Auto merge of #16861 - gterzian:use_microtask_to_await_stable_state, r=jdm

Use microtasks to await a stable state.

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@jdm @KiChjang First pass at using microtasks to await a stable state. I ran into all sorts of problems to get it to compile, I think it's mainly related to the fact that the microtasks are stored in a `Vec`, which meant the `Runnalbe.handler(self: Box<Self>)` couldn't be called while iterating over the Vec... It compiles now although I haven't run any tests. I'm assuming I'm missing something fundamental and was hoping my changes would highlight the problems I run into, and you had a better idea of how to implement this... Perhaps we shouldn't pass a `Runnable` to `await_stable_state` at all?

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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #15375 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/script/script_thread.rs

@@ -670,14 +670,11 @@ impl ScriptThread {
     }
 
     // https://html.spec.whatwg.org/multipage/#await-a-stable-state
-    pub fn await_stable_state<T: Runnable + Send + 'static>(task: T) {
-        //TODO use microtasks when they exist
+    pub fn await_stable_state(task: Microtask) {
         SCRIPT_THREAD_ROOT.with(|root| {
             if let Some(script_thread) = root.get() {
                 let script_thread = unsafe { &*script_thread };
-                let _ = script_thread.chan.send(CommonScriptMsg::RunnableMsg(
-                    ScriptThreadEventCategory::DomEvent,
-                    box task));
+                script_thread.microtask_queue.enqueue(task);
             }
         });
     }