Fix polyline offset arc bulge after joining moved endpoints (#817) (#825)

Co-authored-by: Brandon Thomas <echelon@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/modify.rs

@@ -682,20 +682,40 @@ pub(crate) fn offset_polyline(pl: &Polyline, d: f64) -> Option<Vec<PolyVertex>>
         return None;
     }
     let n = segs.len();
-    let mut verts: Vec<PolyVertex> = Vec::new();
+    // Where each offset segment starts after joining it to the previous one.
+    let mut starts: Vec<Vec2> = Vec::with_capacity(n);
     for i in 0..n {
         let s = segs.get(i)?;
-        let start = if i == 0 && !pl.closed {
+        starts.push(if i == 0 && !pl.closed {
             s.start()
         } else {
             let prev = segs.get((i + n - 1) % n)?;
             join_point(prev, s).unwrap_or(s.start())
+        });
+    }
+    let mut verts: Vec<PolyVertex> = Vec::new();
+    for i in 0..n {
+        let s = segs.get(i)?;
+        let start = *starts.get(i)?;
+        let end = if i + 1 < n {
+            *starts.get(i + 1)?
+        } else if pl.closed {
+            *starts.first()?
+        } else {
+            s.end()
         };
         let bulge = match s {
             Segment::Line(_) => 0.0,
             Segment::Arc { arc, ccw } => {
-                let sw = arc.sweep();
-                cadcraft_geom::arc_to_bulge(if *ccw { sw } else { -sw })
+                // The joined endpoints may have moved along the circle: take the sweep between them.
+                let sweep = if start.near(end, 1e-9) {
+                    arc.sweep()
+                } else if *ccw {
+                    ccw_sweep(arc.center.angle_to(start), arc.center.angle_to(end))
+                } else {
+                    ccw_sweep(arc.center.angle_to(end), arc.center.angle_to(start))
+                };
+                cadcraft_geom::arc_to_bulge(if *ccw { sweep } else { -sweep })
             }
         };
         verts.push(PolyVertex { p: start, bulge, start_width: 0.0, end_width: 0.0 });
@@ -703,7 +723,6 @@ pub(crate) fn offset_polyline(pl: &Polyline, d: f64) -> Option<Vec<PolyVertex>>
     if !pl.closed {
         verts.push(PolyVertex::new(segs.last()?.end()));
     }
-    // Recompute arc bulges where endpoints moved (approximation: keep sweep sign).
     Some(verts)
 }
 

crates/engine/src/cmd/modify2_tests.rs

@@ -707,3 +707,17 @@ fn pedit_preselected_line_asks_to_convert() {
     assert!(s.running.is_none());
     assert!(matches!(kind(&s, &m), EntityKind::Line(_)));
 }
+
+#[test]
+fn offset_polyline_arc_bulge_follows_joined_endpoints() {
+    let mut s = Session::new();
+    // A diagonal line into a CCW semicircle centred on (10, 5); offsetting inward moves the arc start.
+    let pl = pline(&mut s, json!([[0, -5], {"p": [10, 0], "bulge": 1}, [10, 10]]), false);
+    s.execute("offset", &json!({"handle": pl, "distance": 1, "side": [12, 5]})).unwrap();
+    let p = poly(&last(&s));
+    let (a, b) = (p.vertices[1], p.vertices[2]);
+    let (arc, ccw) = cadcraft_geom::bulge_to_arc(a.p, b.p, a.bulge).unwrap();
+    assert!(ccw);
+    assert!(near(arc.center, 10.0, 5.0), "arc centre drifted to {:?}", arc.center);
+    assert!((arc.radius - 4.0).abs() < 1e-6, "radius {}", arc.radius);
+}