Bug 1668966. Only clear PresShell::mResolutionUpdated after the apz transaction. r=kats,botond

We clear PresShell::mResolutionUpdated in nsLayoutUtils::ComputeScrollMetadata here

https://searchfox.org/mozilla-central/rev/7ef5cefd0468b8f509efe38e0212de2398f4c8b3/layout/base/nsLayoutUtils.cpp#9419

but that is incorrect because we call ComputeScrollMetadata twice for one transaction with webrender. This is why ScrollFrameHelper::NotifyApzTransaction exists: to clear things on the layout side after they are sent to the compositor.

Differential Revision: https://phabricator.services.mozilla.com/D92316

layout/base/nsLayoutUtils.cpp

@@ -767,6 +767,10 @@ bool nsLayoutUtils::ShouldDisableApzForElement(nsIContent* aContent) {
 void nsLayoutUtils::NotifyPaintSkipTransaction(ViewID aScrollId) {
   if (nsIScrollableFrame* scrollFrame =
           nsLayoutUtils::FindScrollableFrameFor(aScrollId)) {
+#ifdef DEBUG
+    nsIFrame* f = do_QueryFrame(scrollFrame);
+    MOZ_ASSERT(f && f->PresShell() && !f->PresShell()->IsResolutionUpdated());
+#endif
     scrollFrame->NotifyApzTransaction();
   }
 }
@@ -8537,8 +8541,6 @@ ScrollMetadata nsLayoutUtils::ComputeScrollMetadata(
 
   if (presShell->IsResolutionUpdated()) {
     metadata.SetResolutionUpdated(true);
-    // We only need to tell APZ about the resolution update once.
-    presShell->SetResolutionUpdated(false);
   }
 
   // The cumulative resolution is the resolution at which the scroll frame's

layout/generic/nsGfxScrollFrame.cpp

@@ -4332,6 +4332,9 @@ void ScrollFrameHelper::NotifyApzTransaction() {
   mApzScrollPos = GetScrollPosition();
   mApzAnimationRequested = IsLastScrollUpdateAnimating();
   mScrollUpdates.Clear();
+  if (mIsRoot) {
+    mOuter->PresShell()->SetResolutionUpdated(false);
+  }
 }
 
 Maybe<ScrollMetadata> ScrollFrameHelper::ComputeScrollMetadata(