Send idle messages as part of messaging loop

components/script/dom/document.rs

@@ -3723,12 +3723,7 @@ impl Document {
             receiver.recv().unwrap();
         }
 
-        let result = self.window().reflow(ReflowGoal::UpdateTheRendering, can_gc);
+        self.window().reflow(ReflowGoal::UpdateTheRendering, can_gc)
-
-        self.window()
-            .maybe_send_idle_document_state_to_constellation();
-
-        result
     }
 
     pub(crate) fn id_map(&self) -> Ref<HashMapTracedValues<Atom, Vec<Dom<Element>>>> {

components/script/dom/window.rs

@@ -2345,6 +2345,10 @@ impl Window {
             return;
         }
 
+        if self.Document().needs_rendering_update() {
+            return;
+        }
+
         // When all these conditions are met, notify the constellation
         // that this pipeline is ready to write the image (from the script thread
         // perspective at least).

components/script/script_thread.rs

@@ -1437,6 +1437,18 @@ impl ScriptThread {
         );
     }
 
+    fn maybe_send_idle_document_state_to_constellation(&self) {
+        if !opts::get().wait_for_stable_image {
+            return;
+        }
+
+        for (_, document) in self.documents.borrow().iter() {
+            document
+                .window()
+                .maybe_send_idle_document_state_to_constellation();
+        }
+    }
+
     /// Handle incoming messages from other tasks and the task queue.
     fn handle_msgs(&self, can_gc: CanGc) -> bool {
         // Proritize rendering tasks and others, and gather all other events as `sequential`.
@@ -1647,6 +1659,7 @@ impl ScriptThread {
         }
 
         self.maybe_schedule_rendering_opportunity_after_ipc_message(can_gc);
+        self.maybe_send_idle_document_state_to_constellation();
 
         true
     }