RAL: Eliminate n^2 walk to find repaint containers

Source/core/rendering/RenderBlock.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2007 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2010. 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
  * License as published by the Free Software Foundation; either
@@ skipping 342 matching lines @@
     // If the style has unloaded images, want to notify the ResourceLoadPriorityOptimizer so that
     // network priorities can be set.
     Vector<ImageResource*> images;
     appendImagesFromStyle(images, *newStyle);
     if (images.isEmpty())
         ResourceLoadPriorityOptimizer::resourceLoadPriorityOptimizer()->removeRenderObject(this);
     else
         ResourceLoadPriorityOptimizer::resourceLoadPriorityOptimizer()->addRenderObject(this);
 }
 
-void RenderBlock::repaintTreeAfterLayout()
+void RenderBlock::repaintTreeAfterLayout(RenderLayerModelObject& repaintContainer)
 {
     if (!shouldCheckForInvalidationAfterLayout())
         return;
 
-    RenderBox::repaintTreeAfterLayout();
+    RenderBox::repaintTreeAfterLayout(repaintContainer);
 
     // Take care of positioned objects. This is required as LayoutState keeps a single clip rect.
     if (TrackedRendererListHashSet* positionedObjects = this->positionedObjects()) {
         TrackedRendererListHashSet::iterator end = positionedObjects->end();
         LayoutStateMaintainer statePusher(*this, isTableRow() ? LayoutSize() : locationOffset());
-        for (TrackedRendererListHashSet::iterator it = positionedObjects->begin(); it != end; ++it)
-            (*it)->repaintTreeAfterLayout();
+        for (TrackedRendererListHashSet::iterator it = positionedObjects->begin(); it != end; ++it) {
+            // One of the renderers we're skipping over here may be the child's repaint container,
+            // so we can't pass our own repaint container along.
+            RenderLayerModelObject* repaintContainerForChild = (*it)->containerForRepaint();

[Julien - ping for review, 2014/05/07 17:50:03]

That's kind of crazy that a positioned element doesn't paint into its containing
block or an ancestor of it. It sounds like we could end up improperly painting
due to that.

[leviw_travelin_and_unemployed, 2014/05/07 17:58:48]

Super weird, I agree.

[Ian Vollick, 2014/05/07 22:55:58]

Oh, that is strange. I wonder what bugs this is currently causing..

[eseidel, 2014/05/08 00:43:02]

I'm not sure it is odd.  But I admit to not fully keeping painting in my head. 
http://www.w3.org/TR/CSS2/zindex.html

On 2014/05/07 22:55:58, Ian Vollick wrote:

[ojan, 2014/05/10 17:43:12]

In theory, all the positioned objects in this loop should have the same repaint
container, right? Can we just call containerForRepaint once outside the loop?

[ojan, 2014/05/10 18:02:31]

Hmmm. I guess the repaint container would be different for different position
values. In either case, we'd only need to call conatinerForRepaint once per
position value we encounter.

+            (*it)->repaintTreeAfterLayout(*repaintContainerForChild);
+        }
     }
 }
 
 RenderBlock* RenderBlock::continuationBefore(RenderObject* beforeChild)
 {
     if (beforeChild && beforeChild->parent() == this)
         return this;
 
     RenderBlock* curr = toRenderBlock(continuation());
     RenderBlock* nextToLast = this;
@@ skipping 4590 matching lines @@
 void RenderBlock::showLineTreeAndMark(const InlineBox* markedBox1, const char* markedLabel1, const InlineBox* markedBox2, const char* markedLabel2, const RenderObject* obj) const
 {
     showRenderObject();
     for (const RootInlineBox* root = firstRootBox(); root; root = root->nextRootBox())
         root->showLineTreeAndMark(markedBox1, markedLabel1, markedBox2, markedLabel2, obj, 1);
 }
 
 #endif
 
 } // namespace WebCore