Dimension tool: Shift shows the constrained line while picking the second point (#2432)

The Shift click already snapped the second point to 45-degree steps, but the dashed line from the first point followed the raw pointer, so the constraint looked absent until the click.

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

crates/engine/tests/dimensions.rs

@@ -389,3 +389,21 @@ fn unknown_dimension_options_are_refused_by_commands_but_a_newer_file_still_open
     let doc = vectorcraft_format::load(&serde_json::to_vec(&v).unwrap()).unwrap();
     assert!(doc.node(NodeId(id)).unwrap().extras().dimension.is_some());
 }
+
+/// With Shift down, the dashed line from the first picked point follows the 45-degree step the
+/// second click will pick (#2432), not the raw pointer.
+#[test]
+fn shift_shows_the_constrained_line_while_picking_the_second_point() {
+    use vectorcraft_engine::tools::Overlay;
+    let mut s = session();
+    s.select_tool("dimension", ViewInfo::default()).unwrap();
+    ev(&mut s, PointerKind::Down, 300.0, 300.0);
+    ev(&mut s, PointerKind::Up, 300.0, 300.0);
+    let end = |s: &mut Session, shift: bool| {
+        let e = PointerEvent::new(PointerKind::Move, 372.0, 310.0).with_mods(Mods { shift, ..Mods::default() });
+        s.pointer(&e, ViewInfo::default()).unwrap();
+        s.overlays(ViewInfo::default()).into_iter().find_map(|o| if let Overlay::Line { b, .. } = o { Some(b) } else { None }).unwrap()
+    };
+    assert_eq!(end(&mut s, false).y, 310.0);
+    assert!((end(&mut s, true).y - 300.0).abs() < 1e-9);
+}

crates/tools/src/dimension.rs

@@ -60,9 +60,17 @@ pub struct DimensionTool {
     dragging: bool,
     hover: Option<Target>,
     at: Point,
+    /// Whether Shift was down at the last pointer event: the picked line shows constrained.
+    shift: bool,
     began: bool,
 }
 
+/// `at` as a Shift click picks it: the direction from `last` snapped to 45° steps (from the
+/// constrain angle), the distance kept.
+fn constrained(cx: &ToolContext, last: Point, at: Point) -> Point {
+    last + vectorcraft_geom::constrain_angle_from(at - last, 45.0, cx.constrain_angle)
+}
+
 /// Nearest editable segment, with bounded work and no snapping to dimension artwork itself.
 pub fn pick(cx: &ToolContext, p: Point) -> Option<SegmentRef> {
     let mut best = None;
@@ -217,7 +225,7 @@ impl DimensionTool {
                 if mods.shift
                     && let Some((last, _)) = self.points.last()
                 {
-                    at = *last + vectorcraft_geom::constrain_angle_from(at - *last, 45.0, cx.constrain_angle);
+                    at = constrained(cx, *last, at);
                     reference = reference.filter(|r| r.position(cx.doc) == Some(at));
                 }
                 self.points.push((at, reference));
@@ -293,6 +301,7 @@ impl Tool for DimensionTool {
     }
     fn pointer(&mut self, cx: &ToolContext, ev: &PointerEvent) -> Vec<Action> {
         self.at = ev.pos;
+        self.shift = ev.mods.shift;
         match ev.kind {
             PointerKind::Move => {
                 if self.pending.is_some() {
@@ -393,7 +402,8 @@ impl Tool for DimensionTool {
             segment(edge, &mut out);
         }
         if let Some((p, _)) = self.points.last() {
-            out.push(Overlay::Line { a: *p, b: self.at, color, dashed: true });
+            let b = if self.shift { constrained(cx, *p, self.at) } else { self.at };
+            out.push(Overlay::Line { a: *p, b, color, dashed: true });
         }
         out
     }