Bug 1760222 - Add an API for JS to tell whether the document has scroll-linked effects or not. r=botond

Differential Revision: https://phabricator.services.mozilla.com/D141456

dom/base/nsDOMWindowUtils.cpp

@@ -4736,3 +4736,13 @@ nsDOMWindowUtils::GetSuspendedByBrowsingContextGroup(bool* aResult) {
   *aResult = inner->GetWasSuspendedByGroup();
   return NS_OK;
 }
+
+NS_IMETHODIMP
+nsDOMWindowUtils::GetHasScrollLinkedEffect(bool* aResult) {
+  Document* doc = GetDocument();
+  if (!doc) {
+    return NS_ERROR_FAILURE;
+  }
+  *aResult = doc->HasScrollLinkedEffect();
+  return NS_OK;
+}

dom/interfaces/base/nsIDOMWindowUtils.idl

@@ -2250,6 +2250,14 @@ interface nsIDOMWindowUtils : nsISupports {
 
   // Used for testing to check the suspend status.
   readonly attribute bool suspendedByBrowsingContextGroup;
+
+  // Whether there's any scroll-linked effect in this document. This is only
+  // meaningful after the script in question tried to mutate something in a
+  // scroll event callback until the next refresh driver tick happens.
+  //
+  // See https://firefox-source-docs.mozilla.org/performance/scroll-linked_effects.html
+  // about scroll-linked effects.
+  readonly attribute bool hasScrollLinkedEffect;
 };
 
 [scriptable, uuid(c694e359-7227-4392-a138-33c0cc1f15a6)]