Auto merge of #16883 - jdm:mutationobserver, r=jdm

Mutation Observer API

Rebased from #16668.

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix (partially) #6633
- [X] There are tests for these changes

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components/script/script_thread.rs

@@ -481,6 +481,9 @@ pub struct ScriptThread {
 
     microtask_queue: MicrotaskQueue,
 
+    /// Microtask Queue for adding support for mutation observer microtasks
+    mutation_observer_compound_microtask_queued: Cell<bool>,
+
     /// The unit of related similar-origin browsing contexts' list of MutationObserver objects
     mutation_observers: DOMRefCell<Vec<JS<MutationObserver>>>,
 
@@ -589,6 +592,20 @@ impl ScriptThread {
         })
     }
 
+    pub fn set_mutation_observer_compound_microtask_queued(value: bool) {
+        SCRIPT_THREAD_ROOT.with(|root| {
+            let script_thread = unsafe { &*root.get().unwrap() };
+            script_thread.mutation_observer_compound_microtask_queued.set(value);
+        })
+    }
+
+    pub fn is_mutation_observer_compound_microtask_queued() -> bool {
+        SCRIPT_THREAD_ROOT.with(|root| {
+            let script_thread = unsafe { &*root.get().unwrap() };
+            return script_thread.mutation_observer_compound_microtask_queued.get();
+        })
+    }
+
     pub fn add_mutation_observer(observer: &MutationObserver) {
         SCRIPT_THREAD_ROOT.with(|root| {
             let script_thread = unsafe { &*root.get().unwrap() };
@@ -598,6 +615,13 @@ impl ScriptThread {
         })
     }
 
+    pub fn get_mutation_observers() -> Vec<Root<MutationObserver>> {
+        SCRIPT_THREAD_ROOT.with(|root| {
+            let script_thread = unsafe { &*root.get().unwrap() };
+            script_thread.mutation_observers.borrow().iter().map(|o| Root::from_ref(&**o)).collect()
+        })
+    }
+
     pub fn mark_document_with_no_blocked_loads(doc: &Document) {
         SCRIPT_THREAD_ROOT.with(|root| {
             let script_thread = unsafe { &*root.get().unwrap() };
@@ -750,6 +774,8 @@ impl ScriptThread {
 
             microtask_queue: MicrotaskQueue::default(),
 
+            mutation_observer_compound_microtask_queued: Default::default(),
+
             mutation_observers: Default::default(),
 
             layout_to_constellation_chan: state.layout_to_constellation_chan,