[CSS Blending] Fix blending when mix-blend-mode is set from javascript

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 515 matching lines @@
 {
     if (!RuntimeEnabledFeatures::cssCompositingEnabled())
         return;
 
     bool hadBlendMode = m_blendMode != BlendModeNormal;
     BlendMode newBlendMode = renderer()->style()->blendMode();
     if (newBlendMode != m_blendMode) {
         m_blendMode = newBlendMode;
 
         // Only update the flag if a blend mode is set or unset.
-        if (!hadBlendMode || !hasBlendMode())
-            dirtyAncestorChainBlendedDescendantStatus();
+        if (parent() && (!hadBlendMode || !hasBlendMode()))

[Stephen Chennney, 2013/11/27 13:06:26]

My only concern is that, without a parent, should you still be executing the
former code (that is, calling dirtyAncestor... on this)?

[rosca, 2013/11/27 13:34:08]

If we change the blendMode for an element we should mark as dirty
m_childLayerHasBlendMode for all its ancestors up to the first stacking context.
If it has no ancestor, we'll call setAncestorChainBlendedDescendant [1] at the
time  this element gets added as a child, so that the new ancestors know that
they have a child with blendMode.
The former code was dirtying m_childLayerHasBlendMode for the blending element
itself (B), so it would have recalculated the blending state of B's descendants,
which didn't change actually.

[1]
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit...

+            parent()->dirtyAncestorChainBlendedDescendantStatus();
 
         if (hasCompositedLayerMapping())
             compositedLayerMapping()->setBlendMode(newBlendMode);
     }
 }
 
 void RenderLayer::updateTransform()
 {
     // hasTransform() on the renderer is also true when there is transform-style: preserve-3d or perspective set,
     // so check style too.
@@ skipping 306 matching lines @@
             if (m_hasVisibleDescendant && m_hasSelfPaintingLayerDescendant && hasOutOfFlowPositionedDescendant)
                 break;
         }
 
         m_visibleDescendantStatusDirty = false;
         m_hasSelfPaintingLayerDescendantDirty = false;
         m_hasOutOfFlowPositionedDescendantDirty = false;
     }
 
     if (m_childLayerHasBlendModeStatusDirty) {
+        m_childLayerHasBlendMode = false;
         for (RenderLayer* child = firstChild(); child; child = child->nextSibling()) {
             if (!child->stackingNode()->isStackingContext())
                 child->updateDescendantDependentFlags();
 
             bool childLayerHasBlendMode = child->paintsWithBlendMode() || (child->m_childLayerHasBlendMode && !child->stackingNode()->isStackingContext());
             m_childLayerHasBlendMode |= childLayerHasBlendMode;
 
             if (m_childLayerHasBlendMode)
                 break;
         }
@@ skipping 3287 matching lines @@
     }
 }
 
 void showLayerTree(const WebCore::RenderObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif