Remove clips where we can show that they are unnecessary.

Source/core/rendering/RenderBox.h

Index: Source/core/rendering/RenderBox.h
diff --git a/Source/core/rendering/RenderBox.h b/Source/core/rendering/RenderBox.h
index 045cde10a1f382dfa7d6a3234703a2d95442e9f5..c17b9d39799065837f9343d6ec2d1352689cb356 100644
--- a/Source/core/rendering/RenderBox.h
+++ b/Source/core/rendering/RenderBox.h
@@ -178,21 +178,23 @@ public:
     LayoutSize maxLayoutOverflow() const { return LayoutSize(layoutOverflowRect().maxX(), layoutOverflowRect().maxY()); }
     LayoutUnit logicalLeftLayoutOverflow() const { return style()->isHorizontalWritingMode() ? layoutOverflowRect().x() : layoutOverflowRect().y(); }
     LayoutUnit logicalRightLayoutOverflow() const { return style()->isHorizontalWritingMode() ? layoutOverflowRect().maxX() : layoutOverflowRect().maxY(); }
-    
+
     virtual LayoutRect visualOverflowRect() const { return m_overflow ? m_overflow->visualOverflowRect() : borderBoxRect(); }
     LayoutUnit logicalLeftVisualOverflow() const { return style()->isHorizontalWritingMode() ? visualOverflowRect().x() : visualOverflowRect().y(); }
     LayoutUnit logicalRightVisualOverflow() const { return style()->isHorizontalWritingMode() ? visualOverflowRect().maxX() : visualOverflowRect().maxY(); }
 
     LayoutRect overflowRectForPaintRejection() const;
-    
+
     void addLayoutOverflow(const LayoutRect&);
-    void addVisualOverflow(const LayoutRect&);
-    
+
+    enum VisualOverflowClipBehavior { VisualOverflowNotClipped, VisualOverflowClippedByContentsClip };
+    virtual void addVisualOverflow(const LayoutRect&, VisualOverflowClipBehavior);

[Julien - ping for review, 2013/06/19 19:42:58]

Making this virtual seems to go in the wrong direction. This is required mainly
because you are storing the new rect in RenderBlock where any RenderBox can have
an overflow clip but not all are blocks (e.g. RenderListMarkers).

[jbroman, 2013/08/02 14:28:17]

I don't see how any other kind of box can have an overflow clip (certainly I
can't think of a way to apply an overflow clip to a list marker).

In particular, the only implementation of hasOverflowClip() is in RenderObject,
which queries the bit fields. The only invocation of setHasOverflowClip where
the argument can be true is in RenderBox::updateFromStyle, where it is guarded
by the condition isRenderBlock().

https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit...

Equivalently, the eponymous "overflow" CSS property applies only to block
container elements (which always have renderers which subclass RenderBlock,
AFAICT).

http://www.w3.org/TR/CSS2/visufx.html#overflow

I did originally have the rect in RenderBox, but it was only used for
RenderBlocks, so that seemed silly to penalize ALL boxes everywhere with the
size (including, as you note, list markers, as well as blocks with no overflow
clip).

+
     void addVisualEffectOverflow();
     void addOverflowFromChild(RenderBox* child) { addOverflowFromChild(child, child->locationOffset()); }
     void addOverflowFromChild(RenderBox* child, const LayoutSize& delta);
     void clearLayoutOverflow();
-    
+
     void updateLayerTransform();
 
     LayoutUnit contentWidth() const { return clientWidth() - paddingLeft() - paddingRight(); }
@@ -466,7 +468,7 @@ public:
     LayoutRect clipRect(const LayoutPoint& location, RenderRegion*);
     virtual bool hasControlClip() const { return false; }
     virtual LayoutRect controlClipRect(const LayoutPoint&) const { return LayoutRect(); }
-    bool pushContentsClip(PaintInfo&, const LayoutPoint& accumulatedOffset);
+    bool pushContentsClip(PaintInfo&, const LayoutPoint& accumulatedOffset, const LayoutRect* visualOverflowRect = 0);
     void popContentsClip(PaintInfo&, PaintPhase originalPhase, const LayoutPoint& accumulatedOffset);
 
     virtual void paintObject(PaintInfo&, const LayoutPoint&) { ASSERT_NOT_REACHED(); }