#1284: keep Rove Across Time covered through edits to other keys

The report says a roving key goes back to its original time after other keys are edited. Editing
the last key's value and then its time keeps the roving key at its share of the path, as the
Rove Across Time description says, so this adds a regression test for both edits. It passes on
this branch; the reported sequence is not reproduced yet, so the report still needs the exact
steps.

Claude-Session: https://claude.ai/code/session_0136kZtXrYo5VUWEFxWduwUD

crates/engine/src/tests_timeline.rs

@@ -219,6 +219,25 @@ fn interpolation_dialog_temporal_spatial_and_roving() {
     assert_ne!(prop(&s, l, "transform/position").keys[0].out_interp, Interp::Hold);
 }
 
+/// Editing another key's value or time keeps the roving key at its share of the path (#1284: the
+/// roving key went back to where it was after edits to other keys).
+#[test]
+fn roving_key_follows_value_and_time_edits_to_other_keys() {
+    let (mut s, l) = setup();
+    animate(&mut s, l, "transform/position", &[(0.0, json!([0, 0])), (0.3, json!([300, 0])), (2.0, json!([400, 0]))]);
+    let uid = prop(&s, l, "transform/position").uid;
+    s.execute("keys.select", json!({"keys": [{"layer": l, "prop": uid, "time": 0.3}]})).unwrap();
+    s.execute("keys.interpolation", json!({"roving": true})).unwrap();
+    let roving_at = |s: &Session| prop(s, l, "transform/position").keys[1].time.seconds();
+    assert!((roving_at(&s) - 1.5).abs() < 0.01, "start: {}", roving_at(&s));
+    // The last key's value moves to 800 px: the path is 300 + 500 px, so the roving key is at 300 / 800.
+    s.execute("keys.set", json!({"layer": l, "path": "transform/position", "time": 2.0, "value": [800, 0]})).unwrap();
+    assert!((roving_at(&s) - 0.75).abs() < 0.02, "value edit: {}", roving_at(&s));
+    // The last key moves to 3 s: the same 300 / 800 share of 3 s.
+    s.execute("keys.set", json!({"layer": l, "path": "transform/position", "time": 2.0, "newTime": 3.0})).unwrap();
+    assert!((roving_at(&s) - 1.125).abs() < 0.02, "time edit: {}", roving_at(&s));
+}
+
 #[test]
 fn graph_editor_key_and_handle_edits() {
     let (mut s, l) = setup();