Conditionally create PaintLayer's scrollable area object

third_party/WebKit/Source/core/layout/LayoutBlock.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 269 matching lines @@
         textAutosizer->record(this);
 
     propagateStyleToAnonymousChildren(true);
 
     // It's possible for our border/padding to change, but for the overall logical width or height of the block to
     // end up being the same. We keep track of this change so in layoutBlock, we can know to set relayoutChildren=true.
     m_widthAvailableToChildrenChanged |= oldStyle && diff.needsFullLayout() && needsLayout() && borderOrPaddingLogicalDimensionChanged(*oldStyle, newStyle, LogicalWidth);
     m_heightAvailableToChildrenChanged |= oldStyle && diff.needsFullLayout() && needsLayout() && borderOrPaddingLogicalDimensionChanged(*oldStyle, newStyle, LogicalHeight);
 }
 
+void LayoutBlock::updateFromStyle()
+{
+    LayoutBox::updateFromStyle();
+
+    bool shouldClipOverflow = !styleRef().isOverflowVisible() && allowsOverflowClip();

[Xianzhu, 2016/02/25 19:57:51]

Moved from LayoutBox::updateFromStyle().

+    if (shouldClipOverflow != hasOverflowClip()) {
+        // FIXME: This shouldn't be required if we tracked the visual overflow
+        // generated by positioned children or self painting layers. crbug.com/345403
+        for (LayoutObject* child = firstChild(); child; child = child->nextSibling())
+            child->setMayNeedPaintInvalidation();
+    }
+    setHasOverflowClip(shouldClipOverflow);
+}
+
+bool LayoutBlock::allowsOverflowClip() const
+{
+    bool rootLayerScrolls = document().settings() && document().settings()->rootLayerScrolls();
+    return (rootLayerScrolls || !isLayoutView())
+        // If overflow has been propagated to the viewport, it has no effect here.
+        && node() != document().viewportDefiningElement();

[chrishtr, 2016/02/26 19:42:16]

It seems it would be slightly faster to write:

if (node() == document().viewportDefiningElement()
  return false;

if (!isLayoutView())
  return true;

return document().settings() && document().settings()->rootLayerScrolls()

Since this happens deep in style it seems worth it to optimize.

[Xianzhu, 2016/02/26 20:12:57]

Separated out root scroll logic into LayoutView::allowsOverflowClip() so
LayoutBlock::allowsOverflowClip() becomes:

   return node() != document().viewportDefiningElement();

+}
+
 void LayoutBlock::invalidatePaintOfSubtreesIfNeeded(PaintInvalidationState& childPaintInvalidationState)
 {
     LayoutBox::invalidatePaintOfSubtreesIfNeeded(childPaintInvalidationState);
 
     // Take care of positioned objects. This is required as PaintInvalidationState keeps a single clip rect.
     if (TrackedLayoutBoxListHashSet* positionedObjects = this->positionedObjects()) {
         for (auto* box : *positionedObjects) {
 
             // One of the layoutObjects we're skipping over here may be the child's paint invalidation container,
             // so we can't pass our own paint invalidation container along.
@@ skipping 2562 matching lines @@
 void LayoutBlock::showLineTreeAndMark(const InlineBox* markedBox1, const char* markedLabel1, const InlineBox* markedBox2, const char* markedLabel2, const LayoutObject* obj) const
 {
     showLayoutObject();
     for (const RootInlineBox* root = firstRootBox(); root; root = root->nextRootBox())
         root->showLineTreeAndMark(markedBox1, markedLabel1, markedBox2, markedLabel2, obj, 1);
 }
 
 #endif
 
 } // namespace blink