Clear absolute clip rects when transform changes

Source/core/rendering/RenderLayer.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 486 matching lines @@
 
     bool hadTransform = m_transform;
     if (hasTransform != hadTransform) {
         if (hasTransform)
             m_transform = adoptPtr(new TransformationMatrix);
         else
             m_transform.clear();
 
         // Layers with transforms act as clip rects roots, so clear the cached clip rects here.
         m_clipper.clearClipRectsIncludingDescendants();
+    } else if (hasTransform) {
+        m_clipper.clearClipRectsIncludingDescendants(AbsoluteClipRects);

[Ian Vollick, 2014/06/17 15:57:28]

Doesn't this mean that even if we call updateTransform, and the transform
doesn't change, we'll always clear our AbsoluteClipRects? Should we compare the
old and new transforms?

[ajuma, 2014/06/17 16:01:50]

The caller of updateTransform (RenderLayer::styleChanged) already checks if the
old and new transforms are the same, and only calls updateTransform when they're
not.

[Ian Vollick, 2014/06/17 16:07:25]

Ah, thanks. In that case, maybe we could add an ASSERT here just to make sure
that remains true.

[ajuma, 2014/06/17 17:10:40]

Adding an ASSERT is a good idea, but we don't have access to the old style here,
just the new one. We also can't compare m_transform before and after the
applyTransform below since the transform (expressed as TransformOperations)
could change without changing the resulting matrix (also, having to copy the old
m_transform just for the sake of an ASSERT doesn't seem great).

     }
 
     if (hasTransform) {
         RenderBox* box = renderBox();
         ASSERT(box);
         m_transform->makeIdentity();
         box->style()->applyTransform(*m_transform, box->pixelSnappedBorderBoxRect().size(), RenderStyle::IncludeTransformOrigin);
         makeMatrixRenderable(*m_transform, compositor()->hasAcceleratedCompositing());
     }
 
@@ skipping 3347 matching lines @@
     }
 }
 
 void showLayerTree(const WebCore::RenderObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif