sketch(fix): preserve rectangle constraints through fillets (#1543)

Thanks @Karl-VonRichter for the fix.

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

crates/ui-egui/src/sketch_corner_ui_tests.rs

@@ -378,6 +378,34 @@ fn a_fillet_rounds_every_picked_corner_to_one_radius() {
     assert!((ss.profiles[0].area - (1200.0 - 3.0 * 25.0 * (1.0 - std::f64::consts::PI / 4.0))).abs() < 1e-6, "{}", ss.profiles[0].area);
 }
 
+#[test]
+fn fully_constrained_center_rectangle_corners_can_be_picked_and_filleted() {
+    let mut h = crate::scenario::Harness::new();
+    h.app.run("sketch.create", json!({"plane":"XY"})).unwrap();
+    h.app.run("sketch.rectangle.center", json!({"center":[0,0],"corner":[20,15]})).unwrap();
+    h.app.run("sketch.constraint.coincident", json!({"a":"p5","b":"origin"})).unwrap();
+    h.app.run("sketch.dimension", json!({"entities":["l1"],"value":40})).unwrap();
+    h.app.run("sketch.dimension", json!({"entities":["l2"],"value":30})).unwrap();
+    h.frames(2);
+    h.step(&json!({"start":"sketch.fillet"})).unwrap();
+    h.frames(2);
+    for corner in [[-20, -15, 0], [20, -15, 0], [20, 15, 0], [-20, 15, 0]] {
+        h.step(&json!({"click":{"world":corner}})).unwrap();
+        h.frames(2);
+    }
+    assert_eq!(h.app.dialog.as_ref().map(|d| d.inputs[0].items.len()), Some(4));
+    set_size(&mut h.app, "3");
+    apply(&mut h);
+    assert!(h.app.dialog.is_none(), "{:?}", h.app.status);
+    let info = h.app.run("sketch.inspect", json!({})).unwrap();
+    assert_eq!(info["dof"], 0, "{info}");
+    assert_eq!(info["status"], "solved", "{info}");
+    let st = h.app.session.world_state();
+    let ss = st.sketch(h.app.session.active_sketch.unwrap()).unwrap();
+    assert_eq!(ss.sketch.curves.iter().filter(|c| matches!(c.kind, CurveKind::Arc { .. })).count(), 4);
+    assert!((ss.profiles[0].area - (1200.0 - 36.0 * (1.0 - std::f64::consts::PI / 4.0))).abs() < 1e-6);
+}
+
 #[test]
 fn ordinary_drawing_keeps_origin_snap_and_real_origin_corners_work() {
     let mut h = crate::scenario::Harness::new();

crates/ui-egui/src/sketch_tools.rs

@@ -460,17 +460,18 @@ fn sketch_entity(app: &SolveApp, hits: &[Hit]) -> Option<(String, bool)> {
 }
 
 /// The two editable curves that meet at sketch point `pi`, in curve order, which is the order
-/// the corner commands resolve them in from a `point` parameter. None unless exactly two meet
-/// there and each is one the command can work on: a line always, an arc only for a fillet.
+/// the corner commands resolve them in from a `point` parameter. Construction curves are
+/// ignored when more than two meet. Each side must be a line, or an arc for a fillet.
 pub(crate) fn corner_curves(sk: &solidcraft_engine::sketch::Sketch, pi: usize, arcs_ok: bool) -> Option<(usize, usize)> {
     use solidcraft_engine::sketch::CurveKind;
-    let curves: Vec<_> = sk
-        .curves
-        .iter()
-        .enumerate()
-        .filter(|(ci, c)| !sk.is_linked_curve(*ci) && c.kind.ends().is_some_and(|(a, b)| a == pi || b == pi))
-        .take(3)
-        .collect();
+    let candidates =
+        sk.curves.iter().enumerate().filter(|(ci, c)| !sk.is_linked_curve(*ci) && c.kind.ends().is_some_and(|(a, b)| a == pi || b == pi));
+    let mut curves: Vec<_> = candidates.clone().take(3).collect();
+    // Center rectangles have construction diagonals at their corners. Match the engine's
+    // corner selection: when more than two curves meet, use the profile sides.
+    if curves.len() > 2 {
+        curves = candidates.filter(|(_, c)| !c.construction).take(3).collect();
+    }
     if curves.iter().any(|(_, c)| !matches!(c.kind, CurveKind::Line { .. }) && !(arcs_ok && matches!(c.kind, CurveKind::Arc { .. }))) {
         return None;
     }