Sketch: a tangent arc starts from a trimmed end (#1485) (#1604)

Thanks @DuBearte.

crates/engine/src/cmd/sketch_modify_tests.rs

@@ -310,6 +310,33 @@ fn offset_chain_mirror_patterns_move_scale() {
     assert!(sk.center(sk.curve_index(&cp).unwrap()).unwrap().dist(Vec2::new(-110.0, 0.0)) < 1e-7);
 }
 
+/// #1485: after a circle is trimmed between two construction lines, clicking the trimmed end
+/// picks the construction line's end lying there, not the arc's own; the tangent arc still starts
+/// from the trimmed arc, tangent to it.
+#[test]
+fn a_tangent_arc_starts_from_a_trimmed_end() {
+    let mut s = new_sketch();
+    let c = ids(&run(&mut s, "sketch.circle.center", json!({"center": [0, 0], "radius": 10}))["curves"])[0].clone();
+    run(&mut s, "sketch.line", json!({"points": [[0, 0], [10, 0]], "ids": ["guide_a"]}));
+    run(&mut s, "sketch.line", json!({"points": [[0, 0], [0, 10]], "ids": ["guide_b"]}));
+    run(&mut s, "sketch.construction", json!({"curves": ["guide_a", "guide_b"]}));
+    run(&mut s, "sketch.trim", json!({"curve": c, "at": [7.07, 7.07]}));
+    let sk = sketch(&s);
+    let picked = sk.resolve_point("guide_a.end").unwrap();
+    assert!(sk.point(picked).unwrap().dist(Vec2::new(10.0, 0.0)) < 1e-6);
+    let trimmed = sk.curves.iter().position(|cu| !cu.construction && matches!(cu.kind, CurveKind::Arc { .. })).expect("the trimmed arc");
+    let r = run(&mut s, "sketch.arc.tangent", json!({"start": "guide_a.end", "end": [14, 6]}));
+    let sk = sketch(&s);
+    let a = sk.curve_index(&ids(&r["curves"])[0]).unwrap();
+    let a_ends: Vec<usize> = sk.curves[a].kind.point_ids();
+    let t_ends: Vec<usize> = sk.curves[trimmed].kind.point_ids();
+    assert!(a_ends.iter().any(|q| t_ends.contains(q) && *q != picked), "the new arc shares the trimmed arc's own end");
+    assert!(
+        sk.constraints.iter().any(|k| matches!(k.kind, ConstraintKind::Tangent { a: x, b: y } if (x, y) == (trimmed, a) || (x, y) == (a, trimmed))),
+        "and is tangent to it"
+    );
+}
+
 #[test]
 fn new_creation_tools() {
     let mut s = new_sketch();