Bug 1839580 - For async-parsed stylesheets, fire the load event sooner. r=smaug

This avoids one extra task hop when we finish parsing the stylesheet.

Differential Revision: https://phabricator.services.mozilla.com/D181613

dom/events/AsyncEventDispatcher.h

@@ -190,6 +190,11 @@ class LoadBlockingAsyncEventDispatcher final : public AsyncEventDispatcher {
     }
   }
 
+  // The static version of AsyncEventDispatcher::RunDOMEventWhenSafe should be
+  // preferred when possible, but for LoadBlockingAsyncEventDispatcher it makes
+  // sense to expose the helper so that the load event is blocked appropriately.
+  using AsyncEventDispatcher::RunDOMEventWhenSafe;
+
   LoadBlockingAsyncEventDispatcher(nsINode* aEventNode, dom::Event* aEvent)
       : AsyncEventDispatcher(aEventNode, aEvent),
         mBlockedDoc(aEventNode->OwnerDoc()) {

layout/style/Loader.cpp

@@ -421,17 +421,16 @@ void SheetLoadData::StartPendingLoad() {
                      Loader::PendingLoad::Yes);
 }
 
-void SheetLoadData::ScheduleLoadEventIfNeeded() {
+already_AddRefed<LoadBlockingAsyncEventDispatcher>
+SheetLoadData::PrepareLoadEventIfNeeded() {
   if (!mOwningNodeBeforeLoadEvent) {
-    return;
+    return nullptr;
   }
-
   MOZ_ASSERT(BlocksLoadEvent(), "The rel=preload load event happens elsewhere");
   nsCOMPtr<nsINode> node = std::move(mOwningNodeBeforeLoadEvent);
-  auto* dispatcher = new LoadBlockingAsyncEventDispatcher(
+  return do_AddRef(new LoadBlockingAsyncEventDispatcher(
       node, mLoadFailed ? u"error"_ns : u"load"_ns, CanBubble::eNo,
-      ChromeOnlyDispatch::eNo);
-  dispatcher->PostDOMEvent();
+      ChromeOnlyDispatch::eNo));
 }
 
 nsINode* SheetLoadData::GetRequestingNode() const {
@@ -1557,6 +1556,7 @@ Loader::Completed Loader::ParseSheet(const nsACString& aBytes,
 
 void Loader::NotifyObservers(SheetLoadData& aData, nsresult aStatus) {
   RecordUseCountersIfNeeded(mDocument, aData.mSheet->GetStyleUseCounters());
+  RefPtr loadDispatcher = aData.PrepareLoadEventIfNeeded();
   if (aData.mURI) {
     mLoadsPerformed.PutEntry(SheetLoadDataHashKey(aData));
     aData.NotifyStop(aStatus);
@@ -1565,6 +1565,10 @@ void Loader::NotifyObservers(SheetLoadData& aData, nsresult aStatus) {
     // StyleSheetLoaded callback.
     if (aData.BlocksLoadEvent()) {
       DecrementOngoingLoadCount();
+      if (mPendingLoadCount && mPendingLoadCount == mOngoingLoadCount) {
+        LOG(("  No more loading sheets; starting deferred loads"));
+        StartDeferredLoads();
+      }
     }
   }
   if (!aData.mTitle.IsEmpty() && NS_SUCCEEDED(aStatus)) {
@@ -1579,7 +1583,6 @@ void Loader::NotifyObservers(SheetLoadData& aData, nsresult aStatus) {
           Unused << pageStyleActor;
         }));
   }
-
   if (aData.mMustNotify) {
     if (nsCOMPtr<nsICSSLoaderObserver> observer = std::move(aData.mObserver)) {
       LOG(("  Notifying observer %p for data %p.  deferred: %d", observer.get(),
@@ -1592,11 +1595,12 @@ void Loader::NotifyObservers(SheetLoadData& aData, nsresult aStatus) {
            obs.get(), &aData, aData.ShouldDefer()));
       obs->StyleSheetLoaded(aData.mSheet, aData.ShouldDefer(), aStatus);
     }
-  }
 
-  if (mPendingLoadCount && mPendingLoadCount == mOngoingLoadCount) {
-    LOG(("  No more loading sheets; starting deferred loads"));
-    StartDeferredLoads();
+    if (loadDispatcher) {
+      loadDispatcher->RunDOMEventWhenSafe();
+    }
+  } else if (loadDispatcher) {
+    loadDispatcher->PostDOMEvent();
   }
 }
 

layout/style/SharedStyleSheetCache.cpp

@@ -164,7 +164,6 @@ void SharedStyleSheetCache::LoadCompletedInternal(
         doc->PostStyleSheetApplicableStateChangeEvent(*data->mSheet);
       }
     }
-    data->ScheduleLoadEventIfNeeded();
     aDatasToNotify.AppendElement(data);
 
     NS_ASSERTION(!data->mParentData || data->mParentData->mPendingChildren != 0,

layout/style/SheetLoadData.h

@@ -18,8 +18,9 @@
 #include "nsProxyRelease.h"
 
 namespace mozilla {
+class LoadBlockingAsyncEventDispatcher;
 class StyleSheet;
-}
+}  // namespace mozilla
 class nsICSSLoaderObserver;
 class nsINode;
 class nsIPrincipal;
@@ -79,7 +80,7 @@ class SheetLoadData final
 
   nsIReferrerInfo* ReferrerInfo() const { return mReferrerInfo; }
 
-  void ScheduleLoadEventIfNeeded();
+  already_AddRefed<LoadBlockingAsyncEventDispatcher> PrepareLoadEventIfNeeded();
 
   NotNull<const Encoding*> DetermineNonBOMEncoding(const nsACString& aSegment,
                                                    nsIChannel*) const;
@@ -159,7 +160,8 @@ class SheetLoadData final
   // the original function call that started the load has returned.
   // This applies only to observer notifications; load/error events
   // are fired for any SheetLoadData that has a non-null
-  // mOwningNodeBeforeLoadEvent.
+  // mOwningNodeBeforeLoadEvent (though mMustNotify is used to avoid an event
+  // loop round-trip in that case).
   bool mMustNotify : 1;
 
   // mWasAlternate is true if the sheet was an alternate when the load data was