Bug 1895928 - PresShell::ScheduleViewManagerFlush shouldn't set the layout flush bit. r=smaug

A version of the patch in bug 1895870 caused some WPT tests to time out
because document.fonts.ready never resolves.

This is because I made PresShell::HasPendingReflow look at
mNeedLayoutFlush, and nsIFrame::SchedulePaint() calls
PresShell::ScheduleViewManagerFlush() which sets that bit.

So we end up with mNeedLayoutFlush, but without a flush scheduled, and
the ready promise doesn't resolve until something else schedules a
flush. I fixed the original patch to preserve behavior, but requesting a
repaint shouldn't require a layout flush.

This call was added in bug 598482 part 16:

  https://hg.mozilla.org/mozilla-central/rev/71d77495b606fc145bdceabcda29181978bd9405

We only need the layout flush if there are view geometry updates, so
shift the call up to the view manager.

Differential Revision: https://phabricator.services.mozilla.com/D209938

Author: emilio

layout/base/PresShell.cpp

@@ -3931,11 +3931,9 @@ void PresShell::ScheduleViewManagerFlush() {
     return;
   }
 
-  nsPresContext* presContext = GetPresContext();
-  if (presContext) {
+  if (nsPresContext* presContext = GetPresContext()) {
     presContext->RefreshDriver()->ScheduleViewManagerFlush();
   }
-  SetNeedLayoutFlush();
 }
 
 void PresShell::DispatchSynthMouseMove(WidgetGUIEvent* aEvent) {

view/nsView.cpp

@@ -1081,8 +1081,7 @@ void nsView::DidCompositeWindow(mozilla::layers::TransactionId aTransactionId,
 }
 
 void nsView::RequestRepaint() {
-  PresShell* presShell = mViewManager->GetPresShell();
-  if (presShell) {
+  if (PresShell* presShell = mViewManager->GetPresShell()) {
     presShell->ScheduleViewManagerFlush();
   }
 }

view/nsViewManager.cpp

@@ -455,6 +455,7 @@ void nsViewManager::PostPendingUpdate() {
   nsViewManager* rootVM = RootViewManager();
   rootVM->mHasPendingWidgetGeometryChanges = true;
   if (rootVM->mPresShell) {
+    rootVM->mPresShell->SetNeedLayoutFlush();
     rootVM->mPresShell->ScheduleViewManagerFlush();
   }
 }