Gate display frame callbacks on shared presentation visibility

compositor/src/display_presentation.rs

@@ -44,6 +44,17 @@ impl DisplayPresentation {
         self.outputs.values().any(|record| record.displayed.contains(surface))
     }
 
+    // Membership describes every output; callback ownership is a separate choice.
+    // A stable tie-break avoids alternating owners when refresh rates match.
+    pub fn callback_output(&self, surface: &Id) -> Option<Output> {
+        self.outputs
+            .iter()
+            .filter(|(_, record)| record.displayed.contains(surface))
+            .map(|(output, _)| output)
+            .max_by_key(|output| (output.current_mode().map_or(0, |mode| mode.refresh), output.name()))
+            .cloned()
+    }
+
     pub fn remove_surface(&mut self, surface: &Id) {
         for record in self.outputs.values_mut() {
             record.displayed.remove(surface);
@@ -139,4 +150,20 @@ mod tests {
         states.states.get_mut(&id).unwrap().presentation_state = RenderElementPresentationState::Skipped;
         assert!(visible(&states).is_empty());
     }
+    #[test]
+    fn callback_ownership_is_stable_and_moves_when_an_output_disappears() {
+        let mut record = DisplayPresentation::default();
+        let a = output("a");
+        let b = output("b");
+        let id = Id::new();
+        for output in [&a, &b] {
+            record.queued(output, &scene(&id));
+            record.presented(output);
+        }
+        assert_eq!(record.callback_output(&id), Some(b.clone()));
+        record.remove_output(&b);
+        assert_eq!(record.callback_output(&id), Some(a.clone()));
+        record.remove_surface(&id);
+        assert_eq!(record.callback_output(&id), None);
+    }
 }