Masks: addVertex and setVertex fill in missing tangents of a loaded path (#1250)

A path's vertices and its two tangent arrays deserialize independently,
and nothing on load makes them agree. mask.new pads them, but a path
read from a project file may carry fewer tangents than vertices.
mask.addVertex checked its index against the vertices only and then
inserted into both tangent arrays, and mask.setVertex indexed the
tangent arrays after the same check, so either command panicked on
such a path (Vec insert/index out of bounds).

ShapePath::fit_tangents gives both arrays one entry per vertex (missing
tangents become corners, extras are dropped), and edit_path_counted
calls it on the value it edits, so every mask/shape-path vertex command
works on a consistent path. Other keyframes of an animated path were
already padded by apply_count_op when the vertex count changes.

Tests: two regression tests save a project, cut every mask path's
tangent arrays to one entry (static value and keyframes), reopen it
through Project::from_json and run setVertex and addVertex. Both panic
at mask.rs:209 without the fix.

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

crates/engine/src/commands/mask.rs

@@ -132,6 +132,9 @@ fn edit_path_counted<T>(s: &mut Session, p: &Value, label: &str, f: impl FnOnce(
         let roto = super::paths::is_roto(g);
         let pr = g.get_mut("path").ok_or_else(|| bad(label, "mask has no path"))?;
         let KV::Path(mut sp) = pr.value_at(lt) else { return Err(bad(label, "not a path")) };
+        // A path read from a file may have fewer tangents than vertices; the edits below index
+        // all three arrays by vertex (#1250).
+        sp.fit_tangents();
         let (r, op) = f(&mut sp)?;
         if roto {
             super::paths::roto_smooth(&mut sp);

crates/engine/src/tests_timeline.rs

@@ -331,6 +331,66 @@ fn pen_tool_draws_edits_and_closes_a_mask() {
     assert_eq!(s.active_comp().unwrap().layers[0].masks().unwrap().children.len(), 1);
 }
 
+/// Cut every mask path's tangent arrays in the saved project to one entry (static values and
+/// keyframes alike), then open the result: the shape of a hand-edited or foreign file (#1250).
+fn reopen_with_short_tangents(s: &mut Session) {
+    fn cut(v: &mut Value) {
+        match v {
+            Value::Object(o) => {
+                if o.contains_key("vertices") {
+                    for k in ["in_tangents", "out_tangents"] {
+                        if let Some(Value::Array(a)) = o.get_mut(k) {
+                            a.truncate(1);
+                        }
+                    }
+                }
+                o.values_mut().for_each(cut);
+            }
+            Value::Array(a) => a.iter_mut().for_each(cut),
+            _ => {}
+        }
+    }
+    let mut j: Value = serde_json::from_str(&s.project.to_json()).unwrap();
+    cut(&mut j);
+    let p = effectcraft_project::Project::from_json(&j.to_string()).unwrap();
+    s.replace_project(p, None);
+}
+
+#[test]
+fn vertex_commands_fill_in_missing_tangents_of_a_loaded_path() {
+    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);
+    let path = |s: &Session| prop(s, l, "masks/#1/path").value.as_path().unwrap().clone();
+    assert_eq!(path(&s).in_tangents.len(), 1, "the file really has short tangents");
+    // Both used to index the tangent arrays by vertex and panic.
+    s.execute("mask.setVertex", json!({"layer": l, "mask": m, "index": 2, "in": [0, -20], "out": [0, 20]})).unwrap();
+    s.execute("mask.addVertex", json!({"layer": l, "mask": m, "point": [10, 90]})).unwrap();
+    let p = path(&s);
+    assert_eq!(p.vertices.len(), 4);
+    assert_eq!((p.in_tangents.len(), p.out_tangents.len()), (4, 4));
+    assert_eq!((p.in_tangents[2], p.out_tangents[2]), ([0.0, -20.0], [0.0, 20.0]));
+    assert_eq!(p.in_tangents[1], [0.0, 0.0], "a missing tangent is a corner");
+}
+
+#[test]
+fn vertex_commands_fill_in_missing_tangents_of_a_loaded_animated_path() {
+    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();
+    s.execute("prop.toggleAnimation", json!({"layer": l, "path": "masks/#1/path"})).unwrap();
+    s.execute("time.set", json!({"time": 1.0})).unwrap();
+    s.execute("mask.setVertex", json!({"layer": l, "mask": m, "index": 0, "point": [0, 0]})).unwrap();
+    reopen_with_short_tangents(&mut s);
+    s.execute("time.set", json!({"time": 1.0})).unwrap();
+    s.execute("mask.setVertex", json!({"layer": l, "mask": m, "index": 2, "in": [5, 5]})).unwrap();
+    s.execute("mask.addVertex", json!({"layer": l, "mask": m, "index": 3, "point": [10, 90]})).unwrap();
+    for k in &prop(&s, l, "masks/#1/path").keys {
+        let p = k.value.as_path().unwrap();
+        assert_eq!(p.vertices.len(), 4, "the other key gains the vertex too");
+        assert!(p.in_tangents.len() == 4 && p.out_tangents.len() == 4, "{p:?}");
+    }
+}
+
 #[test]
 fn alt_click_stopwatch_toggles_a_self_reference_expression() {
     let (mut s, l) = setup();

crates/keyframe/src/value.rs

@@ -42,6 +42,16 @@ impl ShapePath {
     pub fn len(&self) -> usize {
         self.vertices.len()
     }
+    /// Give both tangent arrays one entry per vertex: missing tangents become zero (a corner),
+    /// extra ones are dropped. The three arrays are read independently from a project file, so a
+    /// loaded path can disagree with itself (#1250). Returns whether anything changed.
+    pub fn fit_tangents(&mut self) -> bool {
+        let n = self.vertices.len();
+        let changed = self.in_tangents.len() != n || self.out_tangents.len() != n;
+        self.in_tangents.resize(n, [0.0; 2]);
+        self.out_tangents.resize(n, [0.0; 2]);
+        changed
+    }
     /// Number of segments (vertex to next vertex, the closing one included).
     pub fn segment_count(&self) -> usize {
         let n = self.vertices.len();