None, None, *point) {
                 return Task::none();
@@ -404,7 +404,9 @@ impl OpenCADStudio {
             self.push_ucs_to_cmd(i);
         }
         if let StepInput::EntityPick(handle, point) = &input {
-            if !self.dimension_acquisition_allowed(i, None) { return Task::none(); }
+            if !self.dimension_acquisition_allowed(i, None) {
+                return Task::none();
+            }
             let solid_pick = matches!(
                 self.tabs[i].scene.document.get_entity(*handle),
                 Some(
@@ -477,7 +479,11 @@ impl OpenCADStudio {
         }
         let ctrl = self.ctrl_down;
         let shift = self.shift_down;
-        let picked_handle = if let StepInput::EntityPick(h, _) = &input { Some(*h) } else { None };
+        let picked_handle = if let StepInput::EntityPick(h, _) = &input {
+            Some(*h)
+        } else {
+            None
+        };
         let result: Option<CmdResult> = {
             let Some(cmd) = self.tabs[i].active_cmd.as_mut() else {
                 return Task::none();
@@ -2366,17 +2372,15 @@ impl OpenCADStudio {
                 // through a viewport carries the compensation as a negative
                 // DIMLFAC override.
                 if !preserve_base_style {
-                    crate::scene::creation_style::apply_current_creation_styles(&self.tabs[i].scene.document, &mut entity);
+                    crate::scene::creation_style::apply_current_creation_styles(
+                        &self.tabs[i].scene.document,
+                        &mut entity,
+                    );
                 }
                 if !self.apply_viewport_dimension_measurement(i, &mut entity) {
                     return Task::none();
                 }
-                // ...and is deliberately *not* associated in paper space: its
+                // Projected points cannot use direct paper-space source inference.
-                // definition points are projected paper coordinates, so a
-                // paper-space association would bind it to geometry that is
-                // not there. PR3 records the real dimension -> viewport ->
-                // entity chain instead. One gate, shared with
-                // `infer_dimension_sources_guarded`.
                 let association_allowed = self.dimension_association_allowed(i);
                 let inherited_dimension = if preserve_base_style {
                     match &entity {
@@ -2438,20 +2442,15 @@ impl OpenCADStudio {
                 } else {
                     let delta_safe = self.delta_add_safe(i, &entity);
                     let pending = self.begin_undo(i, label, 1, delta_safe);
-                    if let Some(handle) = self.commit_entity_handle_with_dimension_policy(
+                    if let Some(handle) =
-                        entity,
+                        self.commit_entity_handle_with_dimension_policy(entity, preserve_base_style)
-                        preserve_base_style,
+                    {
-                    ) {
                         if association_mode == 2 && association_allowed {
-                            let mut changes = vec![
+                            let mut changes = vec![(handle, crate::scene::ChangeKind::Modified)];
-                                (handle, crate::scene::ChangeKind::Modified),
-                            ];
                             match association {
                                 crate::command::DimensionAssociationInput::Infer(source) => {
                                     let sources: Vec<_> = source.map_or_else(
-                                        || {
+                                        || self.infer_dimension_sources_guarded(i, handle),
-                                            self.infer_dimension_sources_guarded(i, handle)
-                                        },
                                         |source| {
                                             if single_source_dimension {
                                                 vec![Some(source)]
@@ -2481,12 +2480,7 @@ impl OpenCADStudio {
                             changes.dedup_by_key(|(handle, _)| handle.value());
                             self.tabs[i].scene.bump_entities(&changes);
                         } else if association_mode == 2 {
-                            // Measured through a layout viewport: a paper-space
+                            // Preserve the acquired viewport and source paths.
-                            // association would bind projected coordinates to
-                            // geometry that is not on the sheet. Record the real
-                            // chain instead — dimension -> viewport -> block
-                            // path -> entity -> feature — from the AcceptedSnaps
-                            // PR1 captured, in definition-point slot order.
                             self.attach_viewport_dimension_association(i, handle);
                         }
                     }

src/app/commands/mod.rs

@@ -121,7 +121,7 @@ impl OpenCADStudio {
         // is not constrained by ortho/polar relative to a previous command's endpoint.
         self.last_point = None;
         // A new command collects its own points, so the previous command's
-        // accepted snaps must not leak into it (PR1).
+        // accepted snaps must not leak into it.
         self.clear_accepted_snaps();
         // Starting a command restarts the right-click cycle, so its first
         // right-click acts as Enter rather than opening the context menu.

src/app/dim_viewport.rs

@@ -1,5 +1,5 @@
-//! Paper-space dimension acquisition and measurement. Only acquired geometry
+//! Paper-space dimension acquisition and measurement. Acquiring model geometry
-//! carries viewport compensation; free and typed sheet coordinates stay on the sheet.
+//! supplies a viewport scale; sheet-only dimensions retain paper units.
 
 use super::OpenCADStudio;
 use crate::command::DimensionAssociationSource;
@@ -14,7 +14,7 @@ use glam::DVec3;
 pub(crate) enum DimensionMeasureSpace {
     Direct,
     Viewport(ViewportFrame),
-    Mixed,
+    ConflictingViewports,
 }
 
 impl OpenCADStudio {
@@ -26,33 +26,23 @@ impl OpenCADStudio {
         selection.left_dragging = false;
     }
 
-    #[allow(dead_code)] // Consumed by the dependent association change.
-    pub(crate) fn dimension_measuring_snaps(&self, _i: usize) -> Vec<AcceptedSnap> {
-        self.accepted_snaps().to_vec()
-    }
-
     pub(crate) fn dimension_measure_space(&self, i: usize) -> DimensionMeasureSpace {
         let scene = &self.tabs[i].scene;
         if scene.current_layout == "Model" || scene.active_viewport.is_some() {
             return DimensionMeasureSpace::Direct;
         }
         let mut frame: Option<ViewportFrame> = None;
-        let mut direct = false;
+        for f in self.accepted_snaps().iter().filter_map(|snap| snap.frame) {
-        for snap in self.accepted_snaps() {
+            match frame {
-            match snap.frame {
+                None => frame = Some(f),
-                Some(f) => match frame {
+                Some(previous) if previous.viewport == f.viewport => {}
-                    None => frame = Some(f),
+                Some(_) => return DimensionMeasureSpace::ConflictingViewports,
-                    Some(previous) if previous.viewport == f.viewport => {}
-                    Some(_) => return DimensionMeasureSpace::Mixed,
-                },
-                None => direct = true,
             }
         }
-        match (frame, direct) {
+        frame.map_or(
-            (None, _) => DimensionMeasureSpace::Direct,
+            DimensionMeasureSpace::Direct,
-            (Some(f), false) => DimensionMeasureSpace::Viewport(f),
+            DimensionMeasureSpace::Viewport,
-            _ => DimensionMeasureSpace::Mixed,
+        )
-        }
     }
 
     /// Reject an incompatible measuring input before it advances the command.
@@ -68,8 +58,15 @@ impl OpenCADStudio {
         {
             return true;
         }
-        if self.accepted_snaps().iter().any(|s| s.viewport != viewport) {
+        // Sheet geometry and free sheet points may accompany one viewport.
-            self.command_line.push_error(crate::t!("Pick geometry from the same viewport, or use paper-space points for the whole dimension.").as_ref());
+        // A second viewport would make the measurement scale ambiguous.
+        if viewport.is_some_and(|viewport| {
+            self.accepted_snaps()
+                .iter()
+                .filter_map(|s| s.viewport)
+                .any(|previous| previous != viewport)
+        }) {
+            self.command_line.push_error(crate::t!("Model geometry must come from one viewport. Paper-space points can be included.").as_ref());
             return false;
         }
         true
@@ -134,8 +131,8 @@ impl OpenCADStudio {
         );
         let frame = match self.dimension_measure_space(i) {
             DimensionMeasureSpace::Direct => return true,
-            DimensionMeasureSpace::Mixed => {
+            DimensionMeasureSpace::ConflictingViewports => {
-                self.command_line.push_error(crate::t!("Pick geometry from the same viewport, or use paper-space points for the whole dimension.").as_ref());
+                self.command_line.push_error(crate::t!("Model geometry must come from one viewport. Paper-space points can be included.").as_ref());
                 return false;
             }
             DimensionMeasureSpace::Viewport(f) => f,
@@ -214,13 +211,13 @@ impl OpenCADStudio {
         if slots.is_empty() {
             return Vec::new();
         }
-        let measuring = self.dimension_measuring_snaps(i);
+        let measuring = self.accepted_snaps();
         slots
             .iter()
             .enumerate()
             .map(|(index, slot)| {
                 let snap = measuring.get(index)?.clone();
-                let frame = snap.frame?;
+                let frame = snap.frame;
                 let slot = DVec3::new(slot.x, slot.y, slot.z);
                 if snap
                     .paper_point
@@ -232,7 +229,7 @@ impl OpenCADStudio {
                 }
                 Some(AcceptedSnap {
                     paper_point: slot,
-                    model_point: frame.paper_to_model(slot),
+                    model_point: frame.map_or(slot, |frame| frame.paper_to_model(slot)),
                     ..snap
                 })
             })

src/app/mod.rs

@@ -717,21 +717,12 @@ pub(super) struct OpenCADStudio {
     show_layout_tabs: bool,
     /// Last point committed by a drawing command — used as ortho/polar base.
     last_point: Option<glam::DVec3>,
-    /// Viewport frame the live snap marker is currently looking through (PR1).
+    /// Viewport used by the current snap; the displayed point is in paper space.
-    /// Set by the cursor-move handler when the displayed snap came from a
-    /// layout viewport, cleared otherwise. Display/bookkeeping only — the point
-    /// itself is already projected onto the sheet.
     pub(crate) vp_snap_frame: Option<crate::scene::viewport_ref::ViewportFrame>,
-    /// Snaps accepted by the active command's point steps, parallel to the
+    /// Acquired coordinates and source identities in command-step order.
-    /// points it collected (PR1). A command that does not care keeps using the
+    /// Cleared when the command starts or ends.
-    /// plain paper point; dimension commands (PR2) read the model point,
-    /// viewport and frame from here, and PR3 reads `source`. Cleared whenever a
-    /// command starts or ends.
     accepted_snaps: Vec<crate::scene::viewport_ref::AcceptedSnap>,
-    /// Snap resolved by the click currently being handled, with the viewport
+    /// Click result retained until the command accepts its point.
-    /// frame it came through (PR1). Set inside the click handler's point
-    /// resolution so the point-accept path further down can record the
-    /// [`crate::scene::viewport_ref::AcceptedSnap`] without recomputing.
     pending_click_snap: Option<(
         crate::snap::SnapResult,
         Option<crate::scene::viewport_ref::ViewportFrame>,

src/app/properties.rs

@@ -1277,13 +1277,22 @@ impl OpenCADStudio {
                                     &self.tabs[i].scene.document,
                                     d.base().common.handle,
                                 );
-                            let statuses = self.tabs[i].scene.dimension_association_status(d.base().common.handle);
+                            let statuses = self.tabs[i]
-                            let slots = self.tabs[i].scene.dimension_association_slot_points(d.base().common.handle).len();
+                                .scene
+                                .dimension_association_status(d.base().common.handle);
+                            let slots = self.tabs[i]
+                                .scene
+                                .dimension_association_slot_points(d.base().common.handle)
+                                .len();
                             let status = if statuses.is_empty() {
                                 t!("Nonassociative")
-                            } else if statuses.iter().any(|(_, status)| matches!(status, crate::scene::ReferenceStatus::Broken(_))) {
+                            } else if statuses.iter().any(|(_, status)| {
+                                matches!(status, crate::scene::ReferenceStatus::Broken(_))
+                            }) {
                                 t!("Broken reference")
-                            } else if statuses.iter().any(|(_, status)| matches!(status, crate::scene::ReferenceStatus::Unresolved)) {
+                            } else if statuses.iter().any(|(_, status)| {
+                                matches!(status, crate::scene::ReferenceStatus::Unresolved)
+                            }) {
                                 t!("Unresolved reference")
                             } else if statuses.len() < slots {
                                 t!("Partially associated")
@@ -1293,7 +1302,9 @@ impl OpenCADStudio {
                             general.props.push(crate::scene::model::object::Property {
                                 label: t!("Association status").into_owned(),
                                 field: "association_status",
-                                value: crate::scene::model::object::PropValue::ReadOnly(status.into_owned()),
+                                value: crate::scene::model::object::PropValue::ReadOnly(
+                                    status.into_owned(),
+                                ),
                             });
                             general.props.push(crate::scene::model::object::Property {
                                 label: t!("Associative").into_owned(),

src/app/update/viewport.rs

@@ -2059,24 +2059,24 @@ impl OpenCADStudio {
                 )
             };
 
-            // Paper-space snapping THROUGH layout viewports (PR1). `edit_cam`
+            // Paper-space snapping through layout viewports. `edit_cam`
             // is None here, so `cursor_world` is the paper point and the sheet
             // fills the canvas (pane-local == canvas pixels). The viewport hit
             // arrives already projected onto the sheet, so everything
             // downstream keeps working in paper coordinates — LINE still draws
             // at the projected paper point.
             self.vp_snap_frame = None;
-            if edit_cam.is_none() && !needs_entity && !is_gathering && !needs_structure
+            if edit_cam.is_none()
+                && !needs_entity
+                && !is_gathering
+                && !needs_structure
                 && !needs_tan
             {
                 if let Some((vp_hit, frame)) =
                     self.paper_viewport_snap(i, p_full, vp_size, cursor_world)
                 {
-                    let merged = crate::snap::merge_snap(
+                    let merged =
-                        self.tabs[i].snap_result,
+                        crate::snap::merge_snap(self.tabs[i].snap_result, Some(vp_hit), p_full);
-                        Some(vp_hit),
-                        p_full,
-                    );
                     if merged.is_some_and(|hit| hit.viewport.is_some()) {
                         self.vp_snap_frame = Some(frame);
                     }
@@ -3701,7 +3701,7 @@ impl OpenCADStudio {
                         }
                     }
                 }
-                // Paper-space snapping through layout viewports (PR1). Mirrors
+                // Paper-space snapping through layout viewports. Mirrors
                 // the cursor-move pass: the click recomputes snapping from
                 // scratch, so the viewport query has to run here too. The hit
                 // comes back already projected onto the sheet.
@@ -3710,8 +3710,7 @@ impl OpenCADStudio {
                     if let Some((vp_hit, frame)) =
                         self.paper_viewport_snap(i, p_full, (vw, vh), raw)
                     {
-                        let merged =
+                        let merged = crate::snap::merge_snap(snap_hit, Some(vp_hit), p_full);
-                            crate::snap::merge_snap(snap_hit, Some(vp_hit), p_full);
                         if merged.is_some_and(|hit| hit.viewport.is_some()) {
                             click_frame = Some(frame);
                         }
@@ -4046,7 +4045,10 @@ impl OpenCADStudio {
                         }
                     }
 
-                    if !self.dimension_acquisition_allowed(i, None) { self.finish_command_click(i); return Task::none(); }
+                    if !self.dimension_acquisition_allowed(i, None) {
+                        self.finish_command_click(i);
+                        return Task::none();
+                    }
                     let shift = self.shift_down;
                     let result = self.tabs[i].active_cmd.as_mut().map(|c| {
                         // Shift-swap state for TRIM/EXTEND (#336).
@@ -4126,11 +4128,21 @@ impl OpenCADStudio {
                     }
                     result
                 } else if let Some(result) = {
-                    let dimension = self.tabs[i].active_cmd.as_ref().is_some_and(|c| c.measures_through_viewports());
+                    let dimension = self.tabs[i]
+                        .active_cmd
+                        .as_ref()
+                        .is_some_and(|c| c.measures_through_viewports());
                     if dimension {
-                        let per_px = self.tabs[i].scene.camera.borrow().ortho_size() as f64 * 2.0 / (bounds.height as f64).max(1.0);
+                        let per_px = self.tabs[i].scene.camera.borrow().ortho_size() as f64 * 2.0
-                        self.try_dimension_viewport_entity_pick(i, pick_wcs, crate::ui::overlay::pick_box_aperture_px(self.pick_box) as f64 * per_px)
+                            / (bounds.height as f64).max(1.0);
-                    } else { None }
+                        self.try_dimension_viewport_entity_pick(
+                            i,
+                            pick_wcs,
+                            crate::ui::overlay::pick_box_aperture_px(self.pick_box) as f64 * per_px,
+                        )
+                    } else {
+                        None
+                    }
                 } {
                     Some(result)
                 } else if self.tabs[i]
@@ -4139,7 +4151,12 @@ impl OpenCADStudio {
                     .is_some_and(|command| command.entity_pick_accepts_points())
                 {
                     let pending = self.pending_click_snap.take();
-                    if !self.record_accepted_snap(i, pending.map(|(hit, _)| hit), pending.and_then(|(_, frame)| frame), pick_wcs) {
+                    if !self.record_accepted_snap(
+                        i,
+                        pending.map(|(hit, _)| hit),
+                        pending.and_then(|(_, frame)| frame),
+                        pick_wcs,
+                    ) {
                         self.finish_command_click(i);
                         return Task::none();
                     }
@@ -4212,7 +4229,7 @@ impl OpenCADStudio {
                     }
                 }
                 // Record what this point step accepted, alongside the point
-                // itself (PR1): paper point, model point, viewport, frame and
+                // itself: paper point, model point, viewport, frame and
                 // geometry identity. Commands that don't care keep consuming
                 // `world_pt` exactly as before.
                 let pending = self.pending_click_snap.take();
@@ -4221,7 +4238,10 @@ impl OpenCADStudio {
                     pending.map(|(hit, _)| hit),
                     pending.and_then(|(_, frame)| frame),
                     world_pt,
-                ) { self.finish_command_click(i); return Task::none(); }
+                ) {
+                    self.finish_command_click(i);
+                    return Task::none();
+                }
                 self.last_point = Some(world_pt);
                 // The one-shot snap override is spent by this pick —
                 // restore the running osnap configuration (#337).

src/app/update/viewport_snap.rs

@@ -1,24 +1,7 @@
-//! Paper-space snapping THROUGH layout viewports (PR1).
+//! Snap to displayed model geometry while a paper-space command is active.
-//!
+//! Viewport queries use the renderer's resident wires and interaction index,
-//! In a paper layout with no active viewport, `Scene::hit_test_wires` returns
+//! then project the accepted model point onto the sheet. Ordinary selection
-//! only the paper sheet's own entities — viewport contents are deliberately not
+//! continues to use paper-space hit testing.
-//! interactive, and that must stay true for *selection*. Snapping, however,
-//! should see the model geometry a viewport displays, so a paper-space LINE /
-//! DIMENSION can pick a real model endpoint and land on the paper pixel where
-//! it is drawn.
-//!
-//! This module adds that as a SEPARATE query. It never changes
-//! `hit_test_wires`. For each displayed content viewport under the cursor
-//! (top-most first) it runs the EXISTING snap engine against that viewport's
-//! resident model wires, using the viewport's own camera and screen rectangle
-//! (`Scene::viewport_edit_frame_for` — the same adapter MSPACE editing uses),
-//! then maps the accepted model point back onto the sheet with
-//! [`ViewportFrame::model_to_paper`].
-//!
-//! Performance: no projected geometry is rebuilt per pointer event. The wire
-//! set comes from `model_wires_for_viewport_arc`, which is the renderer's own
-//! resident, camera-independent `Arc`, and the cursor-local broad phase reuses
-//! the shared interaction index built over it.
 
 use iced::Point;
 
@@ -34,37 +17,14 @@ const MAX_ACCEPTED_SNAPS: usize = 64;
 
 impl OpenCADStudio {
     /// Snaps accepted by the active command's point steps, oldest first, in
-    /// step order. Consumed by the dimension commands in PR2/PR3.
+    /// step order.
-    #[allow(dead_code)]
     pub(crate) fn accepted_snaps(&self) -> &[AcceptedSnap] {
         &self.accepted_snaps
     }
 
-    /// The most recently accepted snap, if any.
+    /// Infer direct sources only when the measurement uses the active space.
-    #[allow(dead_code)]
+    /// Projected model points must retain their acquired source identities;
-    pub(crate) fn last_accepted_snap(&self) -> Option<&AcceptedSnap> {
+    /// proximity on the sheet could bind them to unrelated paper geometry.
-        self.accepted_snaps.last()
-    }
-
-    /// `Scene::infer_dimension_sources`, suppressed when the dimension being
-    /// created measures through a layout viewport.
-    ///
-    /// Inference works by looking for geometry near the dimension's own
-    /// definition points, in whatever space it is handed. A paper-space
-    /// dimension measured through a viewport has PAPER definition points that
-    /// happen to sit over projected model geometry, so unguarded inference
-    /// would either associate it with unrelated paper-sheet geometry at those
-    /// coordinates, or match model geometry using paper coordinates. Neither
-    /// is correct, and a wrong association is worse than none. Returning an
-    /// empty source list makes `attach_dimension_association` a no-op.
-    ///
-    /// The classification is
-    /// [`OpenCADStudio::dimension_measure_space`] — the *same* one that
-    /// decides whether to compensate the measurement, so the two can never
-    /// disagree. In particular it ignores the trailing dimension-line
-    /// placement click, which routinely lands inside a viewport rectangle for
-    /// an ordinary paper-space dimension and must not suppress its
-    /// association.
     pub(crate) fn infer_dimension_sources_guarded(
         &self,
         tab: usize,
@@ -76,14 +36,8 @@ impl OpenCADStudio {
         self.tabs[tab].scene.infer_dimension_sources(dimension)
     }
 
-    /// The single gate on creating a paper-space association for the
+    /// Whether the dimension can use ordinary active-space associations.
-    /// dimension the active command is committing.
+    /// Placement clicks are excluded by the measurement classification.
-    ///
-    /// `false` exactly when the dimension measures through (or partly through)
-    /// a layout viewport, whatever supplied its sources — inference,
-    /// an explicit object pick, or an explicit source list. Everything in the
-    /// commit path that used to decide this for itself now asks here, so the
-    /// measurement rule and the association rule cannot drift apart.
     pub(crate) fn dimension_association_allowed(&self, tab: usize) -> bool {
         matches!(
             self.dimension_measure_space(tab),
@@ -177,7 +131,7 @@ impl OpenCADStudio {
             ) else {
                 return false;
             };
-            let distance = crate::scene::viewport_dim_seam::feature_pick_distance(
+            let distance = crate::scene::viewport_dimension_pick::feature_pick_distance(
                 &entity,
                 accepted.model_point,
                 Some(source.snap_type),
@@ -207,33 +161,15 @@ impl OpenCADStudio {
         }
     }
 
-    /// Append an already-built accepted snap, keeping the retention cap.
+    /// Retain a bounded history for point and object picks.
-    ///
-    /// Used by paths that are not point steps (the dimension object pick
-    /// resolved through a viewport) and therefore never reach the click
-    /// handler.
     pub(crate) fn push_accepted_snap(&mut self, accepted: AcceptedSnap) {
         if self.accepted_snaps.len() >= MAX_ACCEPTED_SNAPS {
             self.accepted_snaps.remove(0);
         }
         self.accepted_snaps.push(accepted);
     }
-    /// Snap the paper-space cursor to model geometry seen through a layout
+    /// Query visible viewport geometry with a canvas-pixel cursor and sheet point.
-    /// viewport.
+    /// Returns paper coordinates and canvas pixels, plus the source viewport frame.
-    ///
-    /// * `i` — tab index.
-    /// * `cursor_canvas` — cursor in canvas pixels.
-    /// * `canvas` — canvas size in pixels.
-    /// * `cursor_paper` — the paper-space point under the cursor.
-    ///
-    /// Returns the hit expressed in PAPER coordinates (`world` projected onto
-    /// the sheet, `screen` in canvas pixels) together with the frame it came
-    /// through, so the caller can merge it with the ordinary paper-sheet snap
-    /// and later recover the model point.
-    ///
-    /// `None` in the Model layout, while a viewport is active (MSPACE already
-    /// snaps in model space), when snapping is off, or when the cursor is not
-    /// over a displayed viewport.
     pub(in crate::app) fn paper_viewport_snap(
         &mut self,
         i: usize,
@@ -316,10 +252,7 @@ impl OpenCADStudio {
             let Some(hit) = hit.filter(|h| h.snap_type != SnapType::Grid) else {
                 continue;
             };
-            // The wire set is the whole model, not a clipped copy, so the only
+            // The snap aperture can reach outside the visible clip boundary.
-            // way a feature outside the visible part could be offered is via
-            // the aperture reaching past the clip. Reject those: a snap must be
-            // a real feature the viewport actually displays.
             let paper = frame.model_to_paper(hit.world);
             if !self.tabs[i]
                 .scene

src/app/viewport_dimension_tests.rs

@@ -149,24 +149,13 @@ fn viewport_dimension_creation_preserves_active_units_and_roundtrips() {
 }
 
 #[test]
-fn viewport_dimension_mixed_and_degenerate_picks_allow_retry() {
+fn viewport_dimension_conflicting_viewports_and_degenerate_picks_allow_retry() {
     let (mut app, line, frame) = fixture();
     let i = app.active_tab;
     let _ = app.dispatch_command("DIMALIGNED");
     point(&mut app, frame, line, DVec3::ZERO);
     point(&mut app, frame, line, DVec3::ZERO);
     assert_eq!(app.accepted_snaps().len(), 1);
-    let _ = app.feed_command(StepInput::Point(DVec3::new(60.0, 50.0, 0.0)));
-    assert_eq!(app.accepted_snaps().len(), 1);
-    assert_eq!(
-        app.tabs[i]
-            .active_cmd
-            .as_ref()
-            .unwrap()
-            .dimension_acquired_points()
-            .len(),
-        1
-    );
     let other = ViewportFrame {
         viewport: Handle::new(0xFFFF),
         ..frame
@@ -318,7 +307,12 @@ fn viewport_dimension_eligibility_is_shared_and_uses_clipping() {
     let top = scene.add_entity(overlay);
     let paper = DVec3::new(55.0, 50.0, 0.0);
     assert_eq!(
-        scene.viewport_frame_at_paper_point(paper).unwrap().viewport,
+        scene
+            .viewport_frames_at_paper_point(paper)
+            .into_iter()
+            .next()
+            .unwrap()
+            .viewport,
         top
     );
     assert_eq!(
@@ -340,7 +334,12 @@ fn viewport_dimension_eligibility_is_shared_and_uses_clipping() {
         vp.clip_boundary_handle = clip;
     }
     assert_eq!(
-        scene.viewport_frame_at_paper_point(paper).unwrap().viewport,
+        scene
+            .viewport_frames_at_paper_point(paper)
+            .into_iter()
+            .next()
+            .unwrap()
+            .viewport,
         first.viewport
     );
     assert_eq!(
@@ -354,7 +353,7 @@ fn viewport_dimension_eligibility_is_shared_and_uses_clipping() {
     if let Some(EntityType::Viewport(vp)) = scene.document.get_entity_mut(first.viewport) {
         vp.status.is_on = false;
     }
-    assert!(scene.viewport_frame_at_paper_point(paper).is_none());
+    assert!(scene.viewport_frames_at_paper_point(paper).is_empty());
     assert!(scene.dimension_pick_through_viewport(paper, 0.2).is_none());
 }
 
@@ -623,8 +622,8 @@ fn viewport_dimension_spline_tangency_is_parallel_and_validated() {
 }
 
 #[test]
-#[ignore = "Set OPENCAD_VIEWPORT_REGRESSION_DXF to the original local regression drawing"]
+#[ignore = "Set OPENCAD_VIEWPORT_REGRESSION_DXF to an external regression fixture"]
-fn viewport_dimension_original_drawing_keeps_unchanged_measurements() {
+fn viewport_dimension_external_fixture_keeps_unchanged_measurements() {
     use crate::scene::ChangeKind;
     let path = std::env::var("OPENCAD_VIEWPORT_REGRESSION_DXF").expect("regression drawing path");
     let doc = acadrust::DxfReader::from_file(std::path::Path::new(&path))
@@ -820,7 +819,9 @@ fn viewport_dimension_nested_block_path_survives_edit_and_copy() {
     let root = scene.add_entity(EntityType::Insert(root));
     scene.set_current_layout("Dimensions".into());
     let transform =
-        crate::scene::dimension_assoc_chain::block_transform(&scene.document, &[root, nested]);
+        crate::scene::dimension_assoc_chain::walk_chain(&scene.document, &[root, nested, leaf])
+            .unwrap()
+            .transform;
     let model = |x| {
         let v = transform.apply(Vector3::new(x, 0.0, 0.0));
         DVec3::new(v.x, v.y, v.z)
@@ -1095,3 +1096,249 @@ fn viewport_dimension_quadrant_keeps_its_feature_after_translation() {
         .bump_entities(&[(source, ChangeKind::Modified)]);
     assert!((displayed(&app.tabs[i].scene.document) - 400.0).abs() < 1e-5);
 }
+
+fn start_centerline_dimension(
+    command: &str,
+    paper_first: bool,
+    paper_kind: SnapType,
+    factor: f64,
+) -> (OpenCADStudio, Handle, Handle, ViewportFrame) {
+    let (mut app, model, frame) = fixture();
+    let i = app.active_tab;
+    let paper = app.tabs[i]
+        .scene
+        .add_entity(EntityType::Line(Line::from_points(
+            Vector3::new(55.0, 40.0, 0.0),
+            Vector3::new(55.0, 60.0, 0.0),
+        )));
+    let mut style = acadrust::tables::DimStyle::new("CenterlineUnits");
+    style.dimlfac = factor;
+    app.tabs[i].scene.document.dim_styles.add(style).unwrap();
+    app.tabs[i].scene.document.header.current_dimstyle_name = "CenterlineUnits".into();
+    let _ = app.dispatch_command(command);
+    let mut paper_hit = hit(frame, paper, DVec3::new(55.0, 50.0, 0.0));
+    paper_hit.world = DVec3::new(55.0, 50.0, 0.0);
+    paper_hit.model_point = None;
+    paper_hit.viewport = None;
+    paper_hit.snap_type = paper_kind;
+    let mut picks = [
+        (paper_hit, None),
+        (hit(frame, model, DVec3::X * 100.0), Some(frame)),
+    ];
+    if !paper_first {
+        picks.reverse();
+    }
+    for (snap, context) in picks {
+        assert!(app.record_accepted_snap(i, Some(snap), context, snap.world));
+        let result = app.tabs[i]
+            .active_cmd
+            .as_mut()
+            .unwrap()
+            .on_point(snap.world);
+        app.sync_dimension_snaps(i);
+        let _ = app.apply_cmd_result(result);
+    }
+    (app, model, paper, frame)
+}
+
+#[test]
+fn viewport_dimension_paper_centerline_uses_viewport_units_in_either_order() {
+    for command in ["DIMLINEAR", "DIMALIGNED"] {
+        for paper_first in [true, false] {
+            let (mut app, _, paper, _) =
+                start_centerline_dimension(command, paper_first, SnapType::Midpoint, 25.4);
+            let paper_snap = app
+                .accepted_snaps()
+                .iter()
+                .find(|s| s.viewport.is_none())
+                .unwrap();
+            assert_eq!(paper_snap.source.as_ref().unwrap().source.handle, paper);
+            assert_eq!(paper_snap.paper_point, paper_snap.model_point);
+            let _ = app.feed_command(StepInput::Point(DVec3::new(60.0, 80.0, 0.0)));
+            let doc = &app.tabs[app.active_tab].scene.document;
+            assert!(
+                (displayed(doc) - 1270.0).abs() < 1e-3,
+                "{command}: {}",
+                displayed(doc)
+            );
+            app.undo_steps(1);
+            app.redo_steps(1);
+            assert!((displayed(&app.tabs[app.active_tab].scene.document) - 1270.0).abs() < 1e-3);
+        }
+    }
+}
+
+#[test]
+fn viewport_dimension_paper_centerline_tracks_each_owning_space() {
+    use crate::scene::{ChangeKind, ReferenceStatus};
+    for (paper_first, kind) in [(true, SnapType::Midpoint), (false, SnapType::Perpendicular)] {
+        let (mut app, model, paper, frame) =
+            start_centerline_dimension("DIMALIGNED", paper_first, kind, 1.0);
+        let i = app.active_tab;
+        let _ = app.feed_command(StepInput::Point(DVec3::new(60.0, 80.0, 0.0)));
+        let dim = dimension(&app.tabs[i].scene.document).base().common.handle;
+        assert!((displayed(&app.tabs[i].scene.document) - 50.0).abs() < 1e-4);
+        let assoc = app.tabs[i].scene.dimension_association(dim).unwrap();
+        assert!(assoc.trans_space);
+        assert!(assoc
+            .references
+            .iter()
+            .flatten()
+            .any(|r| r.xrefs == vec![paper]));
+        assert!(assoc
+            .references
+            .iter()
+            .flatten()
+            .any(|r| r.xrefs == vec![frame.viewport, model]));
+        assert!(app.tabs[i]
+            .scene
+            .dimension_association_status(dim)
+            .iter()
+            .all(|(_, s)| *s == ReferenceStatus::Resolved));
+        let pending = app.begin_undo(i, "Move paper centerline", 1, true).unwrap();
+        let before = app.tabs[i].scene.document.get_entity_arc(paper);
+        app.tabs[i].scene.record_undo_before(paper, before);
+        let EntityType::Line(line) = app.tabs[i].scene.document.get_entity_mut(paper).unwrap()
+        else {
+            panic!()
+        };
+        line.start.x = 56.0;
+        line.end.x = 56.0;
+        app.tabs[i]
+            .scene
+            .bump_entities(&[(paper, ChangeKind::Modified)]);
+        app.commit_undo_delta(i, pending);
+        assert!((displayed(&app.tabs[i].scene.document) - 40.0).abs() < 1e-4);
+        app.undo_steps(1);
+        assert!((displayed(&app.tabs[i].scene.document) - 50.0).abs() < 1e-4);
+        app.redo_steps(1);
+        let EntityType::Viewport(vp) = app.tabs[i]
+            .scene
+            .document
+            .get_entity_mut(frame.viewport)
+            .unwrap()
+        else {
+            panic!()
+        };
+        vp.view_height = 500.0;
+        vp.custom_scale = 0.2;
+        app.tabs[i].scene.notify_viewport_changed(frame.viewport);
+        // Model endpoint now appears at paper x=70; paper centerline stays x=56.
+        assert!((displayed(&app.tabs[i].scene.document) - 70.0).abs() < 1e-4);
+        let EntityType::Viewport(vp) = app.tabs[i]
+            .scene
+            .document
+            .get_entity_mut(frame.viewport)
+            .unwrap()
+        else {
+            panic!()
+        };
+        vp.view_target.x = 20.0;
+        app.tabs[i].scene.notify_viewport_changed(frame.viewport);
+        assert!((displayed(&app.tabs[i].scene.document) - 50.0).abs() < 1e-4);
+        let EntityType::Line(line) = app.tabs[i].scene.document.get_entity_mut(model).unwrap()
+        else {
+            panic!()
+        };
+        line.end.x = 160.0;
+        app.tabs[i]
+            .scene
+            .bump_entities(&[(model, ChangeKind::Modified)]);
+        assert!((displayed(&app.tabs[i].scene.document) - 110.0).abs() < 1e-4);
+        for ext in ["dxf", "dwg"] {
+            let doc = &app.tabs[i].scene.document;
+            let bytes = crate::io::save_to_bytes(doc, ext, doc.version).unwrap();
+            let mut scene = Scene::new();
+            scene.document = crate::io::load_bytes(&format!("centerline.{ext}"), bytes).unwrap();
+            scene.notify_viewport_changed(frame.viewport);
+            assert!((displayed(&scene.document) - 110.0).abs() < 1e-4, "{ext}");
+            assert!(scene
+                .dimension_association_status(dim)
+                .iter()
+                .all(|(_, s)| *s == ReferenceStatus::Resolved));
+        }
+        let before = app.tabs[i].scene.document.get_entity(dim).unwrap().clone();
+        let model_owner = app.tabs[i]
+            .scene
+            .document
+            .get_entity(model)
+            .unwrap()
+            .common()
+            .owner_handle;
+        app.tabs[i]
+            .scene
+            .document
+            .get_entity_mut(paper)
+            .unwrap()
+            .common_mut()
+            .owner_handle = model_owner;
+        app.tabs[i].scene.notify_viewport_changed(frame.viewport);
+        assert_eq!(app.tabs[i].scene.document.get_entity(dim).unwrap(), &before);
+        assert!(app.tabs[i]
+            .scene
+            .dimension_association_status(dim)
+            .iter()
+            .any(|(_, s)| *s == ReferenceStatus::Unresolved));
+    }
+}
+
+#[test]
+fn viewport_dimension_invalid_paths_leave_no_partial_association() {
+    use crate::scene::viewport_ref::SnapSourceRef;
+    use acadrust::entities::{DimensionAligned, Insert};
+    use acadrust::tables::BlockRecord;
+
+    let (mut app, model, frame) = fixture();
+    let scene = &mut app.tabs[app.active_tab].scene;
+    let mut block = BlockRecord::new("Collapsed");
+    block.handle = scene.document.allocate_handle();
+    let owner = block.handle;
+    scene.document.block_records.add(block).unwrap();
+    let mut leaf = Line::from_points(Vector3::ZERO, Vector3::UNIT_X);
+    leaf.common.owner_handle = owner;
+    let leaf = scene.document.add_entity(EntityType::Line(leaf)).unwrap();
+    scene.set_current_layout("Model".into());
+    let mut insert = Insert::new("Collapsed", Vector3::ZERO);
+    insert.set_x_scale(0.0);
+    insert.set_y_scale(0.0);
+    let root = scene.add_entity(EntityType::Insert(insert));
+    scene.set_current_layout("Dimensions".into());
+    let paper = scene.add_entity(EntityType::Line(Line::from_points(
+        Vector3::new(55.0, 50.0, 0.0),
+        Vector3::new(55.0, 60.0, 0.0),
+    )));
+    for (source, path) in [(model, vec![Handle::new(u64::MAX)]), (leaf, vec![root])] {
+        let dim = scene.add_entity(EntityType::Dimension(Dimension::Aligned(
+            DimensionAligned::new(Vector3::new(50.0, 50.0, 0.0), Vector3::new(55.0, 50.0, 0.0)),
+        )));
+        let invalid = AcceptedSnap {
+            paper_point: DVec3::new(50.0, 50.0, 0.0),
+            model_point: DVec3::ZERO,
+            viewport: Some(frame.viewport),
+            frame: Some(frame),
+            source: Some(SnapSourceRef {
+                source: DimensionAssociationSource::inferred(source),
+                block_path: path,
+                snap_type: SnapType::Endpoint,
+                intersection: None,
+            }),
+        };
+        let mut valid = AcceptedSnap::free(DVec3::new(55.0, 50.0, 0.0));
+        valid.source = Some(SnapSourceRef {
+            source: DimensionAssociationSource::inferred(paper),
+            block_path: Vec::new(),
+            snap_type: SnapType::Endpoint,
+            intersection: None,
+        });
+        scene.attach_viewport_dimension_association(dim, &[Some(invalid), Some(valid)]);
+        let association = scene.dimension_association(dim).unwrap();
+        assert!(!association.trans_space);
+        assert!(association.references[0].is_empty());
+        assert_eq!(association.references[1][0].xrefs, vec![paper]);
+        assert_eq!(scene.dimension_association_sources(dim), vec![paper]);
+        assert_eq!(
+            scene.dimension_association_status(dim),
+            vec![(1, crate::scene::ReferenceStatus::Resolved)]
+        );
+    }
+}

src/command.rs

@@ -2245,23 +2245,23 @@ pub trait CadCommand: Send {
         CmdResult::Cancel
     }
 
-    /// Returns `true` when the command needs entity picking (hit-test) instead of point picking.
+    /// Supports acquiring dimension geometry through a paper-space viewport.
-    /// PR2: this command creates a dimension whose measurement must be
-    /// compensated when its points are picked through a paper-space viewport,
-    /// and whose explicit object pick may land on model geometry displayed
-    /// inside one. See `crate::app::dim_viewport`.
     fn measures_through_viewports(&self) -> bool {
         false
     }
 
     /// Definition points acquired so far, in the command's working space.
     /// Used to retain references only for accepted measuring inputs.
-    fn dimension_acquired_points(&self) -> Vec<DVec3> { Vec::new() }
+    fn dimension_acquired_points(&self) -> Vec<DVec3> {
+        Vec::new()
+    }
 
     /// The next point places annotation rather than acquiring geometry.
-    fn dimension_placement_pending(&self) -> bool { false }
+    fn dimension_placement_pending(&self) -> bool {
-
+        false
+    }
 
+    /// Needs entity hit-testing instead of point input.
     fn needs_entity_pick(&self) -> bool {
         false
     }

src/entities/dim_override.rs

@@ -255,12 +255,9 @@ pub fn set_on_entity(entity: &mut EntityType, code: i16, value: Option<XDataValu
     }
 
     let legacy = common.extended_data.get_record("ACAD_DSTYLE").is_some()
-        && common
+        && common.extended_data.get_record("ACAD").is_some_and(
-            .extended_data
+            |rec| !matches!(rec.values.first(), Some(XDataValue::String(n)) if n == "DSTYLE"),
-            .get_record("ACAD")
+        );
-            .is_some_and(|rec| {
-                !matches!(rec.values.first(), Some(XDataValue::String(n)) if n == "DSTYLE")
-            });
     let app = if legacy { "ACAD_DSTYLE" } else { "ACAD" };
 
     let values = if kept.is_empty() {
@@ -281,8 +278,7 @@ pub fn set_on_entity(entity: &mut EntityType, code: i16, value: Option<XDataValu
 
     let mut rebuilt = ExtendedData::new();
     for record in common.extended_data.records() {
-        if record.application_name != app
+        if record.application_name != app && !(!legacy && record.application_name == "ACAD_DSTYLE")
-            && !(!legacy && record.application_name == "ACAD_DSTYLE")
         {
             rebuilt.add_record(record.clone());
         }

src/entities/dimension.rs

@@ -1638,11 +1638,12 @@ pub(crate) fn dimension_in_paper_space(dimension: &Dimension, document: &CadDocu
     if !owner.is_valid() {
         return false;
     }
-    document.objects.values().any(|object| matches!(object,
+    document.objects.values().any(|object| {
-        acadrust::objects::ObjectType::Layout(layout)
+        matches!(object,
-            if layout.name != "Model" && layout.block_record == owner
+            acadrust::objects::ObjectType::Layout(layout)
-    ))
+                if layout.name != "Model" && layout.block_record == owner
-
+        )
+    })
 }
 
 /// The value a dimension's text reports for its measurement: the raw
@@ -1654,7 +1655,10 @@ pub(crate) fn dimension_in_paper_space(dimension: &Dimension, document: &CadDocu
 /// use when they want the number the user reads.
 pub(crate) fn displayed_measurement(dimension: &Dimension, style: Option<&DimStyle>) -> f64 {
     let raw = dimension.measurement();
-    if matches!(dimension, Dimension::Angular2Ln(_) | Dimension::Angular3Pt(_)) {
+    if matches!(
+        dimension,
+        Dimension::Angular2Ln(_) | Dimension::Angular3Pt(_)
+    ) {
         return raw;
     }
     let lfac = style.map(|s| s.dimlfac).unwrap_or(1.0);

src/modules/annotate/aligned_dim.rs

@@ -161,7 +161,7 @@ impl CadCommand for AlignedDimensionCommand {
         CmdResult::Cancel
     }
 
-    /// PR2: points and object picks may come through a paper-space viewport;
+    /// Points and object picks may come through a paper-space viewport;
     /// the committed dimension then reports the model measurement.
     fn measures_through_viewports(&self) -> bool {
         true
@@ -169,7 +169,8 @@ impl CadCommand for AlignedDimensionCommand {
 
     fn dimension_acquired_points(&self) -> Vec<DVec3> {
         match self.step {
-            Step::First => vec![], Step::Second(p) => vec![p],
+            Step::First => vec![],
+            Step::Second(p) => vec![p],
             Step::DimLine { p1, p2 } => vec![p1, p2],
         }
     }

src/modules/annotate/angular_dim.rs

@@ -482,7 +482,7 @@ impl CadCommand for AngularDimensionCommand {
         CmdResult::Cancel
     }
 
-    /// PR2: points and object picks may come through a paper-space viewport;
+    /// Points and object picks may come through a paper-space viewport;
     /// the committed dimension then reports the model measurement.
     fn measures_through_viewports(&self) -> bool {
         true
@@ -490,11 +490,27 @@ impl CadCommand for AngularDimensionCommand {
 
     fn dimension_acquired_points(&self) -> Vec<DVec3> {
         match self.step {
-            Step::Vertex => vec![], Step::FirstRay(p) => vec![p],
+            Step::Vertex => vec![],
-            Step::SecondRay { vertex, first } | Step::CircleSecondRay { vertex, first, .. } => vec![vertex, first],
+            Step::FirstRay(p) => vec![p],
-            Step::SecondLine { first_start, first_end, .. } => vec![first_start, first_end],
+            Step::SecondRay { vertex, first } | Step::CircleSecondRay { vertex, first, .. } => {
-            Step::ArcPoint3 { vertex, first, second } => vec![vertex, first, second],
+                vec![vertex, first]
-            Step::ArcPoint2 { first_start, first_end, second_start, second_end } => vec![first_start, first_end, second_start, second_end],
+            }
+            Step::SecondLine {
+                first_start,
+                first_end,
+                ..
+            } => vec![first_start, first_end],
+            Step::ArcPoint3 {
+                vertex,
+                first,
+                second,
+            } => vec![vertex, first, second],
+            Step::ArcPoint2 {
+                first_start,
+                first_end,
+                second_start,
+                second_end,
+            } => vec![first_start, first_end, second_start, second_end],
         }
     }
 

src/modules/annotate/diameter_dim.rs

@@ -132,7 +132,7 @@ impl CadCommand for DiameterDimensionCommand {
         CmdResult::Cancel
     }
 
-    /// PR2: points and object picks may come through a paper-space viewport;
+    /// Points and object picks may come through a paper-space viewport;
     /// the committed dimension then reports the model measurement.
     fn measures_through_viewports(&self) -> bool {
         true
@@ -140,7 +140,8 @@ impl CadCommand for DiameterDimensionCommand {
 
     fn dimension_acquired_points(&self) -> Vec<DVec3> {
         match self.step {
-            Step::SelectObject => vec![], Step::DimLine(source) => vec![dvec(source.point_at_angle(source.start_angle))],
+            Step::SelectObject => vec![],
+            Step::DimLine(source) => vec![dvec(source.point_at_angle(source.start_angle))],
         }
     }
 

src/modules/annotate/linear_dim.rs

@@ -217,7 +217,7 @@ impl CadCommand for LinearDimensionCommand {
         CmdResult::Cancel
     }
 
-    /// PR2: points and object picks may come through a paper-space viewport;
+    /// Points and object picks may come through a paper-space viewport;
     /// the committed dimension then reports the model measurement.
     fn measures_through_viewports(&self) -> bool {
         true
@@ -225,7 +225,8 @@ impl CadCommand for LinearDimensionCommand {
 
     fn dimension_acquired_points(&self) -> Vec<DVec3> {
         match self.step {
-            Step::FirstPoint => vec![], Step::SecondPoint(p) => vec![p],
+            Step::FirstPoint => vec![],
+            Step::SecondPoint(p) => vec![p],
             Step::DimensionLine { first, second } => vec![first, second],
         }
     }

src/modules/annotate/radius_dim.rs

@@ -130,7 +130,7 @@ impl CadCommand for RadiusDimensionCommand {
         CmdResult::Cancel
     }
 
-    /// PR2: points and object picks may come through a paper-space viewport;
+    /// Points and object picks may come through a paper-space viewport;
     /// the committed dimension then reports the model measurement.
     fn measures_through_viewports(&self) -> bool {
         true
@@ -138,7 +138,8 @@ impl CadCommand for RadiusDimensionCommand {
 
     fn dimension_acquired_points(&self) -> Vec<DVec3> {
         match self.step {
-            Step::SelectObject => vec![], Step::DimLine(source) => vec![dvec(source.point_at_angle(source.start_angle))],
+            Step::SelectObject => vec![],
+            Step::DimLine(source) => vec![dvec(source.point_at_angle(source.start_angle))],
         }
     }
 

src/scene/dimension_assoc.rs

@@ -370,15 +370,7 @@ fn source_marker(entity: &EntityType, point: Vector3) -> Option<i32> {
 
 // ── Chain-aware reference resolution ──────────────────────────────────────
 
-/// How a resolved feature point travels from the entity it belongs to up into
+/// Transform an entity-local feature through its block path and optional viewport.
-/// the space the dimension is drawn in.
-///
-/// `transform` is the block-instance path (entity-local -> model).
-/// `frame` is the layout viewport the chain looks through, when it has one;
-/// with a frame the dimension lives on the sheet and every point it holds is
-/// a *paper* point, while the value it displays is a *model* length. Keeping
-/// both directions in one place is what stops the viewport compensation from
-/// being applied twice (see `viewport_ref::measurement_rule`).
 #[derive(Clone, Copy, Debug)]
 pub(crate) struct ChainMap {
     pub transform: Transform,
@@ -528,21 +520,10 @@ fn hint_error(entity: &EntityType, point: Vector3) -> f64 {
     f64::INFINITY
 }
 
-/// The stored osnap point, brought into the entity's own coordinates.
+/// Interpret a stored point as a source-space or dimension-space hint.
-///
+/// Validate both interpretations against the geometry before using the hint
-/// Which space the file wrote it in is not recoverable from the record: our
+/// to disambiguate a feature; the stored point is never the resolved result.
-/// own pre-PR3 writer stored a model/WCS point, AutoCAD's trans-space records
+fn local_hint(map: &ChainMap, entity: &EntityType, stored: Vector3) -> Option<Vector3> {
-/// hold the point in the space the dimension is drawn in, and both appear in
-/// the same drawing. Rather than guess, both readings are mapped down to the
-/// entity and the one that actually lands on the geometry wins. The point is
-/// only ever a *hint* — it disambiguates candidates, it is never the answer.
-fn local_hint(
-    scene: &Scene,
-    map: &ChainMap,
-    entity: &EntityType,
-    stored: Vector3,
-) -> Option<Vector3> {
-    let _ = scene;
     let inverse = chain::invert(&map.transform)?;
     let mut candidates = vec![inverse.apply(stored)];
     if map.frame.is_some() {
@@ -610,7 +591,7 @@ pub fn resolve_reference_chain(
             .max(reference.osnap_point.z.abs())
             < 1e40
         {
-            local_hint(scene, &map, entity, reference.osnap_point)
+            local_hint(&map, entity, reference.osnap_point)
         } else {
             None
         };
@@ -636,9 +617,8 @@ pub fn resolve_reference_chain(
     })
 }
 
-/// The pre-PR3 marker/parameter resolution, unchanged in behaviour but now
+/// Resolve a marker/parameter in entity-local coordinates.
-/// evaluated in the *entity's own* coordinates rather than assumed to be in
+/// The caller applies the block and viewport transforms.
-/// model space. Lifting through the block path is the caller's job.
 fn resolve_local_by_marker(
     entity: &EntityType,
     reference: &AssocDimensionReference,
@@ -1224,8 +1204,7 @@ impl Scene {
     }
 
     /// Persist the association for a dimension whose points were picked
-    /// through [`AcceptedSnap`]s — the entry point PR2 calls once it has
+    /// through [`AcceptedSnap`]s, preserving each source and its owning space.
-    /// created a paper-space dimension that measures model geometry.
     ///
     /// `snaps` is positional: one slot per definition point, in the same order
     /// [`dimension_reference_points`] returns them (linear/aligned: first,
@@ -1245,13 +1224,7 @@ impl Scene {
         else {
             return;
         };
-        // A radial dimension has ONE reference, and it is an
+        // Radial references identify a circle/arc and angle, rather than a vertex.
-        // `AcDbOsnapPointRef` of osnap type 10 carrying the *radial* marker
-        // (which circle / which polyline arc segment) and the angle around it,
-        // not a vertex marker. `source_reference` would return the generic
-        // curve-parameter marker `-2`, which `radial_source_for_marker` cannot
-        // find, so radial goes down its own path — the same one
-        // `attach_dimension_association_sources` uses for model space.
         let radial_dimension = matches!(
             dimension_entity,
             Dimension::Radius(_) | Dimension::Diameter(_) | Dimension::LargeRadial(_)
@@ -1278,18 +1251,17 @@ impl Scene {
             let mut xrefs = Vec::new();
             if let Some(viewport) = snap.viewport {
                 xrefs.push(viewport);
-                trans_space = true;
             }
             xrefs.extend(source.block_path.iter().copied());
             xrefs.push(entity_handle);
-            reactor_targets.extend(xrefs.iter().copied());
+            let Ok(walked) = chain::walk_chain(&self.document, &xrefs) else {
-
+                continue;
+            };
+            let Some(inverse) = chain::invert(&walked.transform) else {
+                continue;
+            };
+            let local = inverse.apply(dvec3(snap.model_point));
             let osnap_type = chain::osnap_type_for(source.snap_type);
-            let local = Self::point_in_entity_space(
-                &self.document,
-                &source.block_path,
-                dvec3(snap.model_point),
-            );
             let marker_parameter = source
                 .source
                 .marker
@@ -1314,6 +1286,8 @@ impl Scene {
                 }
                 reactor_targets.extend(intersection_objects.iter().copied());
             }
+            trans_space |= snap.viewport.is_some();
+            reactor_targets.extend(xrefs.iter().copied());
             associativity |= 1 << index;
             references[index].push(AssocDimensionReference {
                 class_name: "AcDbOsnapPointRef".to_string(),
@@ -1323,8 +1297,8 @@ impl Scene {
                 main_subent_type: 1,
                 main_gs_marker: marker,
                 osnap_distance: parameter,
-                // Stored in MODEL space. `local_hint` reads either convention,
+                // Store the source's owning-space point: model coordinates for
-                // so an imported paper-space point still resolves.
+                // viewport features, paper coordinates for direct sheet features.
                 osnap_point: dvec3(snap.model_point),
                 ..AssocDimensionReference::default()
             });
@@ -1351,12 +1325,7 @@ impl Scene {
         dimension: Handle,
         snaps: &[Option<AcceptedSnap>],
     ) {
-        let Some(snap) = snaps
+        let Some(snap) = snaps.iter().flatten().find(|snap| snap.source.is_some()) else {
-            .iter()
-            .flatten()
-            .find(|snap| snap.source.is_some())
-            .cloned()
-        else {
             return;
         };
         let source = snap.source.as_ref().expect("filtered above");
@@ -1364,14 +1333,17 @@ impl Scene {
         let Some(entity) = self.document.get_entity(entity_handle) else {
             return;
         };
-        // The acquired point in the source entity's own coordinates, so the
+        let mut xrefs = Vec::new();
-        // radial geometry and the angle are both computed where the entity
+        xrefs.extend(snap.viewport);
-        // lives.
+        xrefs.extend(source.block_path.iter().copied());
-        let local = Self::point_in_entity_space(
+        xrefs.push(entity_handle);
-            &self.document,
+        let Ok(walked) = chain::walk_chain(&self.document, &xrefs) else {
-            &source.block_path,
+            return;
-            dvec3(snap.model_point),
+        };
-        );
+        let Some(inverse) = chain::invert(&walked.transform) else {
+            return;
+        };
+        let local = inverse.apply(dvec3(snap.model_point));
         let radial = match source.source.marker {
             Some(marker) => radial_source_for_marker(entity, marker),
             None => radial_source_at(entity, local),
@@ -1382,14 +1354,7 @@ impl Scene {
         } else {
             radial.angle_at(dpoint(local))
         };
-        let mut xrefs = Vec::new();
+        let trans_space = snap.viewport.is_some();
-        let mut trans_space = false;
-        if let Some(viewport) = snap.viewport {
-            xrefs.push(viewport);
-            trans_space = true;
-        }
-        xrefs.extend(source.block_path.iter().copied());
-        xrefs.push(entity_handle);
         let reactor_targets = xrefs.clone();
         let reference = AssocDimensionReference {
             class_name: "AcDbOsnapPointRef".to_string(),
@@ -1431,28 +1396,64 @@ impl Scene {
         }
     }
 
-    /// A model-space point brought down into the coordinates of the entity at
+    /// Select one measurement viewport while allowing direct references to
-    /// the end of `block_path`. Identity for top-level geometry.
+    /// geometry owned by the dimension's paper layout. A model-space chain
-    fn point_in_entity_space(
+    /// without its viewport cannot be interpreted as a paper-space anchor.
-        document: &acadrust::CadDocument,
+    fn association_measurement_frame(
-        block_path: &[Handle],
+        &self,
-        point: Vector3,
+        association: &AssocDimensionAssociation,
-    ) -> Vector3 {
+    ) -> Result<Option<ViewportFrame>, ()> {
-        if block_path.is_empty() {
+        let mut viewport = None;
-            return point;
+        for reference in association.references.iter().flatten() {
+            for handles in [&reference.xrefs, &reference.intersection_objects] {
+                let Some(root) = handles.first() else {
+                    continue;
+                };
+                if matches!(
+                    self.document.get_entity(*root),
+                    Some(EntityType::Viewport(_))
+                ) {
+                    if viewport.is_some_and(|previous| previous != *root) {
+                        return Err(());
+                    }
+                    viewport = Some(*root);
+                }
+            }
         }
-        chain::invert(&chain::block_transform(document, block_path))
+        let Some(viewport) = viewport else {
-            .map(|inverse| inverse.apply(point))
+            return if association.trans_space {
-            .unwrap_or(point)
+                Err(())
+            } else {
+                Ok(None)
+            };
+        };
+        let owner = self
+            .document
+            .get_entity(association.dimension)
+            .ok_or(())?
+            .common()
+            .owner_handle;
+        for reference in association.references.iter().flatten() {
+            for handles in [&reference.xrefs, &reference.intersection_objects] {
+                let Some(root) = handles.first() else {
+                    continue;
+                };
+                if self
+                    .document
+                    .get_entity(*root)
+                    .ok_or(())?
+                    .common()
+                    .owner_handle
+                    != owner
+                {
+                    return Err(());
+                }
+            }
+        }
+        self.viewport_frame(viewport).map(Some).ok_or(())
     }
 
-    /// Per-slot health of a dimension's association, for the UI and for tests.
+    /// Resolve each reference's current status without changing its saved data.
-    ///
-    /// Nothing here is persisted: brokenness is *derived* from whether the
-    /// chain still resolves. That is what makes undo restore a link for free —
-    /// the erased source comes back and the same query starts answering
-    /// [`ReferenceStatus::Resolved`] again, with the reference record itself
-    /// never having been rewritten.
     pub fn dimension_association_status(&self, dimension: Handle) -> Vec<(usize, ReferenceStatus)> {
         let Some(association) = self.dimension_association(dimension) else {
             return Vec::new();
@@ -1485,8 +1486,15 @@ impl Scene {
                     .is_some()
             });
         let measurement_supported = measurement_supported
-            && association.references.iter().flatten()
+            && association
-                .filter(|r| chain::needs_from_point(r.osnap_type)).count() <= 1;
+                .references
+                .iter()
+                .flatten()
+                .filter(|r| chain::needs_from_point(r.osnap_type))
+                .count()
+                <= 1;
+        let measurement_supported =
+            measurement_supported && self.association_measurement_frame(association).is_ok();
         let points = self.dimension_association_slot_points(dimension);
         association
             .references
@@ -1519,7 +1527,7 @@ impl Scene {
     ///
     /// Layer visibility and viewport clipping are deliberately not consulted:
     /// hiding a layer or clipping a viewport changes what a *new* snap may
-    /// acquire (PR1's concern), never what an existing association points at.
+    /// acquire, never what an existing association points at.
     pub fn dimension_association_sources(&self, dimension: Handle) -> Vec<Handle> {
         let Some(association) = self.dimension_association(dimension) else {
             return Vec::new();
@@ -1553,15 +1561,8 @@ impl Scene {
         })
     }
 
-    /// Duplicate dimension associations across a COPY/MIRROR/ARRAY handle map.
+    /// Preserve an association only when the dimension and every source-chain
-    ///
+    /// handle were copied. Remap the complete chain to the new entities.
-    /// A copied dimension only keeps an association when *every* handle in
-    /// every chain was copied too — the viewport, each INSERT on the path and
-    /// the source entity. Then the whole chain is remapped onto the copies, so
-    /// the copy measures the copied geometry. Copying a dimension on its own
-    /// leaves it with no association at all, which is exactly what the model
-    /// space path did before this: an all-or-nothing rule, never a copy that
-    /// silently drives off the original's geometry.
     pub(crate) fn copy_dimension_associations(
         &mut self,
         handle_map: &rustc_hash::FxHashMap<Handle, Handle>,
@@ -1591,7 +1592,7 @@ impl Scene {
                 if !complete {
                     return None;
                 }
-                let remap = |handles: &Vec<Handle>| -> Vec<Handle> {
+                let remap = |handles: &[Handle]| -> Vec<Handle> {
                     handles
                         .iter()
                         .map(|handle| handle_map.get(handle).copied().unwrap_or(*handle))
@@ -1640,15 +1641,7 @@ impl Scene {
         }
     }
 
-    /// Report a layout viewport as changed so the dimensions drawn through it
+    /// Refresh dimensions when their viewport moves, pans, zooms, or twists.
-    /// are regenerated.
-    ///
-    /// A viewport move / pan / zoom / twist changes where model geometry lands
-    /// on the sheet without touching that geometry, so nothing in the ordinary
-    /// entity-change path would notice. Every reference that looks through a
-    /// viewport carries that viewport's handle as `xrefs[0]`, so announcing the
-    /// viewport itself is all that is needed — the existing dependency lookup
-    /// in `refresh_associative_dimensions` finds the associations from there.
     pub fn notify_viewport_changed(&mut self, viewport: Handle) {
         if matches!(
             self.document.get_entity(viewport),
@@ -1750,17 +1743,25 @@ impl Scene {
                             .chain(reference.intersection_objects.iter())
                             .any(|handle| changed.contains(handle))
                     })
-                    .then_some(association.clone())
+                    .then(|| association.clone())
             })
             .collect();
 
         let mut refreshed = Vec::new();
         for association in associations {
+            let Ok(frame) = self.association_measurement_frame(&association) else {
+                continue;
+            };
             // Dependent snaps need a resolved independent definition point.
             // Two such features require a coupled solve; old points are not
             // evidence of a solution after the geometry changes.
-            if association.references.iter().flatten()
+            if association
-                .filter(|r| chain::needs_from_point(r.osnap_type)).count() > 1
+                .references
+                .iter()
+                .flatten()
+                .filter(|r| chain::needs_from_point(r.osnap_type))
+                .count()
+                > 1
             {
                 continue;
             }
@@ -1806,27 +1807,20 @@ impl Scene {
             {
                 continue;
             }
-            let viewports: std::collections::HashSet<_> =
+            // Every point is already in paper space: direct sheet references
-                resolved.iter().flatten().map(|r| r.viewport).collect();
+            // stay there, and model references are projected individually.
-            if viewports.len() > 1 {
+            // Only the measurement factor is shared by the whole dimension.
-                continue;
+            let map = frame
-            }
-            // The map of the first live reference governs how the whole
-            // dimension is placed and measured — all of a dimension's points
-            // are in one space.
-            let map = resolved
-                .iter()
-                .flatten()
-                .next()
-                .and_then(|value| value.viewport)
-                .and_then(|handle| self.viewport_frame(handle))
                 .map(|frame| ChainMap {
                     transform: Transform::identity(),
                     frame: Some(frame),
                 })
                 .unwrap_or_else(ChainMap::identity);
-            let model_points: [Option<Vector3>; 4] =
+            let model_points: [Option<Vector3>; 4] = std::array::from_fn(|index| {
-                std::array::from_fn(|index| resolved[index].map(|value| value.model));
+                resolved[index]
+                    .filter(|value| value.viewport.is_some())
+                    .map(|value| value.model)
+            });
             let resolved: [Option<Vector3>; 4] =
                 std::array::from_fn(|index| resolved[index].map(|value| value.space));
 
@@ -2117,18 +2111,7 @@ impl Scene {
                 Dimension::Linear(linear) => linear_measurement(linear),
                 _ => dimension.measurement(),
             };
-            // `actual_measurement` stays RAW, in the dimension's own space —
+            // Store the raw sheet measurement; the formatter applies DIMLFAC once.
-            // paper, for a dimension drawn on the sheet through a viewport.
-            // The viewport compensation is carried by the negative DIMLFAC
-            // override and applied once, by the formatter. See
-            // `viewport_ref::measurement_rule`: substituting the model
-            // distance here (or multiplying by `MeasurementScale::model_factor`)
-            // would make a refreshed dimension read a different number from an
-            // identical freshly-created one.
-            //
-            // The similarity property of a viewport frame is what makes this
-            // exact: paper_span * (1 / scale) is the model span, for a rotated
-            // linear projection just as much as for a free distance.
             debug_assert!(
                 map.frame.is_none()
                     || model_points
@@ -2176,12 +2159,7 @@ impl Scene {
     }
 }
 
-/// How far the midpoint of a dimension's measured points moved.
+/// Average displacement of the dimension's resolved definition points.
-///
-/// Used to carry the sheet annotation — the dimension line's standoff and a
-/// user-dragged text position — along with the geometry. Slots that did not
-/// resolve keep their old point, so a one-ended edit shifts the annotation by
-/// half the movement, which is what keeps the dimension line centred.
 fn centroid_shift(before: [Option<Vector3>; 2], after: [Option<Vector3>; 2]) -> Vector3 {
     let mut delta = Vector3::new(0.0, 0.0, 0.0);
     let mut count = 0.0;
@@ -2194,8 +2172,6 @@ fn centroid_shift(before: [Option<Vector3>; 2], after: [Option<Vector3>; 2]) ->
     if count == 0.0 {
         return Vector3::new(0.0, 0.0, 0.0);
     }
-    // Averaged over the slots that actually moved, not over both, so a
-    // dimension with one broken reference still tracks the live one.
     Vector3::new(delta.x / count, delta.y / count, delta.z / count)
 }
 

src/scene/dimension_assoc_chain.rs

@@ -1,38 +1,8 @@
-//! Full reference chains for associative dimensions.
+//! Resolve a dimension feature through `[viewport?, insert..., entity]`.
-//!
+//! `xrefs` stores the primary chain; `intersection_objects` stores the second
-//! An `AssocDimensionReference` addresses a *feature* — an osnap point on a
+//! intersection source. Features are evaluated in entity-local coordinates and
-//! piece of geometry — through a chain of handles:
+//! transformed through their block instances. The caller projects viewport
-//!
+//! references onto the sheet. All arithmetic uses `f64`.
-//! ```text
-//! dimension -> [viewport] -> [insert ...] -> entity -> feature
-//! ```
-//!
-//! `AssocDimensionReference::xrefs` is that chain, outermost first. AutoCAD
-//! calls the same list `mainObjectIds`; `intersection_objects`
-//! (`intersectObjectIds`) is the second chain an INTERSECTION / APPARENT
-//! INTERSECTION osnap needs.
-//!
-//! The pre-PR3 resolver only ever read `xrefs[0]`, which is correct for a
-//! model-space dimension on top-level geometry and wrong for everything else:
-//! a paper-space dimension measuring through a layout viewport stores the
-//! VIEWPORT as `xrefs[0]`, so the resolver stopped at a viewport entity, found
-//! no curve on it and gave up. Nine of the ten association objects in the
-//! 115448 regression drawing are of that shape.
-//!
-//! This module owns:
-//!  * [`walk_chain`] — turn `xrefs` into a viewport, a block-instance path
-//!    with its accumulated transform, and the innermost entity.
-//!  * [`feature_point`] — evaluate the osnap feature on that entity, in the
-//!    entity's own coordinates, then lift it through the block path into
-//!    model space.
-//!
-//! Mapping model -> paper through the viewport frame is deliberately *not*
-//! done here; the caller owns it, because only the caller knows whether the
-//! association is `trans_space` and which [`ViewportFrame`] is current.
-//!
-//! All arithmetic is `f64`.
-//!
-//! [`ViewportFrame`]: super::viewport_ref::ViewportFrame
 
 use acadrust::objects::AssocDimensionReference;
 use acadrust::types::{Handle, Matrix4, Transform, Vector3};
@@ -45,12 +15,8 @@ use crate::entities::curve::entity_curve;
 /// `AcDb::OsnapMode`, the value stored in
 /// [`AssocDimensionReference::osnap_type`].
 ///
-/// These are the AutoCAD ObjectARX codes, not our own [`crate::snap::SnapType`]
+/// These are ObjectARX codes, independent of [`crate::snap::SnapType`].
-/// discriminants. The pre-PR3 writer already emitted `END` for generic points
+#[allow(dead_code)] // Include codes preserved from unsupported imported modes.
-/// and `NEAR` for a point taken at a parameter on a circle, so those two keep
-/// meaning exactly what they used to.
-#[allow(dead_code)] // The full code table is kept so a reader can see what a
-                    // stored `osnap_type` means, including the modes we pass through untouched.
 pub(crate) mod osnap {
     pub const NONE: u8 = 0;
     pub const END: u8 = 1;
@@ -72,7 +38,7 @@ pub(crate) mod osnap {
 ///
 /// Modes with no ObjectARX equivalent (grid, object pick) map to
 /// [`osnap::NONE`]; a reference carrying `NONE` resolves by marker/parameter
-/// alone, exactly like the pre-PR3 records.
+/// alone.
 pub(crate) fn osnap_type_for(snap: crate::snap::SnapType) -> u8 {
     use crate::snap::SnapType as S;
     match snap {
@@ -166,7 +132,11 @@ pub(crate) fn walk_chain(
             return Err(ChainError::Invalid);
         };
         // Arrays need an instance index that this reference format does not carry.
-        if insert.row_count > 1 || insert.column_count > 1 || block_path.contains(&handle) {
+        if insert.row_count > 1
+            || insert.column_count > 1
+            || block_path.len() >= 8
+            || block_path.contains(&handle)
+        {
             return Err(ChainError::Invalid);
         }
         let block = document
@@ -182,64 +152,23 @@ pub(crate) fn walk_chain(
     Err(ChainError::Empty)
 }
 
-/// Accumulated entity-local -> model transform for an ordered INSERT path.
+/// Invert an affine block transform, rejecting singular or nonfinite matrices.
-pub(crate) fn block_transform(document: &CadDocument, block_path: &[Handle]) -> Transform {
-    let mut transform = Transform::identity();
-    for handle in block_path {
-        if let Some(EntityType::Insert(insert)) = document.get_entity(*handle) {
-            transform = transform.compose(&crate::scene::render_graph::insert_transform(
-                document, insert,
-            ));
-        }
-    }
-    transform
-}
-
-/// Affine inverse of `transform`.
-///
-/// `acadrust::types::Matrix4` has no inverse of its own, and a block path only
-/// ever produces affine matrices (translate / rotate / scale), so this inverts
-/// the 3x3 linear part by cofactors and back-substitutes the translation. A
-/// degenerate (zero-scale) insert returns `None` rather than exploding.
 pub(crate) fn invert(transform: &Transform) -> Option<Transform> {
     let m = &transform.matrix.m;
-    let a = [
+    let matrix = glam::DMat4::from_cols_array_2d(&std::array::from_fn(|column| {
-        [m[0][0], m[0][1], m[0][2]],
+        std::array::from_fn(|row| m[row][column])
-        [m[1][0], m[1][1], m[1][2]],
+    }));
-        [m[2][0], m[2][1], m[2][2]],
+    if !matrix.is_finite() || matrix.determinant().abs() < 1e-18 {
-    ];
-    let det = a[0][0] * (a[1][1] * a[2][2] - a[1][2] * a[2][1])
-        - a[0][1] * (a[1][0] * a[2][2] - a[1][2] * a[2][0])
-        + a[0][2] * (a[1][0] * a[2][1] - a[1][1] * a[2][0]);
-    if !det.is_finite() || det.abs() < 1e-18 {
         return None;
     }
-    let inv_det = 1.0 / det;
+    let inverse = matrix.inverse();
-    let mut inverse = [[0.0f64; 3]; 3];
+    if !inverse.is_finite() {
-    for row in 0..3 {
+        return None;
-        for col in 0..3 {
-            // Cofactor of (col, row) — transposed, which is the adjugate.
-            let (r0, r1) = ((col + 1) % 3, (col + 2) % 3);
-            let (c0, c1) = ((row + 1) % 3, (row + 2) % 3);
-            inverse[row][col] = (a[r0][c0] * a[r1][c1] - a[r0][c1] * a[r1][c0]) * inv_det;
-        }
     }
-    let t = [m[0][3], m[1][3], m[2][3]];
+    let columns = inverse.to_cols_array_2d();
-    let translation = [
+    Some(Transform::from_matrix(Matrix4 {
-        -(inverse[0][0] * t[0] + inverse[0][1] * t[1] + inverse[0][2] * t[2]),
+        m: std::array::from_fn(|row| std::array::from_fn(|column| columns[column][row])),
-        -(inverse[1][0] * t[0] + inverse[1][1] * t[1] + inverse[1][2] * t[2]),
+    }))
-        -(inverse[2][0] * t[0] + inverse[2][1] * t[1] + inverse[2][2] * t[2]),
-    ];
-    Some(Transform {
-        matrix: Matrix4 {
-            m: [
-                [inverse[0][0], inverse[0][1], inverse[0][2], translation[0]],
-                [inverse[1][0], inverse[1][1], inverse[1][2], translation[1]],
-                [inverse[2][0], inverse[2][1], inverse[2][2], translation[2]],
-                [0.0, 0.0, 0.0, 1.0],
-            ],
-        },
-    })
 }
 
 // ── Feature evaluation ────────────────────────────────────────────────────
@@ -482,9 +411,7 @@ fn nearest_point(
     hint: Option<Vector3>,
 ) -> Option<Vector3> {
     let planar = entity_curve(entity)?;
-    // Circles and arcs carry an *angle* in `osnap_distance` — that is the
+    // Circles and arcs store an angle; other curves use a normalized parameter.
-    // convention the pre-PR3 writer established and imported files share it.
-    // Every other curve carries the kernel's normalised 0..1 parameter.
     match &planar.curve {
         KernelCurve::Circle(_) | KernelCurve::Arc(_) => match entity {
             EntityType::Circle(circle) => Some(circle.point_at_angle_wcs(reference.osnap_distance)),
@@ -506,14 +433,8 @@ fn nearest_point(
 
 // ── Tangent ───────────────────────────────────────────────────────────────
 
-/// Tangent point on `entity` for the touch line through `context.from`.
+/// Tangent point for the line through `context.from`.
-///
+/// Spline tangency solves `(C(u) - P) x C'(u) = 0` and validates the residual.
-/// Circles and arcs have a closed form. A spline does not, so the stored
-/// parameter (`osnap_distance`) seeds a Newton refinement of the tangency
-/// condition `(C(u) - P) · C'(u) = 0`; with no usable parameter the seed is
-/// the point on the spline nearest the stored osnap point. That is what makes
-/// the 115448 drawing's tangent-to-spline reference survive an edit to the
-/// spline instead of silently freezing at its imported location.
 fn tangent_point(
     entity: &EntityType,
     reference: &AssocDimensionReference,

src/scene/mod.rs

@@ -43,7 +43,7 @@ mod limits;
 mod modify;
 mod mspace;
 pub mod viewport_ref;
-pub mod viewport_dim_seam;
+pub mod viewport_dimension_pick;
 pub mod named_parameters;
 mod page_setup;
 mod paper;

src/scene/mspace.rs

@@ -83,14 +83,7 @@ impl Scene {
         self.viewport_edit_frame_for(self.active_viewport?, canvas_px)
     }
 
-    /// [`viewport_edit_frame`] for an explicitly-named viewport, rather than
+    /// Camera and canvas rectangle for snapping through an inactive viewport.
-    /// only the active (MSPACE) one.
-    ///
-    /// Paper-space snapping through viewports (PR1) needs the same camera +
-    /// screen rectangle pair for a viewport the user has NOT entered, so the
-    /// existing snap engine can run against that viewport's model wires on the
-    /// pixels the GPU actually drew them on. `viewport_edit_frame` is now a
-    /// thin wrapper over this.
     pub fn viewport_edit_frame_for(
         &self,
         vp_handle: Handle,
@@ -101,15 +94,12 @@ impl Scene {
         Some((cam, full))
     }
 
-    /// Planar paper <-> model mapping for a layout viewport, derived from the
+    /// Planar paper/model mapping derived from the viewport's display camera.
-    /// *same* camera the renderer displays its contents with
+    /// Oblique and perspective views have no supported planar mapping.
-    /// ([`camera_for_viewport`]) so a snap computed through the frame lands on
+    pub fn viewport_frame(
-    /// the pixels the GPU drew.
+        &self,
-    ///
+        vp_handle: Handle,
-    /// Returns `None` for a viewport whose view is not an in-plane orthographic
+    ) -> Option<crate::scene::viewport_ref::ViewportFrame> {
-    /// top view (oblique / isometric / perspective viewports are unsupported by
-    /// the planar frame — callers fall back to no viewport snapping there).
-    pub fn viewport_frame(&self, vp_handle: Handle) -> Option<crate::scene::viewport_ref::ViewportFrame> {
         use crate::scene::viewport_ref::ViewportFrame;
 
         let (paper_center, vp_height, locked) = match self.document.get_entity(vp_handle) {
@@ -120,7 +110,7 @@ impl Scene {
             ),
             _ => return None,
         };
-        if vp_height.abs() < 1e-9 {
+        if !paper_center.is_finite() || !vp_height.is_finite() || vp_height < 1e-9 {
             return None;
         }
         let cam = self.camera_for_viewport(vp_handle)?;
@@ -133,7 +123,7 @@ impl Scene {
         // an oblique 3-D view: unsupported.
         let right = (cam.rotation * glam::Vec3::X).as_dvec3();
         let up = (cam.rotation * glam::Vec3::Y).as_dvec3();
-        if right.z.abs() > 1e-6 || up.z.abs() > 1e-6 {
+        if !right.is_finite() || !up.is_finite() || right.z.abs() > 1e-6 || up.z.abs() > 1e-6 {
             return None;
         }
         let normal = right.cross(up);
@@ -142,7 +132,7 @@ impl Scene {
         }
         // Model units visible across the viewport's height -> paper per model.
         let model_height = cam.ortho_size() as f64 * 2.0;
-        if model_height.abs() < 1e-12 {
+        if !model_height.is_finite() || model_height < 1e-12 {
             return None;
         }
         let scale = vp_height / model_height;
@@ -162,23 +152,28 @@ impl Scene {
 
     /// Visible, supported content viewports at a sheet point, topmost first.
     /// Shared by snapping and explicit dimension object picking.
-    pub fn viewport_frames_at_paper_point(&self, paper: glam::DVec3) -> Vec<crate::scene::viewport_ref::ViewportFrame> {
+    pub fn viewport_frames_at_paper_point(
-        if self.current_layout == "Model" || self.active_viewport.is_some() { return Vec::new(); }
+        &self,
-        self.layout_content_viewports().iter().rev().filter_map(|&handle| {
+        paper: glam::DVec3,
-            if !self.viewport_displays_content(handle) || !self.viewport_displays_paper_point(handle, paper.truncate()) {
+    ) -> Vec<crate::scene::viewport_ref::ViewportFrame> {
-                return None;
+        if self.current_layout == "Model" || self.active_viewport.is_some() {
-            }
+            return Vec::new();
-            self.viewport_frame(handle)
+        }
-        }).collect()
+        self.layout_content_viewports()
+            .iter()
+            .rev()
+            .filter_map(|&handle| {
+                if !self.viewport_displays_content(handle)
+                    || !self.viewport_displays_paper_point(handle, paper.truncate())
+                {
+                    return None;
+                }
+                self.viewport_frame(handle)
+            })
+            .collect()
     }
 
-    /// Does `paper` lie inside the viewport's displayed area?
+    /// Test the viewport rectangle and optional nonrectangular clip boundary.
-    ///
-    /// Always bounded by the viewport's paper rectangle; a viewport with a
-    /// `clip_boundary_handle` (non-rectangular / clipped viewport) additionally
-    /// has to contain the point inside that boundary polygon. Used so a
-    /// paper-space snap never reports a model feature the viewport does not
-    /// actually show.
     pub fn viewport_displays_paper_point(&self, vp_handle: Handle, paper: glam::DVec2) -> bool {
         let Some(EntityType::Viewport(vp)) = self.document.get_entity(vp_handle) else {
             return false;

src/scene/project.rs

@@ -674,10 +674,7 @@ pub(crate) fn clip_boundary_polygon_for_document(
     z: f32,
 ) -> Vec<[f32; 3]> {
     use std::f64::consts::TAU;
-    let Some(entity) = document
+    let Some(entity) = document.get_entity(handle) else {
-        .entities()
-        .find(|e| e.common().handle == handle)
-    else {
         return vec![];
     };
     // Circles and ellipses tessellate directly — their `to_render` returns a

src/scene/viewport_dimension_pick.rs

@@ -1,20 +1,5 @@
-//! Object-pick resolution for paper-space dimensions taken through a layout
+//! Pick dimension geometry through a layout viewport. Resolve block instances
-//! viewport.
+//! in model space, then project the picked entity onto the paper sheet.
-//!
-//! Paper-space hit testing only sees the sheet's own entities, so a dimension
-//! command's explicit "select object" pick that lands inside a content
-//! viewport finds nothing. [`Scene::dimension_pick_through_viewport`] maps the
-//! click into model space through the viewport's
-//! [`crate::scene::viewport_ref::ViewportFrame`], hit-tests the viewport's
-//! resident model wires (descending into block instances and baking the
-//! instance transform), and hands back a copy of the picked entity already
-//! projected onto the sheet.
-//!
-//! The frame itself is built by [`Scene::viewport_frame`] (see
-//! `crate::scene::mspace`), which derives it from the *live viewport camera*
-//! so it always agrees with the pixels the renderer drew. It returns `None`
-//! for oblique / perspective viewports, and every entry point here then
-//! declines the pick rather than guessing a planar mapping.
 
 use super::*;
 
@@ -27,7 +12,7 @@ use glam::DVec3;
 
 /// A model entity resolved by clicking inside a paper-space content viewport.
 #[derive(Clone, Debug)]
-pub struct ViewportDimPick {
+pub struct ViewportDimensionPick {
     /// The viewport the click looked through.
     pub frame: ViewportFrame,
     /// Handle of the innermost model entity that owns the picked geometry.
@@ -46,18 +31,6 @@ pub struct ViewportDimPick {
 }
 
 impl Scene {
-    pub fn content_viewport_at_paper_point(&self, paper: DVec3) -> Option<Handle> {
-        self.viewport_frames_at_paper_point(paper)
-            .first()
-            .map(|f| f.viewport)
-    }
-
-    pub fn viewport_frame_at_paper_point(&self, paper: DVec3) -> Option<ViewportFrame> {
-        self.viewport_frames_at_paper_point(paper)
-            .into_iter()
-            .next()
-    }
-
     /// Resolve an explicit dimension object pick made on the sheet but landing
     /// inside a content viewport, against the *model* geometry that viewport
     /// displays.
@@ -67,14 +40,13 @@ impl Scene {
         &self,
         paper: DVec3,
         aperture_paper: f64,
-    ) -> Option<ViewportDimPick> {
+    ) -> Option<ViewportDimensionPick> {
         for frame in self.viewport_frames_at_paper_point(paper) {
             let model_point = frame.paper_to_model(paper);
             let aperture_model =
                 (aperture_paper.max(0.0) * frame.paper_to_model_length_factor()).max(1e-9);
 
-            let Some(top) =
+            let Some(top) = self.nearest_model_wire_handle(&frame, model_point, aperture_model)
-                self.nearest_model_wire_handle(frame.viewport, model_point, aperture_model)
             else {
                 continue;
             };
@@ -93,7 +65,7 @@ impl Scene {
                 &EntityTransform::Affine(viewport_model_to_paper_transform(&frame)),
             );
             let entity_handle = paper_entity.as_entity().handle();
-            return Some(ViewportDimPick {
+            return Some(ViewportDimensionPick {
                 frame,
                 entity_handle: if entity_handle.is_valid() {
                     entity_handle
@@ -114,10 +86,11 @@ impl Scene {
     /// draws, so this matches what the user sees.
     fn nearest_model_wire_handle(
         &self,
-        viewport: Handle,
+        frame: &ViewportFrame,
         model_point: DVec3,
         aperture: f64,
     ) -> Option<Handle> {
+        let viewport = frame.viewport;
         let camera = self.camera_for_viewport(viewport)?;
         let bounds = iced::Rectangle {
             x: 0.0,
@@ -150,9 +123,6 @@ impl Scene {
                 .unwrap_or(DVec3::ZERO);
             let distance = wire_polyline_nearest(wire, offset, model_point);
             if let Some((distance, nearest)) = distance {
-                let Some(frame) = self.viewport_frame(viewport) else {
-                    continue;
-                };
                 if !self.viewport_displays_paper_point(
                     viewport,
                     frame.model_to_paper(nearest).truncate(),
@@ -188,19 +158,14 @@ impl Scene {
     ) -> Option<(EntityType, Vec<Handle>)> {
         let entity = self.document.get_entity(handle)?;
         match entity {
-            EntityType::Insert(_) => {
+            EntityType::Insert(insert) => self.descend_block_instance(
-                let mut path = Vec::new();
+                insert,
-                let found = self.descend_block_instance(
+                Transform::identity(),
-                    entity.clone(),
+                model_point,
-                    Transform::identity(),
+                0,
-                    model_point,
+                viewport,
-                    &mut path,
+                kind,
-                    0,
+            ),
-                    viewport,
-                    kind,
-                )?;
-                Some((found.0, found.1))
-            }
             other => {
                 feature_pick_distance(other, model_point, kind)?;
                 Some((other.clone(), Vec::new()))
@@ -212,21 +177,17 @@ impl Scene {
     /// Returns the entity already transformed into WCS plus the INSERT path.
     fn descend_block_instance(
         &self,
-        insert_entity: EntityType,
+        insert: &acadrust::entities::Insert,
         outer: Transform,
         model_point: DVec3,
-        path: &mut Vec<Handle>,
         depth: usize,
         viewport: Option<Handle>,
         kind: Option<crate::snap::SnapType>,
     ) -> Option<(EntityType, Vec<Handle>)> {
         const MAX_DEPTH: usize = 8;
-        if depth > MAX_DEPTH {
+        if depth >= MAX_DEPTH {
             return None;
         }
-        let EntityType::Insert(insert) = &insert_entity else {
-            return None;
-        };
         if insert.row_count > 1 || insert.column_count > 1 {
             return None;
         }
@@ -234,10 +195,9 @@ impl Scene {
         let local = crate::scene::render_graph::insert_transform(&self.document, insert);
         let combined = local.then(&outer);
         let block = self.document.block_records.get(&insert.block_name)?;
-        let children: Vec<Handle> = block.entity_handles.clone();
 
         let mut best: Option<(f64, EntityType, Vec<Handle>)> = None;
-        for child in children {
+        for &child in &block.entity_handles {
             let Some(child_entity) = self.document.get_entity(child) else {
                 continue;
             };
@@ -249,13 +209,11 @@ impl Scene {
             if common.invisible || layer.is_some_and(|l| l.flags.off || l.flags.frozen) || frozen {
                 continue;
             }
-            if matches!(child_entity, EntityType::Insert(_)) {
+            if let EntityType::Insert(child_insert) = child_entity {
-                let mut nested = Vec::new();
                 if let Some((entity, mut nested_path)) = self.descend_block_instance(
-                    child_entity.clone(),
+                    child_insert,
                     combined,
                     model_point,
-                    &mut nested,
                     depth + 1,
                     viewport,
                     kind,
@@ -300,8 +258,6 @@ impl Scene {
             }
         }
         let (_, entity, found_path) = best?;
-        path.clear();
-        path.extend(found_path.iter().copied());
         Some((entity, found_path))
     }
 }

src/scene/viewport_ref.rs

@@ -187,17 +187,8 @@ impl AcceptedSnap {
     }
 }
 
-/// The two independent factors that turn a raw measured distance into the
+/// Length dimensions store raw distances in their owning space. The displayed
-/// number a linear/aligned/radial dimension displays.
+/// value uses `user_lfac * viewport_compensation`; angular dimensions are exempt.
-///
-/// * `user_lfac` is the drawing/style's DIMLFAC as the user intends it
-///   (always positive here; the sign convention of DXF DIMLFAC is resolved by
-///   [`MeasurementScale::from_dimlfac`]).
-/// * `viewport_compensation` undoes the viewport's paper-per-model scale for
-///   a dimension placed in paper space but measuring model geometry. It is
-///   `1.0` for model-space dimensions and for paper-only dimensions.
-///
-/// Angular dimensions never use either factor.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub struct MeasurementScale {
     pub user_lfac: f64,
@@ -252,59 +243,13 @@ impl MeasurementScale {
         common.extended_data = data;
     }
 
-    pub const IDENTITY: Self = Self {
-        user_lfac: 1.0,
-        viewport_compensation: 1.0,
-    };
-
-    /// Resolve the DXF DIMLFAC convention:
-    /// * `dimlfac > 0`: applies to every dimension.
-    /// * `dimlfac < 0`: `|dimlfac|` applies only to dimensions created in
-    ///   paper space that measure model geometry through a viewport; model
-    ///   space dimensions use `1.0`.
-    /// * `dimlfac == 0`: treated as `1.0`.
-    ///
-    /// `viewport` supplies the compensation when the dimension measures
-    /// through a viewport; pass `None` for model-space or paper-only
-    /// dimensions.
-    pub fn from_dimlfac(dimlfac: f64, viewport: Option<&ViewportFrame>) -> Self {
-        let through_viewport = viewport.is_some();
-        let user_lfac = if dimlfac == 0.0 {
-            1.0
-        } else if dimlfac > 0.0 {
-            dimlfac
-        } else if through_viewport {
-            -dimlfac
-        } else {
-            1.0
-        };
-        let viewport_compensation = viewport
-            .map(ViewportFrame::paper_to_model_length_factor)
-            .unwrap_or(1.0);
-        Self {
-            user_lfac,
-            viewport_compensation,
-        }
-    }
-
     /// Combined factor applied to a distance measured on the paper sheet.
     pub fn paper_factor(&self) -> f64 {
         self.user_lfac * self.viewport_compensation
     }
 
-    /// Factor applied to a distance already measured in model space (the
+    /// Positive DIMLFAC applies everywhere; negative DIMLFAC applies only on
-    /// viewport compensation must not be applied again).
+    /// paper. Zero means one. Saved viewport compensation is already in DIMLFAC.
-    pub fn model_factor(&self) -> f64 {
-        self.user_lfac
-    }
-
-    /// The DXF DIMLFAC sign convention, resolved without a [`ViewportFrame`].
-    ///
-    /// Rendering an *existing* dimension only knows whether the entity lives
-    /// in a paper-space layout; the viewport compensation is already baked
-    /// into the persisted magnitude. This is the same rule
-    /// as [`MeasurementScale::from_dimlfac`] with `viewport.is_some()`
-    /// replaced by `paper_space`.
     pub fn user_lfac_for_space(dimlfac: f64, paper_space: bool) -> f64 {
         if dimlfac == 0.0 {
             1.0
@@ -318,39 +263,6 @@ impl MeasurementScale {
     }
 }
 
-/// # Where the viewport compensation lives, once
-///
-/// A dimension that sits on a paper layout but measures model geometry seen
-/// through a viewport has **paper** definition points and displays a **model**
-/// number. Exactly one rule governs that, everywhere:
-///
-/// > `Dimension::actual_measurement` (DXF group 42) is always the **raw**
-/// > geometric measurement in the dimension's *own* space — paper, for a
-/// > viewport dimension. The viewport compensation lives **only** in the
-/// > magnitude of the negative DIMLFAC override
-/// > ([`MeasurementScale::viewport_dimlfac_override`]), and is applied
-/// > **once**, at format time, by the linear formatter.
-///
-/// So for a 1:10 viewport and a user DIMLFAC of 1: a 100-unit model line is
-/// 10 paper units, `actual_measurement` is `10`, the override is `-10`,
-/// [`MeasurementScale::user_lfac_for_space`] resolves that to `10` for a
-/// paper-space dimension, and the text reads `100`.
-///
-/// The consequence for association refresh
-/// (`Scene::refresh_associative_dimensions`) is that after re-placing a
-/// viewport dimension's definition points on the sheet it must write the
-/// **paper** measurement back — it must *not* substitute the model distance,
-/// and it must *not* multiply by [`MeasurementScale::model_factor`]. Doing
-/// either makes a refreshed dimension read a different number from an
-/// identical freshly-created one (`user_lfac` too many times). This is the
-/// rule that makes `attach_viewport_dimension_association` + refresh agree
-/// with creation by construction, and it is why `actual_measurement` stays
-/// raw: that is also what the DXF format specifies and what export writes.
-///
-/// Angular dimensions are exempt on both counts — a similarity preserves
-/// angles, so there is no compensation and no override.
-pub mod measurement_rule {}
-
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -385,34 +297,10 @@ mod tests {
     }
 
     #[test]
-    fn negative_dimlfac_only_applies_through_viewports() {
+    fn dimlfac_sign_depends_on_dimension_space() {
-        let f = frame(0.5, 0.0);
+        for (factor, paper, model) in [(-2.0, 2.0, 1.0), (0.0, 1.0, 1.0), (3.5, 3.5, 3.5)] {
-        let through = MeasurementScale::from_dimlfac(-2.0, Some(&f));
+            assert_eq!(MeasurementScale::user_lfac_for_space(factor, true), paper);
-        assert_eq!(through.user_lfac, 2.0);
+            assert_eq!(MeasurementScale::user_lfac_for_space(factor, false), model);
-        assert!((through.viewport_compensation - 2.0).abs() < 1e-12);
-        assert!((through.paper_factor() - 4.0).abs() < 1e-12);
-        assert!((through.model_factor() - 2.0).abs() < 1e-12);
-
-        let model = MeasurementScale::from_dimlfac(-2.0, None);
-        assert_eq!(model, MeasurementScale::IDENTITY);
-
-        let positive = MeasurementScale::from_dimlfac(3.0, None);
-        assert_eq!(positive.user_lfac, 3.0);
-        assert_eq!(positive.viewport_compensation, 1.0);
-    }
-
-    #[test]
-    fn user_lfac_for_space_matches_from_dimlfac() {
-        let f = frame(0.5, 0.0);
-        for lfac in [-2.0, 0.0, 1.0, 3.5] {
-            assert_eq!(
-                MeasurementScale::user_lfac_for_space(lfac, true),
-                MeasurementScale::from_dimlfac(lfac, Some(&f)).user_lfac
-            );
-            assert_eq!(
-                MeasurementScale::user_lfac_for_space(lfac, false),
-                MeasurementScale::from_dimlfac(lfac, None).user_lfac
-            );
         }
     }
 }

src/snap.rs

@@ -9,9 +9,9 @@ use iced::time::Instant;
 use iced::{Point, Rectangle};
 
 use cadkernel::geom2d::Curve;
+use acadrust::types::Handle;
 
 use crate::command::{DimensionAssociationSource, TangentObject};
-use acadrust::types::Handle;
 use crate::scene::model::wire_model::{SnapHint, TangentGeom, WireModel};
 use crate::scene::pick::interaction_index::WireSource;
 const DEFAULT_OSNAP_RADIUS_PX: f32 = 15.0;
@@ -90,7 +90,7 @@ pub struct SnapResult {
     /// Layout viewport this snap was seen *through*, when the snap ran against
     /// model geometry displayed by a paper-space viewport. `None` for ordinary
     /// model-space / paper-sheet snaps. Filled by the paper-space viewport snap
-    /// query, never by the engine itself. (PR1)
+    /// query, never by the engine itself.
     pub viewport: Option<Handle>,
     /// Owning entity of the geometry the snap landed on, when the engine could
     /// attribute the feature to a wire.
@@ -102,7 +102,7 @@ pub struct SnapResult {
     /// the instance path needs the document and the accepted model point, so
     /// it happens once at accept time, in
     /// `OpenCADStudio::resolve_source_block_path`, which fills
-    /// [`crate::scene::viewport_ref::SnapSourceRef::block_path`]. (PR1)
+    /// [`crate::scene::viewport_ref::SnapSourceRef::block_path`].
     pub source: Option<DimensionAssociationSource>,
     /// Second real object at an intersection, when both objects are known.
     pub secondary_source: Option<DimensionAssociationSource>,
@@ -996,10 +996,10 @@ impl Snapper {
             extension_base2: None,
             extension_origin: None,
             extension_dir: None,
-                        viewport: None,
+            viewport: None,
-                        source: None,
+            source: None,
-                        secondary_source: None,
+            secondary_source: None,
-                        model_point: None,
+            model_point: None,
         })
     }
 
@@ -1234,7 +1234,8 @@ impl Snapper {
 
         let mut try_pt = |world: glam::DVec3,
                           snap_type: SnapType,
-                          src: Option<DimensionAssociationSource>, secondary: Option<DimensionAssociationSource>| {
+                          src: Option<DimensionAssociationSource>,
+                          secondary: Option<DimensionAssociationSource>| {
             let screen = world_to_screen(world, view_rot, eye, bounds);
             if !in_bounds(screen) {
                 return;
@@ -1261,10 +1262,10 @@ impl Snapper {
                     extension_base2: None,
                     extension_origin: None,
                     extension_dir: None,
-                        viewport: None,
+                    viewport: None,
-                        source: src,
+                    source: src,
-                        secondary_source: secondary,
+                    secondary_source: secondary,
-                        model_point: None,
+                    model_point: None,
                 });
             }
         };
@@ -1318,7 +1319,9 @@ impl Snapper {
                     try_pt(
                         DVec3::from_array(point),
                         SnapType::Endpoint,
-                        wire_source(wire), None);
+                        wire_source(wire),
+                        None,
+                    );
                 }
                 // Tessellated open curves have no key-vertex set. Their only
                 // endpoints are first/last, so testing candidate wires remains
@@ -1338,7 +1341,9 @@ impl Snapper {
                         try_pt(
                             wp_f64(wire, wire.points.len() - 1),
                             SnapType::Endpoint,
-                            wire_source(wire), None);
+                            wire_source(wire),
+                            None,
+                        );
                     }
                 }
             } else {
@@ -1361,7 +1366,9 @@ impl Snapper {
                                 try_pt(
                                     wp_f64(wire, wire.points.len() - 1),
                                     SnapType::Endpoint,
-                                    src, None);
+                                    src,
+                                    None,
+                                );
                             }
                         }
                     }
@@ -1412,7 +1419,9 @@ impl Snapper {
                     try_pt(
                         nearest_on_segment(cursor_world, seg.a, seg.b),
                         SnapType::Nearest,
-                        wires.source_wire(seg.wire).and_then(wire_source), None);
+                        wires.source_wire(seg.wire).and_then(wire_source),
+                        None,
+                    );
                 }
             } else {
                 for wire in in_range_wires.iter() {
@@ -1436,7 +1445,9 @@ impl Snapper {
                             try_pt(
                                 foot,
                                 SnapType::Perpendicular,
-                                wires.source_wire(seg.wire).and_then(wire_source), None);
+                                wires.source_wire(seg.wire).and_then(wire_source),
+                                None,
+                            );
                         }
                     }
                 } else {
@@ -1465,7 +1476,9 @@ impl Snapper {
                             try_pt(
                                 point,
                                 SnapType::Intersection,
-                                wires.source_wire(segment.wire).and_then(wire_source), None);
+                                wires.source_wire(segment.wire).and_then(wire_source),
+                                None,
+                            );
                         }
                     }
                 }
@@ -1563,7 +1576,7 @@ impl Snapper {
                     for &wire_j in &local_wires[idx + 1..] {
                         if let Some(points) = exact_curve_intersections(wire_i, wire_j) {
                             // Two wires meet here; attribute the feature to the
-                            // first. PR3 resolves the true pair.
+                            // first. Association resolution requires both source paths.
                             let src = wire_source(wire_i);
                             for point in points {
                                 try_pt(point, SnapType::Intersection, src, wire_source(wire_j));
@@ -1600,7 +1613,9 @@ impl Snapper {
                                 try_pt(
                                     pt,
                                     SnapType::Intersection,
-                                    wires.source_wire(a.wire).and_then(wire_source), wires.source_wire(b.wire).and_then(wire_source));
+                                    wires.source_wire(a.wire).and_then(wire_source),
+                                    wires.source_wire(b.wire).and_then(wire_source),
+                                );
                                 if exact_cursor {
                                     break 'intersection_sweep;
                                 }
@@ -1646,7 +1661,12 @@ impl Snapper {
                                     continue;
                                 }
                                 if let Some(pt) = seg_intersect_3d(a0, a1, b0, b1) {
-                                    try_pt(pt, SnapType::Intersection, wire_source(wire_i), wire_source(wire_j));
+                                    try_pt(
+                                        pt,
+                                        SnapType::Intersection,
+                                        wire_source(wire_i),
+                                        wire_source(wire_j),
+                                    );
                                 }
                             }
                         }
@@ -1772,7 +1792,9 @@ impl Snapper {
                                 try_pt(
                                     segment_a.a + ta as f64 * (segment_a.b - segment_a.a),
                                     SnapType::ApparentIntersection,
-                                    wires.source_wire(segment_a.wire).and_then(wire_source), wires.source_wire(segment_b.wire).and_then(wire_source));
+                                    wires.source_wire(segment_a.wire).and_then(wire_source),
+                                    wires.source_wire(segment_b.wire).and_then(wire_source),
+                                );
                                 if dist2(apparent_screen, cursor_screen) <= f32::EPSILON {
                                     break 'apparent_sweep;
                                 }
@@ -1812,7 +1834,9 @@ impl Snapper {
                                     try_pt(
                                         wa0 + ta as f64 * (wa1 - wa0),
                                         SnapType::ApparentIntersection,
-                                        wire_source(wire_i), wire_source(wire_j));
+                                        wire_source(wire_i),
+                                        wire_source(wire_j),
+                                    );
                                 }
                             }
                         }
@@ -2033,10 +2057,10 @@ impl Snapper {
                             extension_base2: None,
                             extension_origin: None,
                             extension_dir: None,
-                        viewport: None,
+                            viewport: None,
-                        source: wire_source(wire),
+                            source: wire_source(wire),
-                        secondary_source: None,
+                            secondary_source: None,
-                        model_point: None,
+                            model_point: None,
                         });
                     }
                 }
@@ -2206,19 +2230,13 @@ fn snap_tier(t: SnapType) -> u8 {
     }
 }
 
-/// Candidate ordering: tier first, then cursor distance, and only when two
+/// Merge paper and viewport snaps using the engine's ordering.
-/// candidates are effectively equidistant (coincident features) the classic
+/// Both screen positions must use canvas pixels.
-/// sub-priority — so an Endpoint still beats an Intersection sitting on the
+pub fn merge_snap(
-/// exact same point. Distances are screen-px²; 4.0 ≈ a 2 px coincidence band.
+    a: Option<SnapResult>,
-/// Choose between two independently-computed snap results using the SAME
+    b: Option<SnapResult>,
-/// priority-then-distance rule the engine applies internally
+    cursor: Point,
-/// ([`snap_better`]): a higher-priority snap inside the aperture wins even
+) -> Option<SnapResult> {
-/// when a lower-priority one is nearer, and distance only breaks ties.
-///
-/// Used to merge the ordinary paper-sheet snap with the viewport snap taken
-/// through a layout viewport (PR1). Both `screen` fields must be expressed in
-/// the same pixel frame.
-pub fn merge_snap(a: Option<SnapResult>, b: Option<SnapResult>, cursor: Point) -> Option<SnapResult> {
     match (a, b) {
         (Some(a), Some(b)) => {
             let (at, asub) = (snap_tier(a.snap_type), snap_priority(a.snap_type));
@@ -2235,6 +2253,10 @@ pub fn merge_snap(a: Option<SnapResult>, b: Option<SnapResult>, cursor: Point) -
     }
 }
 
+/// Candidate ordering: tier first, then cursor distance, and only when two
+/// candidates are effectively equidistant (coincident features) the classic
+/// sub-priority — so an Endpoint still beats an Intersection sitting on the
+/// exact same point. Distances are screen-px²; 4.0 ≈ a 2 px coincidence band.
 fn snap_better(tier: u8, d2: f32, sub: u8, best: (u8, f32, u8)) -> bool {
     let (bt, bd2, bsub) = best;
     if tier != bt {

tests/fixtures/dim_assoc/README.md

@@ -1,16 +1,10 @@
 # Viewport association fixture
 
-`viewport_associations.dxf` is a synthetic paper-space fixture for line, circle,
+`viewport_associations.dxf` contains line, circle, arc, polyline, spline,
-arc, polyline, spline, block-instance, and intersection references through one
+block-instance, and intersection references through a 1:2 viewport.
-1:2 viewport. `src/app/viewport_dimension_tests.rs` loads it and exercises actual
+`src/app/viewport_dimension_tests.rs` tests refresh and intersection source edits.
-association refresh and an edit to the second intersection source.
 
-Each dimension carries `DIMLFAC=-2` and `ACAD_DIMASSOC_CALC_DIMLFAC=-2`. These are
+Each dimension stores `DIMLFAC=-2` and `ACAD_DIMASSOC_CALC_DIMLFAC=-2` so its paper
-necessary for its paper definition points and saved model measurements to agree;
+definition points measure in model units. The spline tangent from `(0, 300)`
-the original fixture omitted both factors. The spline tangent also uses the
+touches `(90, 283.125)` at parameter `0.75`; tests verify its tangency and preserve
-actual touch point `(90, 283.125)` at parameter `0.75`, measured from `(0, 300)`.
+the dimension if an edited reference cannot resolve.
-The original endpoint `(120, 300)` was not tangent. Tests verify the cross product
-with the spline derivative and preservation after an unsupported reference edit.
-
-The larger original `115448.dxf` is private and is not included. An opt-in test
-uses `OPENCAD_VIEWPORT_REGRESSION_DXF` to read a local copy without modifying it.