Auto merge of #20041 - paavininanda:mutflags, r=jdm

Added extra bool in Window object to know about its Mutation Observers

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- Added Cell<bool> type variable in Window object to know whether its mutation observers are present/absent
- Added get and set functions for this variable
- Added supporting test files to check for performance

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #16936.

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- [x] There are tests for these changes, but these are manual tests for checking correctness of added code

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components/script/dom/mutationobserver.rs

@@ -9,7 +9,7 @@ use dom::bindings::codegen::Bindings::MutationObserverBinding::MutationCallback;
 use dom::bindings::codegen::Bindings::MutationObserverBinding::MutationObserverBinding::MutationObserverMethods;
 use dom::bindings::codegen::Bindings::MutationObserverBinding::MutationObserverInit;
 use dom::bindings::error::{Error, Fallible};
-use dom::bindings::reflector::{Reflector, reflect_dom_object};
+use dom::bindings::reflector::{Reflector, reflect_dom_object, DomObject};
 use dom::bindings::root::DomRoot;
 use dom::bindings::str::DOMString;
 use dom::mutationrecord::MutationRecord;
@@ -67,6 +67,7 @@ impl MutationObserver {
     }
 
     pub fn Constructor(global: &Window, callback: Rc<MutationCallback>) -> Fallible<DomRoot<MutationObserver>> {
+        global.set_exists_mut_observer();
         let observer = MutationObserver::new(global, callback);
         ScriptThread::add_mutation_observer(&*observer);
         Ok(observer)
@@ -105,6 +106,9 @@ impl MutationObserver {
 
     /// <https://dom.spec.whatwg.org/#queueing-a-mutation-record>
     pub fn queue_a_mutation_record(target: &Node, attr_type: Mutation) {
+        if !target.global().as_window().get_exists_mut_observer() {
+            return;
+        }
         // Step 1
         let mut interestedObservers: Vec<(DomRoot<MutationObserver>, Option<DOMString>)> = vec![];
         // Step 2 & 3

components/script/dom/window.rs

@@ -285,9 +285,20 @@ pub struct Window {
     /// The Webrender Document id associated with this window.
     #[ignore_malloc_size_of = "defined in webrender_api"]
     webrender_document: DocumentId,
+
+    /// Flag to identify whether mutation observers are present(true)/absent(false)
+    exists_mut_observer: Cell<bool>,
 }
 
 impl Window {
+    pub fn get_exists_mut_observer(&self) -> bool {
+        self.exists_mut_observer.get()
+    }
+
+    pub fn set_exists_mut_observer(&self) {
+        self.exists_mut_observer.set(true);
+    }
+
     #[allow(unsafe_code)]
     pub fn clear_js_runtime_for_script_deallocation(&self) {
         unsafe {
@@ -1814,6 +1825,7 @@ impl Window {
             test_worklet: Default::default(),
             paint_worklet: Default::default(),
             webrender_document,
+            exists_mut_observer: Cell::new(false),
         });
 
         unsafe {