Skip PaintPhaseDescendantBlockBackgroundsOnly phase if possible

third_party/WebKit/Source/core/layout/LayoutBox.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 1453 matching lines @@
         layoutView = layoutView->frame()->ownerLayoutObject()->view();
     mapToVisibleRectInAncestorSpace(layoutView, rect, nullptr);
     return rect.intersects(LayoutRect(layoutView->frameView()->scrollableArea()->visibleContentRectDouble()));
 }
 
 PaintInvalidationReason LayoutBox::invalidatePaintIfNeeded(PaintInvalidationState& paintInvalidationState, const LayoutBoxModelObject& paintInvalidationContainer)
 {
     if (isFloating())
         paintInvalidationState.enclosingSelfPaintingLayer(*this).setNeedsPaintPhaseFloat();
 
+    if (hasBoxDecorationBackground() || (hasOverflowClip() && scrollableArea()->hasOverflowControls())) {

[chrishtr, 2016/02/01 17:43:49]

Why the is this logic the right logic? In particular the part about controls and
overflow clip. Please also add a comment explaining.

[Xianzhu, 2016/02/01 17:58:39]

Added comment:
// We also paint overflow controls in background phase.

+        PaintLayer& layer = paintInvalidationState.enclosingSelfPaintingLayer(*this);
+        if (layer.layoutObject() != this)
+            layer.setNeedsPaintPhaseDescendantBlockBackgrounds();
+    }
+
     PaintInvalidationReason fullInvalidationReason = fullPaintInvalidationReason();
     // If the current paint invalidation reason is PaintInvalidationDelayedFull, then this paint invalidation can delayed if the
     // LayoutBox in question is not on-screen. The logic to decide whether this is appropriate exists at the site of the original
     // paint invalidation that chose PaintInvalidationDelayedFull.
     if (fullInvalidationReason == PaintInvalidationDelayedFull) {
         if (!intersectsVisibleViewport())
             return PaintInvalidationDelayedFull;
 
         // Reset state back to regular full paint invalidation if the object is onscreen.
         setShouldDoFullPaintInvalidation(PaintInvalidationFull);
@@ skipping 3156 matching lines @@
 
 void LayoutBox::clearPercentHeightDescendants()
 {
     for (LayoutObject* curr = slowFirstChild(); curr; curr = curr->nextInPreOrder(this)) {
         if (curr->isBox())
             toLayoutBox(curr)->removeFromPercentHeightContainer();
     }
 }
 
 } // namespace blink