HATCH treats rooms enclosed by multilines as islands (#1642)

A pick inside a multiline wall found the band between the wall's lines
as the outer boundary, but the rooms the inner lines enclose were
dropped as islands: island_curve accepted lines, arcs, polylines and
splines, not multilines. The rooms were filled along with the walls. A
multiline's lines can now make up an island like the other open curves.

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

crates/engine/src/cmd/hatch.rs

@@ -113,7 +113,8 @@ fn closed_loop(d: &Drawing, e: &Entity) -> Option<Vec<PolyVertex>> {
 }
 
 /// Open curves that can make up an island together (a square drawn as four LINEs, two ARCs
-/// forming a circle). Closed objects are islands on their own (see `closed_loop`).
+/// forming a circle, the inner lines of multiline walls around a room). Closed objects are
+/// islands on their own (see `closed_loop`).
 fn island_curve(d: &Drawing, e: &Entity) -> bool {
     matches!(
         e.kind,
@@ -123,6 +124,7 @@ fn island_curve(d: &Drawing, e: &Entity) -> bool {
             | EntityKind::LwPolyline(_)
             | EntityKind::Polyline3d(_)
             | EntityKind::Spline(_)
+            | EntityKind::MLine(_)
     ) && closed_loop(d, e).is_none()
 }
 

crates/engine/tests/it/hatch_options.rs

@@ -249,3 +249,13 @@ fn hatchedit_refuses_unknown_patterns_and_region_boundaries() {
     // Region boundaries need REGION objects; the hatch is left as it was.
     assert!(s.execute("hatchedit", &json!({ "handles": [h], "recreate": "region" })).is_err());
 }
+
+#[test]
+fn multiline_wall_hatch_keeps_the_room_as_an_island() {
+    // A closed multiline wall: picking inside the wall hatches the band between its outer and
+    // inner lines; the room the inner lines enclose is an island, not filled.
+    let mut s = Session::new();
+    s.execute("mline", &json!({ "points": [[0, 0], [10, 0], [10, 10], [0, 10]], "closed": true, "scale": 1, "justification": "zero" })).unwrap();
+    let h = handle(&s.execute("hatch", &json!({ "points": [[0, 5]], "pattern": "SOLID" })).unwrap());
+    assert_eq!(hatch(&s, &h).loops.len(), 2, "outer face + the room as an island");
+}