fix(3d): seat axis steps on 3D snaps; raised bases follow the cursor (#1030)

Height and distance steps (PUSHPULL, CYLINDER, CONE, PYRAMID, BOX,
EXTRUDE) replaced the point with the cursor's screen mapping onto the
command axis, discarding an acquired 3D object snap. A genuine snap now
supplies its own coordinate along the axis, as axis grips already do.

Base steps after the first base point (BOX/WEDGE opposite corner and cube
size, CYLINDER/CONE/PYRAMID radius) read the cursor on the base plane
through that point. A base started on a raised face intersected the ray
with world XY instead, so in oblique views the footprint ran far past the
mouse.

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

src/app/update/viewport.rs

@@ -87,6 +87,28 @@ pub(in crate::app) fn snap_keeps_elevation(hit: Option<crate::snap::SnapType>) -
     matches!(hit, Some(t) if t != crate::snap::SnapType::Grid)
 }
 
+/// Point on a command's construction axis (height / distance steps). A
+/// genuine object snap supplies its own coordinate along the axis, so a
+/// height or push distance seats on existing geometry — as axis grips do.
+/// Otherwise the cursor maps onto the axis on screen.
+fn command_axis_point(
+    snap: Option<crate::snap::SnapResult>,
+    cursor: Point,
+    bounds: iced::Rectangle,
+    view: glam::Mat4,
+    eye: glam::DVec3,
+    origin: glam::DVec3,
+    direction: glam::DVec3,
+) -> Option<glam::DVec3> {
+    match snap.filter(|hit| snap_keeps_elevation(Some(hit.snap_type))) {
+        Some(hit) => {
+            let axis = direction.try_normalize()?;
+            Some(origin + axis * (hit.world - origin).dot(axis))
+        }
+        None => cursor_on_projected_axis(cursor, bounds, view, eye, origin, direction),
+    }
+}
+
 fn cursor_on_projected_axis(
     cursor: Point,
     bounds: iced::Rectangle,
@@ -2393,7 +2415,15 @@ impl OpenCADStudio {
                     .as_ref()
                     .and_then(|command| command.cursor_axis())
                     .and_then(|(origin, direction)| {
-                        cursor_on_projected_axis(p, bounds, view_rot, eye, origin, direction)
+                        command_axis_point(
+                            self.tabs[i].snap_result,
+                            p,
+                            bounds,
+                            view_rot,
+                            eye,
+                            origin,
+                            direction,
+                        )
                     })
                     .unwrap_or(effective)
             };
@@ -3955,7 +3985,7 @@ impl OpenCADStudio {
                     .as_ref()
                     .and_then(|command| command.cursor_axis())
                     .and_then(|(origin, direction)| {
-                        cursor_on_projected_axis(p, bounds, view_rot, eye, origin, direction)
+                        command_axis_point(snap_hit, p, bounds, view_rot, eye, origin, direction)
                     })
                     .unwrap_or(pt);
                 // A click while dynamic-input fields hold typed values

src/modules/model/cylinder_cmd.rs

@@ -462,6 +462,16 @@ impl CadCommand for CylinderCommand {
         self.plane = plane;
     }
 
+    /// The radius step reads the cursor on the base plane through the
+    /// centre, so a base started on a raised face follows the mouse.
+    fn cursor_plane(&self) -> Option<(DVec3, DVec3)> {
+        if !matches!(self.step, Step::BaseRadius | Step::BaseDiameter) {
+            return None;
+        }
+        let frame = self.frame?;
+        Some((frame.z.normalize_or_zero(), frame.origin))
+    }
+
     fn cursor_axis(&self) -> Option<(DVec3, DVec3)> {
         matches!(self.step, Step::Height).then(|| {
             let frame = self.frame.unwrap_or(self.plane);

src/modules/model/primitive_cmd.rs

@@ -2098,6 +2098,25 @@ impl CadCommand for PrimitiveCommand {
         self.plane = plane;
     }
 
+    /// Base steps after the first base point read the cursor on the base
+    /// plane through that point, so a base started on a raised face follows
+    /// the mouse instead of the ray's hit on the working plane below.
+    fn cursor_plane(&self) -> Option<(DVec3, DVec3)> {
+        let origin = if self.shape == Shape::Cone {
+            matches!(self.cone_step, ConeStep::BaseRadius | ConeStep::BaseDiameter)
+                .then_some(self.cone_frame?.origin)
+        } else if self.shape == Shape::Pyramid {
+            (self.pyramid_step == PyramidStep::BaseRadius).then_some(self.pyramid_frame?.origin)
+        } else if self.shape.rectangular() {
+            matches!(self.box_step, BoxStep::OppositeCorner | BoxStep::CubeSize)
+                .then(|| self.pts.first().map(|&first| self.plane.to_world(first)))
+                .flatten()
+        } else {
+            None
+        }?;
+        Some((self.plane.z.normalize_or_zero(), origin))
+    }
+
     fn cursor_axis(&self) -> Option<(DVec3, DVec3)> {
         if self.shape == Shape::Cone {
             return matches!(self.cone_step, ConeStep::Height | ConeStep::HeightAfterTopRadius)