Sketch UI: click a constraint glyph to select it, Delete removes the constraint; glyphs sit half way along their curve; lighter randomized sketch test

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AS5Tn3xieewUSg3uxrMVdL

crates/engine/src/cmd/sketch_constraints.rs

@@ -403,9 +403,19 @@ fn toggle_driven(s: &mut Session, p: &Value) -> Result<Value> {
 /// or at its point.
 fn anchor(sk: &Sketch, k: &ConstraintKind) -> Option<Vec2> {
     use ConstraintKind::*;
+    // Half way along the curve.
     let mid = |c: usize| -> Option<Vec2> {
         let poly = sk.polyline(c);
-        poly.get(poly.len() / 2).copied()
+        let total: f64 = poly.windows(2).map(|w| w[0].dist(w[1])).sum();
+        let mut acc = 0.0;
+        for w in poly.windows(2) {
+            let l = w[0].dist(w[1]);
+            if acc + l >= total * 0.5 && l > 0.0 {
+                return Some(w[0].lerp(w[1], (total * 0.5 - acc) / l));
+            }
+            acc += l;
+        }
+        poly.first().copied()
     };
     let pt = |p: usize| sk.point(p);
     match *k {

crates/engine/src/cmd/sketch_fuzz_tests.rs

@@ -41,12 +41,12 @@ fn entities(s: &Session) -> (Vec<String>, Vec<String>, Vec<String>) {
 
 #[test]
 fn random_sketch_editing_never_breaks() {
-    for seed in 1..=4u64 {
+    for seed in 1..=2u64 {
         let mut r = Rng(0x9E37_79B9_7F4A_7C15 ^ seed);
         let mut s = Session::default();
         let _ = s.execute("PrimitiveBox", &json!({"length": 30, "width": 20, "height": 10}));
         let _ = s.execute("SketchCreate", &json!({"plane": {"face": [15, 10, 10]}}));
-        for step in 0..150 {
+        for step in 0..120 {
             let (curves, points, cons) = entities(&s);
             let c = |r: &mut Rng| r.pick(&curves).cloned().unwrap_or_default();
             let p = |r: &mut Rng| r.pick(&points).cloned().unwrap_or_default();