Add a UseCounter for clip CSS or -webkit-clip-path that clips positioned descendants.

Source/core/paint/DeprecatedPaintLayerClipper.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 47 matching lines @@
     if (position == FixedPosition) {
         clipRects.setPosClipRect(clipRects.fixedClipRect());
         clipRects.setOverflowClipRect(clipRects.fixedClipRect());
         clipRects.setFixed(true);
     } else if (position == RelativePosition) {
         clipRects.setPosClipRect(clipRects.overflowClipRect());
     } else if (position == AbsolutePosition) {
         clipRects.setOverflowClipRect(clipRects.posClipRect());
     }
 }
+
 static void applyClipRects(const ClipRectsContext& context, LayoutObject& layoutObject, LayoutPoint offset, ClipRects& clipRects)
 {
     ASSERT(layoutObject.hasOverflowClip() || layoutObject.hasClip());
     LayoutView* view = layoutObject.view();
     ASSERT(view);
     if (clipRects.fixed() && context.rootLayer->layoutObject() == view)
         offset -= toIntSize(view->frameView()->scrollPosition());
     if (layoutObject.hasOverflowClip()) {
         ClipRect newOverflowClip = toLayoutBox(layoutObject).overflowClipRect(offset, context.scrollbarRelevancy);
         newOverflowClip.setHasRadius(layoutObject.style()->hasBorderRadius());
         clipRects.setOverflowClipRect(intersection(newOverflowClip, clipRects.overflowClipRect()));
         if (layoutObject.isPositioned())
             clipRects.setPosClipRect(intersection(newOverflowClip, clipRects.posClipRect()));
         if (layoutObject.isLayoutView())
             clipRects.setFixedClipRect(intersection(newOverflowClip, clipRects.fixedClipRect()));
     }
     if (layoutObject.hasClip()) {
         LayoutRect newClip = toLayoutBox(layoutObject).clipRect(offset);
-        clipRects.setPosClipRect(intersection(newClip, clipRects.posClipRect()));
-        clipRects.setOverflowClipRect(intersection(newClip, clipRects.overflowClipRect()));
-        clipRects.setFixedClipRect(intersection(newClip, clipRects.fixedClipRect()));
+        {
+            const ClipRect& rect& = intersection(newClip, clipRects.posClipRect());

[pdr., 2015/09/05 00:07:48]

One too many ampersands!

[chrishtr, 2015/09/05 00:21:16]

Fixed.

+            rect.setIsClippedByClipCss();
+            clipRects.setPosClipRect(rect);
+        }
+
+        {
+            const ClipRect& rect = intersection(newClip, clipRects.overflowClipRect());
+            rect.setIsClippedByClipCss();
+            clipRects.setOverflowClipRect(rect);
+        }
+
+        {
+            const ClipRect& rect = intersection(newClip, clipRects.fixedClipRect());
+            rect.setIsClippedByClipCss();
+            clipRects.setFixedClipRect(rect);
+        }
     }
 }
 
 DeprecatedPaintLayerClipper::DeprecatedPaintLayerClipper(LayoutBoxModelObject& layoutObject)
     : m_layoutObject(layoutObject)
 {
 }
 
 ClipRects* DeprecatedPaintLayerClipper::clipRectsIfCached(const ClipRectsContext& context) const
 {
@@ skipping 154 matching lines @@
             if (shouldRespectOverflowClip(context))
                 backgroundRect.intersect(bounds);
         }
     }
 
     // CSS clip (different than clipping due to overflow) can clip to any box, even if it falls outside of the border box.
     if (m_layoutObject.hasClip()) {
         // Clip applies to *us* as well, so go ahead and update the damageRect.
         LayoutRect newPosClip = toLayoutBox(m_layoutObject).clipRect(offset);
         backgroundRect.intersect(newPosClip);
+        backgroundRect.setIsClippedByClipCss();
         foregroundRect.intersect(newPosClip);
+        foregroundRect.setIsClippedByClipCss();
         outlineRect.intersect(newPosClip);
+        outlineRect.setIsClippedByClipCss();
     }
 }
 
 void DeprecatedPaintLayerClipper::calculateClipRects(const ClipRectsContext& context, ClipRects& clipRects) const
 {
     bool rootLayerScrolls = m_layoutObject.document().settings() && m_layoutObject.document().settings()->rootLayerScrolls();
     if (!m_layoutObject.layer()->parent() && !rootLayerScrolls) {
         // The root layer's clip rect is always infinite.
         clipRects.reset(LayoutRect(LayoutRect::infiniteIntRect()));
         return;
@@ skipping 98 matching lines @@
     if (context.respectOverflowClip == IgnoreOverflowClip)
         return false;
 
     if (layer->isRootLayer() && context.respectOverflowClipForViewport == IgnoreOverflowClip)
         return false;
 
     return true;
 }
 
 } // namespace blink