feat(offset): open on the live preview for pre-selected objects; signed typed distance

With objects already selected, OFFSET opens at the side step with the
preview following the cursor instead of asking for a distance first
(#422). At the side step a typed distance places the offset at once on
the cursor's side, and a negative one on the other side; Enter at the
start repeats the last distance (#418). Erase and Layer, which lived on
the skipped distance step, are offered there too. With nothing selected
the distance-first flow is unchanged.

The opposite side is found by mirroring the cursor through its nearest
point on the outline, capped at half the offset distance so it cannot
cross a narrow gap back to the cursor's own side. Two new prompts are
added to the catalog and every locale.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/modules/draw/modify/offset.rs

@@ -9,6 +9,10 @@
 //     2. Pick object to offset (Line, Arc, Circle, LwPolyline)
 //     3. Pick a point on the side to offset toward, or choose Multiple
 //        to keep offsetting the newly created result at the same distance
+//
+//   With objects already selected, the command opens at step 3 instead, the
+//   preview following the cursor. At step 3 a typed distance places the
+//   offset at once on the cursor's side; a negative one, on the other side.
 
 use crate::entities::curve::{entity_curve, lwpolyline_world_xy};
 use crate::modules::draw::modify::spline_ops::spline_sample_xy;
@@ -331,13 +335,18 @@ fn compute_offsets(entity: &EntityType, dist: f64, side_pt: Vec3) -> Vec<EntityT
 // distance for every supported entity type.
 
 fn perp_distance(entity: &EntityType, pt: Vec3) -> f64 {
+    closest_on_wire(entity, pt).map_or(0.0, |(_, d)| d)
+}
+
+/// Nearest point of the entity outline to `pt`, and its distance.
+fn closest_on_wire(entity: &EntityType, pt: Vec3) -> Option<([f64; 2], f64)> {
     let pts = entity_wire_pts(entity);
     if pts.len() < 2 {
-        return 0.0;
+        return None;
     }
     let px = pt.x as f64;
     let py = pt.y as f64;
-    let mut best = f64::INFINITY;
+    let mut best = ([px, py], f64::INFINITY);
     for w in pts.windows(2) {
         let ax = w[0][0] as f64;
         let ay = w[0][1] as f64;
@@ -354,11 +363,35 @@ fn perp_distance(entity: &EntityType, pt: Vec3) -> f64 {
         let cx = ax + t * dx;
         let cy = ay + t * dy;
         let d = ((px - cx).powi(2) + (py - cy).powi(2)).sqrt();
-        if d < best {
-            best = d;
+        if d < best.1 {
+            best = ([cx, cy], d);
         }
     }
-    best
+    Some(best)
+}
+
+// ── Opposite side ─────────────────────────────────────────────────────────
+//
+// A negative typed distance offsets away from the cursor. Rather than teach
+// every offset function a sign, the cursor is mirrored through its nearest
+// point on the outline, which puts it on the other side for every type: the
+// other half-plane of a line, inside instead of outside a circle or closed
+// loop. The mirror stops at half the offset distance so that, in a narrow
+// part of a shape, it cannot cross the far wall and come back to the
+// cursor's own side.
+
+fn far_side(entity: &EntityType, pt: Vec3, dist: f64) -> Option<Vec3> {
+    let ([qx, qy], d) = closest_on_wire(entity, pt)?;
+    if d < 1e-9 {
+        // On the outline itself: there is no side to be opposite to.
+        return None;
+    }
+    let step = d.min(dist * 0.5) / d;
+    Some(Vec3::new(
+        (qx + (qx - pt.x as f64) * step) as f32,
+        (qy + (qy - pt.y as f64) * step) as f32,
+        pt.z,
+    ))
 }
 
 // ── Wire preview points ─────────────────────────────────────────────────────
@@ -440,9 +473,13 @@ pub struct OffsetCommand {
     /// Unlike the command's opening snapshot, this also includes objects
     /// created by earlier offsets while the command remains active.
     picked: Option<EntityType>,
-    /// Pre-selected offsettable objects (pick-first, #422); consumed when the
-    /// distance step resolves.
-    preselected: Vec<EntityType>,
+    /// Last cursor position the preview saw. A distance typed at the side
+    /// step places the offset relative to it.
+    cursor: Option<DVec3>,
+    /// The command opened on pre-selected objects (#422) and has made nothing
+    /// yet: Enter takes the last distance, and Erase / Layer, which the
+    /// skipped distance step would have offered, are offered here.
+    drag_start: bool,
     /// A Yes/No or Current/Source answer is being collected for this option.
     awaiting: Option<Await>,
     /// Erase option: replace the source object with its offset.
@@ -485,7 +522,8 @@ impl OffsetCommand {
             all_entities,
             entity_index,
             picked: None,
-            preselected: Vec::new(),
+            cursor: None,
+            drag_start: false,
             awaiting: None,
             erase_source: false,
             layer_current: false,
@@ -494,38 +532,28 @@ impl OffsetCommand {
         }
     }
 
-    /// Pick-first flow (#422): the distance step still comes first, then the
-    /// pre-selected objects go straight to the side step.
+    /// Pick-first flow (#422): the objects are already chosen, so the command
+    /// opens at the side step with the preview following the cursor. Clicking
+    /// offsets through the cursor; typing a distance places it at that
+    /// distance; Enter repeats the last distance (#418).
     pub fn with_selection(all_entities: Vec<EntityType>, targets: Vec<EntityType>) -> Self {
-        let entity_index = ModifyEntityIndex::build(&all_entities);
-        Self {
-            step: Step::Distance,
-            all_entities,
-            entity_index,
-            picked: None,
-            preselected: targets,
-            awaiting: None,
-            erase_source: false,
-            layer_current: false,
-            targets_are_sources: false,
-            made: 0,
+        let mut cmd = Self::new(all_entities);
+        if !targets.is_empty() {
+            cmd.step = Step::PickSide {
+                targets,
+                locked: None,
+                multiple: false,
+            };
+            cmd.targets_are_sources = true;
+            cmd.drag_start = true;
         }
+        cmd
     }
 
     /// Leave the distance step with the given mode (Some = locked distance,
-    /// None = through): pre-selected objects jump to the side step, otherwise
-    /// the object-pick loop starts.
+    /// None = through) for the object-pick loop.
     fn advance_from_distance(&mut self, locked: Option<f64>) -> CmdResult {
-        if self.preselected.is_empty() {
-            self.step = Step::SelectObject { locked };
-        } else {
-            self.step = Step::PickSide {
-                targets: std::mem::take(&mut self.preselected),
-                locked,
-                multiple: false,
-            };
-            self.targets_are_sources = true;
-        }
+        self.step = Step::SelectObject { locked };
         CmdResult::NeedPoint
     }
 
@@ -554,6 +582,73 @@ impl OffsetCommand {
             t!("OFFSET distance = %{d}", d = d).into_owned()
         )
     }
+
+    /// Offset the side step's targets with `pt` choosing the side: toward it,
+    /// or away from it when `flip` is set. Each target offsets by its own
+    /// through-distance, or by the locked magnitude.
+    fn place(&mut self, pt: DVec3, flip: bool) -> CmdResult {
+        let (locked, targets, multiple) = match &self.step {
+            Step::PickSide {
+                locked,
+                targets,
+                multiple,
+            } => (*locked, targets.clone(), *multiple),
+
+            _ => return CmdResult::NeedPoint,
+        };
+        let per_target: Vec<(Handle, Vec<EntityType>)> = targets
+            .iter()
+            .filter_map(|entity| {
+                let mag = locked.unwrap_or_else(|| perp_distance(entity, pt.as_vec3()));
+                if mag < 1e-9 {
+                    return None;
+                }
+                let side = if flip {
+                    far_side(entity, pt.as_vec3(), mag)?
+                } else {
+                    pt.as_vec3()
+                };
+                let offsets = compute_offsets(entity, mag, side);
+                (!offsets.is_empty()).then(|| (entity.common().handle, offsets))
+            })
+            .collect();
+        let mut news: Vec<EntityType> = per_target
+            .iter()
+            .flat_map(|(_, offsets)| offsets.iter().cloned())
+            .collect();
+        if news.is_empty() {
+            return CmdResult::NeedPoint;
+        }
+        self.made += 1;
+        self.drag_start = false;
+        // Erase option: the offset replaces its source object. Only the
+        // objects picked from the drawing qualify — a Multiple chain's
+        // targets are the offsets just made, which are not replaced again.
+        let erase = self.erase_source && self.targets_are_sources;
+        self.targets_are_sources = false;
+        if multiple {
+            // Multiple mode chains from the result just created, matching
+            // OFFSET's repeated fixed-distance behavior (parallel/concentric
+            // series instead of duplicate copies at the first offset).
+            self.step = Step::PickSide {
+                targets: news.clone(),
+                locked,
+                multiple: true,
+            };
+        } else {
+            // Classic loop (#418): commit this offset and go back to the object
+            // pick at the same distance, until Enter / Esc finishes.
+            self.step = Step::SelectObject { locked };
+        }
+        if erase {
+            return CmdResult::ReplaceManyContinue(per_target);
+        }
+        if news.len() == 1 {
+            CmdResult::CommitEntity(news.pop().unwrap())
+        } else {
+            CmdResult::CommitEntities(news)
+        }
+    }
 }
 
 impl CadCommand for OffsetCommand {
@@ -602,6 +697,23 @@ impl CadCommand for OffsetCommand {
                 } else {
                     std::borrow::Cow::Borrowed("")
                 };
+                if self.drag_start {
+                    let d = format!("{:.4}", defaults::get_offset_dist());
+                    return if *multiple {
+                        t!(
+                            "OFFSET%{n} Multiple  Click through point or type distance, negative for the other side [Erase/Layer] <%{d}>:",
+                            n = n,
+                            d = d
+                        )
+                    } else {
+                        t!(
+                            "OFFSET%{n}  Click through point or type distance, negative for the other side [Multiple/Erase/Layer] <%{d}>:",
+                            n = n,
+                            d = d
+                        )
+                    }
+                    .into_owned();
+                }
                 match (locked, multiple) {
                     (Some(d), false) => {
                         let d = format!("{:.4}", d);
@@ -658,6 +770,16 @@ impl CadCommand for OffsetCommand {
                 opts.push(CmdOption::enter("Exit"));
                 opts
             }
+            Step::PickSide { multiple, .. } if self.drag_start => {
+                let mut opts = Vec::new();
+                if !multiple {
+                    opts.push(CmdOption::new("Multiple", "M"));
+                }
+                opts.push(CmdOption::new("Erase", "E"));
+                opts.push(CmdOption::new("Layer", "L"));
+                opts.push(CmdOption::enter(&format!("{:.4}", defaults::get_offset_dist())));
+                opts
+            }
             Step::PickSide { multiple, .. } => {
                 let mut opts = Vec::new();
                 if !multiple {
@@ -777,19 +899,23 @@ impl CadCommand for OffsetCommand {
         {
             return self.undo_last();
         }
+        // Erase / Layer belong to the distance step, or to the side step when
+        // a pre-selection skipped the distance step.
+        if matches!(self.step, Step::Distance) || self.drag_start {
+            if upper == "E" || upper == "ERASE" {
+                self.awaiting = Some(Await::Erase);
+                return Some(CmdResult::NeedPoint);
+            }
+            if upper == "L" || upper == "LAYER" {
+                self.awaiting = Some(Await::Layer);
+                return Some(CmdResult::NeedPoint);
+            }
+        }
         match &mut self.step {
             Step::Distance => {
                 if t.eq_ignore_ascii_case("t") || t.eq_ignore_ascii_case("through") {
                     return Some(self.advance_from_distance(None));
                 }
-                if upper == "E" || upper == "ERASE" {
-                    self.awaiting = Some(Await::Erase);
-                    return Some(CmdResult::NeedPoint);
-                }
-                if upper == "L" || upper == "LAYER" {
-                    self.awaiting = Some(Await::Layer);
-                    return Some(CmdResult::NeedPoint);
-                }
                 if let Some(d) = crate::entities::common::parse_typed_length(&t) {
                     return Some(self.accept_distance(d));
                 }
@@ -804,15 +930,20 @@ impl CadCommand for OffsetCommand {
                     *multiple = true;
                     return Some(CmdResult::NeedPoint);
                 }
-                if !t.is_empty() {
-                    if let Some(d) = crate::entities::common::parse_typed_length(&t) {
-                        let d = d.abs().max(1e-9);
-                        defaults::set_offset_dist(d);
-                        *locked = Some(d);
-                    }
+                let Some(d) = crate::entities::common::parse_typed_length(&t) else {
+                    return Some(CmdResult::NeedPoint);
+                };
+                let mag = d.abs().max(1e-9);
+                defaults::set_offset_dist(mag);
+                *locked = Some(mag);
+                // The cursor has already chosen the side, so the value places
+                // the offset now: on the cursor's side, or the other side when
+                // negative. Before the cursor has been seen, the click still
+                // chooses.
+                match self.cursor {
+                    Some(cursor) => Some(self.place(cursor, d < 0.0)),
+                    None => Some(CmdResult::NeedPoint),
                 }
-                // Stay on the side step — the click chooses which side.
-                Some(CmdResult::NeedPoint)
             }
             _ => None,
         }
@@ -863,66 +994,11 @@ impl CadCommand for OffsetCommand {
             _ => {}
         }
 
-        let (locked, targets, multiple) = match &self.step {
-            Step::PickSide {
-                locked,
-                targets,
-                multiple,
-            } => (*locked, targets.clone(), *multiple),
-
-            _ => return CmdResult::NeedPoint,
-        };
-        // Each target offsets by its own through-distance (or the locked
-        // magnitude), toward the clicked side.
-        let per_target: Vec<(Handle, Vec<EntityType>)> = targets
-            .iter()
-            .filter_map(|entity| {
-                let mag = locked.unwrap_or_else(|| perp_distance(entity, pt.as_vec3()));
-                if mag < 1e-9 {
-                    return None;
-                }
-                let offsets = compute_offsets(entity, mag, pt.as_vec3());
-                (!offsets.is_empty()).then(|| (entity.common().handle, offsets))
-            })
-            .collect();
-        let mut news: Vec<EntityType> = per_target
-            .iter()
-            .flat_map(|(_, offsets)| offsets.iter().cloned())
-            .collect();
-        if news.is_empty() {
-            return CmdResult::NeedPoint;
-        }
-        self.made += 1;
-        // Erase option: the offset replaces its source object. Only the
-        // objects picked from the drawing qualify — a Multiple chain's
-        // targets are the offsets just made, which are not replaced again.
-        let erase = self.erase_source && self.targets_are_sources;
-        self.targets_are_sources = false;
-        if multiple {
-            // Multiple mode chains from the result just created, matching
-            // OFFSET's repeated fixed-distance behavior (parallel/concentric
-            // series instead of duplicate copies at the first offset).
-            self.step = Step::PickSide {
-                targets: news.clone(),
-                locked,
-                multiple: true,
-            };
-        } else {
-            // Classic loop (#418): commit this offset and go back to the object
-            // pick at the same distance, until Enter / Esc finishes.
-            self.step = Step::SelectObject { locked };
-        }
-        if erase {
-            return CmdResult::ReplaceManyContinue(per_target);
-        }
-        if news.len() == 1 {
-            CmdResult::CommitEntity(news.pop().unwrap())
-        } else {
-            CmdResult::CommitEntities(news)
-        }
+        self.place(pt, false)
     }
 
     fn on_preview_wires(&mut self, pt: DVec3) -> Vec<WireModel> {
+        self.cursor = Some(pt);
         if let Step::ReferenceSecond { first } = &self.step {
             return vec![WireModel::solid(
                 "offset_reference_distance".into(),
@@ -979,6 +1055,17 @@ impl CadCommand for OffsetCommand {
                 let d = defaults::get_offset_dist();
                 self.accept_distance(d)
             }
+            // Opened on a pre-selection, the same Enter offsets by the last
+            // distance on the cursor's side.
+            Step::PickSide { .. } if self.drag_start => {
+                let Some(cursor) = self.cursor else {
+                    return CmdResult::NeedPoint;
+                };
+                if let Step::PickSide { locked, .. } = &mut self.step {
+                    *locked = Some(defaults::get_offset_dist());
+                }
+                self.place(cursor, false)
+            }
             _ => CmdResult::Cancel,
         }
     }
@@ -1100,3 +1187,127 @@ mod option_tests {
         assert!(matches!(cmd.on_point(DVec3::new(5.0, 5.0, 0.0)), CmdResult::CommitEntity(_)));
     }
 }
+
+#[cfg(test)]
+mod drag_tests {
+    use super::*;
+    use codec::types::Vector2;
+
+    fn line_y0() -> EntityType {
+        let mut line = codec::entities::Line::from_coords(0.0, 0.0, 0.0, 10.0, 0.0, 0.0);
+        line.common.handle = Handle::new(1);
+        EntityType::Line(line)
+    }
+
+    /// OFFSET opened on `target`, with the cursor hovering at `cursor`.
+    fn dragging(target: EntityType, cursor: [f64; 2]) -> OffsetCommand {
+        let mut cmd = OffsetCommand::with_selection(vec![target.clone()], vec![target]);
+        cmd.on_preview_wires(DVec3::new(cursor[0], cursor[1], 0.0));
+        cmd
+    }
+
+    fn committed_line_y(result: Option<CmdResult>) -> f64 {
+        match result {
+            Some(CmdResult::CommitEntity(EntityType::Line(l))) => l.start.y,
+            _ => panic!("expected one committed line"),
+        }
+    }
+
+    #[test]
+    fn preselection_opens_on_the_live_preview() {
+        let mut cmd = OffsetCommand::with_selection(vec![line_y0()], vec![line_y0()]);
+        assert!(matches!(cmd.step, Step::PickSide { locked: None, .. }));
+        assert_eq!(cmd.options().into_iter().map(|o| o.keyword).collect::<Vec<_>>(), ["M", "E", "L", ""]);
+        // The preview passes through the cursor before anything is typed.
+        assert_eq!(cmd.on_preview_wires(DVec3::new(5.0, 3.0, 0.0)).len(), 1);
+        assert_eq!(cmd.dyn_live_value(DVec3::new(5.0, 3.0, 0.0)), Some(3.0));
+        // A click still offsets through the cursor.
+        assert!(matches!(cmd.on_point(DVec3::new(5.0, 3.0, 0.0)), CmdResult::CommitEntity(_)));
+    }
+
+    #[test]
+    fn typed_distance_places_on_the_cursor_side() {
+        let mut cmd = dragging(line_y0(), [5.0, 3.0]);
+        assert!((committed_line_y(cmd.on_text_input("2")) - 2.0).abs() < 1e-9);
+        // The loop continues at the typed distance.
+        assert!(matches!(cmd.step, Step::SelectObject { locked: Some(d) } if d == 2.0));
+    }
+
+    #[test]
+    fn negative_distance_places_on_the_other_side() {
+        let mut cmd = dragging(line_y0(), [5.0, 3.0]);
+        assert!((committed_line_y(cmd.on_text_input("-2")) + 2.0).abs() < 1e-9);
+        assert_eq!(defaults::get_offset_dist(), 2.0);
+    }
+
+    #[test]
+    fn negative_distance_inside_a_circle_grows_it() {
+        let circle = EntityType::Circle(CircleEnt {
+            center: codec::types::Vector3::new(0.0, 0.0, 0.0),
+            radius: 10.0,
+            ..Default::default()
+        });
+        let mut cmd = dragging(circle.clone(), [3.0, 0.0]);
+        match cmd.on_text_input("-1") {
+            Some(CmdResult::CommitEntity(EntityType::Circle(c))) => assert!((c.radius - 11.0).abs() < 1e-9),
+            _ => panic!("expected one committed circle"),
+        }
+        let mut cmd = dragging(circle, [3.0, 0.0]);
+        match cmd.on_text_input("1") {
+            Some(CmdResult::CommitEntity(EntityType::Circle(c))) => assert!((c.radius - 9.0).abs() < 1e-9),
+            _ => panic!("expected one committed circle"),
+        }
+    }
+
+    // A U whose arms sit 1 apart. From inside the left arm the nearest wall
+    // faces the gap; mirroring the cursor its full 1.9 across would land in
+    // the right arm, back inside the loop. -0.25 must still go outward.
+    #[test]
+    fn negative_distance_does_not_mirror_across_a_gap() {
+        let u = EntityType::LwPolyline(LwPolyline {
+            vertices: [
+                [0.0, 0.0], [9.0, 0.0], [9.0, 10.0], [5.0, 10.0],
+                [5.0, 2.0], [4.0, 2.0], [4.0, 10.0], [0.0, 10.0],
+            ]
+            .iter()
+            .map(|&[x, y]| LwVertex::new(Vector2::new(x, y)))
+            .collect(),
+            is_closed: true,
+            ..Default::default()
+        });
+        let mut cmd = dragging(u, [2.1, 6.0]);
+        match cmd.on_text_input("-0.25") {
+            Some(CmdResult::CommitEntity(EntityType::LwPolyline(p))) => {
+                let min_x = p.vertices.iter().map(|v| v.location.x).fold(f64::INFINITY, f64::min);
+                assert!((min_x + 0.25).abs() < 1e-6, "offset went inward: min x {min_x}");
+            }
+            _ => panic!("expected one committed polyline"),
+        }
+    }
+
+    #[test]
+    fn enter_repeats_the_last_distance_on_the_cursor_side() {
+        defaults::set_offset_dist(4.0);
+        let mut cmd = dragging(line_y0(), [5.0, -1.0]);
+        match cmd.on_enter() {
+            CmdResult::CommitEntity(EntityType::Line(l)) => assert!((l.start.y + 4.0).abs() < 1e-9),
+            _ => panic!("expected one committed line"),
+        }
+        // After that, Enter finishes as before.
+        assert!(matches!(cmd.on_enter(), CmdResult::Cancel));
+    }
+
+    #[test]
+    fn erase_is_offered_without_the_distance_step() {
+        let mut cmd = dragging(line_y0(), [5.0, 3.0]);
+        cmd.on_text_input("E");
+        cmd.on_text_input("Y");
+        assert!(matches!(cmd.on_text_input("2"), Some(CmdResult::ReplaceManyContinue(_))));
+    }
+
+    #[test]
+    fn without_preselection_the_distance_comes_first() {
+        let cmd = OffsetCommand::new(vec![line_y0()]);
+        assert!(matches!(cmd.step, Step::Distance));
+    }
+}