Offset elliptical arcs towards the picked side (#1475)

Found while testing crates/engine/src/cmd/modify.rs:584-597 (offset_kind, EntityKind::Ellipse branch)

Co-authored-by: Claude Haiku 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/modify.rs

@@ -586,10 +586,14 @@ pub(crate) fn offset_kind(k: &EntityKind, dist: f64, side: Vec2) -> Option<Entit
         EntityKind::Ellipse(e) => {
             // Approximate with a polyline offset of the tessellation.
             let ge = cadcraft_geom::Ellipse { center: e.center.xy(), major: e.major.xy(), ratio: e.ratio, start: e.start, end: e.end };
+            // Inside = on the centre side of the full ellipse, even for an open arc.
             let inside = {
-                let mut pts = Vec::new();
-                ge.tessellate(e.major.xy().len() * 1e-3, &mut pts);
-                cadcraft_geom::point_in_polygon(&pts, side)
+                let a = e.major.xy().len();
+                let b = a * e.ratio.abs();
+                let u = e.major.xy().normalized();
+                let d = side - e.center.xy();
+                let (x, y) = (d.dot(u) * b, d.dot(u.perp()) * a);
+                x * x + y * y < (a * b) * (a * b)
             };
             // offset_points shifts along the left normal, which points into a counter-clockwise ellipse.
             let mut pts = Vec::new();

crates/engine/src/cmd/modify2_tests.rs

@@ -754,3 +754,18 @@ fn offset_polyline_arc_bulge_follows_joined_endpoints() {
     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);
 }
+
+#[test]
+fn offset_elliptical_arc_follows_picked_side() {
+    let max_x = |side: Value| {
+        let mut s = Session::new();
+        let e = h(&s.execute("ellipse", &json!({"center": [0, 0], "major": [10, 0], "ratio": 0.5, "start": 0, "end": 90})).unwrap());
+        s.execute("offset", &json!({"handle": e, "distance": 1, "side": side})).unwrap();
+        let p = poly(&last(&s));
+        p.vertices.iter().map(|v| v.p.x).fold(f64::MIN, f64::max)
+    };
+    let centre = max_x(json!([0, 0]));
+    assert!((centre - 9.0).abs() < 0.05, "centre side went outward: max x = {centre}");
+    let outer = max_x(json!([20, 20]));
+    assert!((outer - 11.0).abs() < 0.05, "outer side went inward: max x = {outer}");
+}