Fix #2091 clear stale key and vertex selections around mask copy/paste

Selecting a mask group now clears the previous key selection, so Copy/Cut act
on the mask. Pasting masks clears the source mask's vertex selection, so Clear
removes the pasted mask rather than points of the source.

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

crates/engine/src/cmd/prop.rs

@@ -794,6 +794,8 @@ fn select_prop(s: &mut Session, p: &Value) -> Result<Value> {
     if !add {
         s.state.selected_props.clear();
         s.state.selected_vertices.clear();
+        // Groups (masks, effects) have no keys of their own: don't leave the old selection's behind.
+        s.state.selected_keys.clear();
     }
     if !s.state.selected_layers.contains(&lid) {
         s.state.selected_layers = vec![lid];
@@ -811,9 +813,6 @@ fn select_prop(s: &mut Session, p: &Value) -> Result<Value> {
         && let Some(pr) = s.active_comp().and_then(|c| c.layer(lid)).and_then(|l| l.props.find(uid))
     {
         let keys: Vec<KeyRef> = pr.keys.iter().map(|k| KeyRef { layer: lid, prop: uid, time: k.time }).collect();
-        if !add {
-            s.state.selected_keys.clear();
-        }
         s.state.selected_keys.extend(keys);
     }
     Ok(Value::Null)

crates/engine/src/cmd/prop_groups.rs

@@ -183,6 +183,8 @@ pub(crate) fn paste_masks(s: &mut Session, p: &Value) -> Result<Value> {
         }
         proj.next_id = next + 1;
         st.selected_props = out.clone();
+        // The source mask's points (selected with it) must not outlive its selection.
+        st.selected_vertices.clear();
         Ok(out)
     })?;
     Ok(json!({"masks": pasted.iter().map(|(_, u)| *u).collect::<Vec<_>>()}))