Conditionally create PaintLayer's scrollable area object

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

Index: third_party/WebKit/Source/core/layout/LayoutBlock.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBlock.cpp b/third_party/WebKit/Source/core/layout/LayoutBlock.cpp
index 9793a7e6714b61b6a6e07379afc26ae23cf00e83..597235f810dec3ea1ef63ce0f4f632b19a60f45a 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBlock.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBlock.cpp
@@ -287,6 +287,28 @@ void LayoutBlock::styleDidChange(StyleDifference diff, const ComputedStyle* oldS
     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);