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..8e19b3a046f9c539b1de5ee8e535696a70100d18 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBlock.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBlock.cpp
@@ -287,6 +287,26 @@ 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();
+    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
+{
+    // If overflow has been propagated to the viewport, it has no effect here.
+    return node() != document().viewportDefiningElement();
+}
+
 void LayoutBlock::invalidatePaintOfSubtreesIfNeeded(PaintInvalidationState& childPaintInvalidationState)
 {
     LayoutBox::invalidatePaintOfSubtreesIfNeeded(childPaintInvalidationState);