Auto merge of #22451 - kingdido999:send-idle-message-once, r=jdm

Avoid sending idle message more than once after a reflow is complete

<!-- Please describe your changes on the following line: -->
A boolean variable is introduced to prevent sending SetDocumentState idle message multiple times after a reflow is complete.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #22410 (GitHub issue number if applicable)

<!-- Either: -->
- [x] These changes do not require tests because automated test is not possible.

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/22451)
<!-- Reviewable:end -->

components/script/dom/window.rs

@@ -289,6 +289,10 @@ pub struct Window {
     /// Webrender API Sender
     #[ignore_malloc_size_of = "defined in webrender_api"]
     webrender_api_sender: RenderApiSender,
+
+    /// Indicate whether a SetDocumentStatus message has been sent after a reflow is complete.
+    /// It is used to avoid sending idle message more than once, which is unneccessary.
+    has_sent_idle_message: Cell<bool>,
 }
 
 impl Window {
@@ -1541,12 +1545,15 @@ impl Window {
                 elem.has_class(&atom!("reftest-wait"), CaseSensitivity::CaseSensitive)
             });
 
-            let ready_state = document.ReadyState();
+            let has_sent_idle_message = self.has_sent_idle_message.get();
-
+            let is_ready_state_complete = document.ReadyState() == DocumentReadyState::Complete;
             let pending_images = self.pending_layout_images.borrow().is_empty();
-            if ready_state == DocumentReadyState::Complete && !reftest_wait && pending_images {
+
+            if !has_sent_idle_message && is_ready_state_complete && !reftest_wait && pending_images
+            {
                 let event = ScriptMsg::SetDocumentState(DocumentState::Idle);
                 self.send_to_constellation(event);
+                self.has_sent_idle_message.set(true);
             }
         }
 
@@ -2094,6 +2101,7 @@ impl Window {
             webrender_document,
             exists_mut_observer: Cell::new(false),
             webrender_api_sender,
+            has_sent_idle_message: Cell::new(false),
         });
 
         unsafe { WindowBinding::Wrap(runtime.cx(), win) }