Auto merge of #22389 - jdm:anim-complete, r=emilio,ferjm

Cancel animations that affect nodes that do not participate in layout.

- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #22378
- [x] There are tests for these changes

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components/script/dom/webidls/Window.webidl

@@ -173,3 +173,8 @@ partial interface Window {
    readonly attribute TestRunner testRunner;
    //readonly attribute EventSender eventSender;
 };
+
+partial interface Window {
+   [Pref="css.animations.testing.enabled"]
+   readonly attribute unsigned long runningAnimationCount;
+};

components/script/dom/window.rs

@@ -74,7 +74,7 @@ use devtools_traits::{ScriptToDevtoolsControlMsg, TimelineMarker, TimelineMarker
 use dom_struct::dom_struct;
 use embedder_traits::EmbedderMsg;
 use euclid::{Point2D, Rect, Size2D, TypedPoint2D, TypedScale, TypedSize2D, Vector2D};
-use ipc_channel::ipc::IpcSender;
+use ipc_channel::ipc::{channel, IpcSender};
 use ipc_channel::router::ROUTER;
 use js::jsapi::JSAutoCompartment;
 use js::jsapi::JSContext;
@@ -1144,6 +1144,12 @@ impl WindowMethods for Window {
         self.test_runner.or_init(|| TestRunner::new(self.upcast()))
     }
 
+    fn RunningAnimationCount(&self) -> u32 {
+        let (sender, receiver) = channel().unwrap();
+        let _ = self.layout_chan.send(Msg::GetRunningAnimations(sender));
+        receiver.recv().unwrap_or(0) as u32
+    }
+
     // https://html.spec.whatwg.org/multipage/#dom-name
     fn SetName(&self, name: DOMString) {
         self.window_proxy().set_name(name);