Remove clips where we can show that they are unnecessary.

Source/core/rendering/RenderBox.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2006, 2007 Apple Inc. 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
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 22 matching lines @@
 class RenderBoxRegionInfo;
 class RenderRegion;
 struct PaintInfo;
 
 enum SizeType { MainOrPreferredSize, MinSize, MaxSize };
 enum AvailableLogicalHeightType { ExcludeMarginBorderPadding, IncludeMarginBorderPadding };
 enum OverlayScrollbarSizeRelevancy { IgnoreOverlayScrollbarSize, IncludeOverlayScrollbarSize };
 
 enum ShouldComputePreferred { ComputeActual, ComputePreferred };
 
+enum ContentsClipBehavior { ContentsClipRequired, ContentsClipAutomatic };

[Julien - ping for review, 2013/08/08 21:36:50]

Those names could use some improvement: "automatic" is a very undefined word.

Here are some better names:
* ForceContentsClip, SkipContentClipIfPossible
* ContentsClipForced, ContentClipMayBeIgnored

[jbroman, 2013/08/09 15:34:35]

Took your first suggestion.

+
 class RenderBox : public RenderBoxModelObject {
 public:
     explicit RenderBox(ContainerNode*);
     virtual ~RenderBox();
 
     // hasAutoZIndex only returns true if the element is positioned or a flex-item since
     // position:static elements that are not flex-items get their z-index coerced to auto.
     virtual bool requiresLayer() const OVERRIDE { return isRoot() || isPositioned() || createsGroup() || hasClipPath() || hasOverflowClip() || hasTransform() || hasHiddenBackface() || hasReflection() || style()->specifiesColumns() || !style()->hasAutoZIndex() || isFloatingWithShapeOutside(); }
 
     virtual bool backgroundIsKnownToBeOpaqueInRect(const LayoutRect& localRect) const OVERRIDE;
@@ skipping 126 matching lines @@
     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;
 
+    LayoutRect contentsVisualOverflowRect() const { return m_overflow ? m_overflow->contentsVisualOverflowRect() : LayoutRect(); }
+
     void addLayoutOverflow(const LayoutRect&);
     void addVisualOverflow(const LayoutRect&);
 
+    // Clipped by the contents clip, if one exists.
+    void addContentsVisualOverflow(const LayoutRect&);
+
     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(); }
     LayoutUnit contentHeight() const { return clientHeight() - paddingTop() - paddingBottom(); }
     LayoutUnit contentLogicalWidth() const { return style()->isHorizontalWritingMode() ? contentWidth() : contentHeight(); }
@@ skipping 261 matching lines @@
 
     bool hasUnsplittableScrollingOverflow() const;
     bool isUnsplittableForPagination() const;
 
     virtual LayoutRect localCaretRect(InlineBox*, int caretOffset, LayoutUnit* extraWidthToEndOfLine = 0);
 
     virtual LayoutRect overflowClipRect(const LayoutPoint& location, RenderRegion*, OverlayScrollbarSizeRelevancy = IgnoreOverlayScrollbarSize);
     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, ContentsClipBehavior = ContentsClipRequired);

[Julien - ping for review, 2013/08/08 21:36:50]

There are 3 callers, let's avoid the default parameter.

[jbroman, 2013/08/09 15:34:35]

Done.

     void popContentsClip(PaintInfo&, PaintPhase originalPhase, const LayoutPoint& accumulatedOffset);
 
     virtual void paintObject(PaintInfo&, const LayoutPoint&) { ASSERT_NOT_REACHED(); }
     virtual void paintBoxDecorations(PaintInfo&, const LayoutPoint&);
     virtual void paintMask(PaintInfo&, const LayoutPoint&);
     virtual void imageChanged(WrappedImagePtr, const IntRect* = 0);
 
     // Called when a positioned object moves but doesn't necessarily change size.  A simplified layout is attempted
     // that just updates the object's position. If the size does change, the object remains dirty.
     bool tryLayoutDoingPositionedMovementOnly()
@@ skipping 259 matching lines @@
         if (UNLIKELY(m_inlineBoxWrapper != 0))
             deleteLineBoxWrapper();
     }
 
     m_inlineBoxWrapper = boxWrapper;
 }
 
 } // namespace WebCore
 
 #endif // RenderBox_h