Bug 1672527 - Don't update overflow on frames that haven't been reflowed yet. r=emilio

This adds NS_FRAME_FIRST_REFLOW to the frame state flags
we're NOT updating overflow for.  This is a minor performance
optimization.

Differential Revision: https://phabricator.services.mozilla.com/D94395

layout/base/RestyleManager.cpp

@@ -1247,6 +1247,14 @@ static nsIContent* NextSiblingWhichMayHaveFrame(nsIContent* aContent) {
   return nullptr;
 }
 
+// If |aFrame| is dirty or has dirty children, or has never been reflowed,
+// then we can skip updating overflows since that will happen when it's
+// reflowed.
+static inline bool CanSkipOverflowUpdates(const nsIFrame* aFrame) {
+  return aFrame->HasAnyStateBits(
+      NS_FRAME_IS_DIRTY | NS_FRAME_HAS_DIRTY_CHILDREN | NS_FRAME_FIRST_REFLOW);
+}
+
 void RestyleManager::ProcessRestyledFrames(nsStyleChangeList& aChangeList) {
   NS_ASSERTION(!nsContentUtils::IsSafeToRunScript(),
                "Someone forgot a script blocker");
@@ -1484,7 +1492,7 @@ void RestyleManager::ProcessRestyledFrames(nsStyleChangeList& aChangeList) {
     } else {
       NS_ASSERTION(frame, "This shouldn't happen");
 
-      if (!frame->FrameMaintainsOverflow()) {
+      if (!frame->FrameMaintainsOverflow() || CanSkipOverflowUpdates(frame)) {
         // frame does not maintain overflow rects, so avoid calling
         // FinishAndStoreOverflow on it:
         hint &=
@@ -1637,10 +1645,7 @@ void RestyleManager::ProcessRestyledFrames(nsStyleChangeList& aChangeList) {
             // the outer svg's anonymous child frame (instead of to the
             // anonymous child's children).
 
-            // If |hintFrame| is dirty or has dirty children, we don't bother
-            // updating overflows since that will happen when it's reflowed.
-            if (!hintFrame->HasAnyStateBits(NS_FRAME_IS_DIRTY |
-                                            NS_FRAME_HAS_DIRTY_CHILDREN)) {
+            if (!CanSkipOverflowUpdates(hintFrame)) {
               mOverflowChangedTracker.AddFrame(
                   hintFrame, OverflowChangedTracker::CHILDREN_CHANGED);
             }
@@ -1651,10 +1656,7 @@ void RestyleManager::ProcessRestyledFrames(nsStyleChangeList& aChangeList) {
             for (; childFrame; childFrame = childFrame->GetNextSibling()) {
               MOZ_ASSERT(childFrame->IsFrameOfType(nsIFrame::eSVG),
                          "Not expecting non-SVG children");
-              // If |childFrame| is dirty or has dirty children, we don't bother
-              // updating overflows since that will happen when it's reflowed.
-              if (!childFrame->HasAnyStateBits(NS_FRAME_IS_DIRTY |
-                                               NS_FRAME_HAS_DIRTY_CHILDREN)) {
+              if (!CanSkipOverflowUpdates(childFrame)) {
                 mOverflowChangedTracker.AddFrame(
                     childFrame, OverflowChangedTracker::CHILDREN_CHANGED);
               }
@@ -1668,10 +1670,7 @@ void RestyleManager::ProcessRestyledFrames(nsStyleChangeList& aChangeList) {
             }
           }
         }
-        // If |frame| is dirty or has dirty children, we don't bother updating
-        // overflows since that will happen when it's reflowed.
-        if (!frame->HasAnyStateBits(NS_FRAME_IS_DIRTY |
-                                    NS_FRAME_HAS_DIRTY_CHILDREN)) {
+        if (!CanSkipOverflowUpdates(frame)) {
           if (hint & (nsChangeHint_UpdateOverflow |
                       nsChangeHint_UpdatePostTransformOverflow)) {
             OverflowChangedTracker::ChangeKind changeKind;