Masks: deleting vertices fills in missing tangents first, like add and set (#1250)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/commands/mask.rs

@@ -290,6 +290,7 @@ fn edit_vertices_counted(s: &mut Session, p: &Value, label: &str, removes: bool,
             let roto = super::paths::is_roto(g);
             let Some(pr) = g.get_mut("path") else { continue };
             let KV::Path(mut sp) = pr.value_at(lt) else { continue };
+            sp.fit_tangents();
             f(&mut sp, idx);
             if roto {
                 super::paths::roto_smooth(&mut sp);

crates/engine/src/tests_timeline.rs

@@ -373,6 +373,17 @@ fn vertex_commands_fill_in_missing_tangents_of_a_loaded_path() {
     assert_eq!(p.in_tangents[1], [0.0, 0.0], "a missing tangent is a corner");
 }
 
+#[test]
+fn deleting_vertices_of_a_loaded_path_with_short_tangents_does_not_panic() {
+    let (mut s, l) = setup();
+    let m = s.execute("mask.new", json!({"layer": l, "vertices": [[10, 10], [90, 10], [90, 90]]})).unwrap()["mask"].as_u64().unwrap();
+    reopen_with_short_tangents(&mut s);
+    s.execute("mask.selectVertices", json!({"vertices": [{"layer": l, "mask": m, "index": 2}]})).unwrap();
+    s.execute("mask.deleteVertices", json!({})).unwrap();
+    let p = prop(&s, l, "masks/#1/path").value.as_path().unwrap().clone();
+    assert_eq!((p.vertices.len(), p.in_tangents.len(), p.out_tangents.len()), (2, 2, 2));
+}
+
 #[test]
 fn vertex_commands_fill_in_missing_tangents_of_a_loaded_animated_path() {
     let (mut s, l) = setup();