Clip scrollbars to box bounds when they don't fit.

third_party/WebKit/Source/core/layout/compositing/GraphicsLayerTreeBuilder.cpp

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 79 matching lines @@
             infoForChildren.childLayersOfEnclosingCompositedLayer->append(currentCompositedLayerMapping->foregroundLayer());
     }
 
     PaintLayerStackingNodeIterator iterator(*layer.stackingNode(), NormalFlowChildren | PositiveZOrderChildren);
     while (PaintLayerStackingNode* curNode = iterator.next())
         rebuild(*curNode->layer(), infoForChildren);
 
     if (hasCompositedLayerMapping) {
         bool parented = false;
         if (layer.layoutObject()->isLayoutPart())
             parented = PaintLayerCompositor::parentFrameContentLayers(toLayoutPart(layer.layoutObject()));

[chrishtr, 2015/11/20 01:20:42]

So you figured out what this function does? Could you rename it to some thing
clearer and
remove the FIXME next to its definition?

[skobes, 2015/11/20 01:35:21]

Done.

(It's less confusing when you realize "parent" was meant as a verb.)

 
         if (!parented)
-            currentCompositedLayerMapping->parentForSublayers()->setChildren(layerChildren);
-
-        // If the layer has a clipping layer the overflow controls layers will be siblings of the clipping layer.
-        // Otherwise, the overflow control layers are normal children.
-        // FIXME: Why isn't this handled in CLM updateInternalHierarchy?
-        if (!currentCompositedLayerMapping->hasClippingLayer() && !currentCompositedLayerMapping->hasScrollingLayer()) {
-            if (GraphicsLayer* overflowControlLayer = currentCompositedLayerMapping->layerForHorizontalScrollbar()) {
-                overflowControlLayer->removeFromParent();
-                currentCompositedLayerMapping->parentForSublayers()->addChild(overflowControlLayer);
-            }
-
-            if (GraphicsLayer* overflowControlLayer = currentCompositedLayerMapping->layerForVerticalScrollbar()) {
-                overflowControlLayer->removeFromParent();
-                currentCompositedLayerMapping->parentForSublayers()->addChild(overflowControlLayer);
-            }
-
-            if (GraphicsLayer* overflowControlLayer = currentCompositedLayerMapping->layerForScrollCorner()) {
-                overflowControlLayer->removeFromParent();
-                currentCompositedLayerMapping->parentForSublayers()->addChild(overflowControlLayer);
-            }
-        }
+            currentCompositedLayerMapping->setSublayers(layerChildren);

[chrishtr, 2015/11/20 01:20:42]

How does this restore the various graphics layers that used to be handled in
lines 105-123? IOW why is layerChildren the right set of layers? A little
confused by that.

[skobes, 2015/11/20 01:35:21]

They aren't in layerChildren.  The relevant code is inside
CompositedLayerMapping::setSublayers:

    // If we have scrollbars, but are not using composited scrolling, then
parentForSublayers may return m_graphicsLayer.
    // In that case, the above call to setChildren has clobbered the overflow
controls host layer, so we need to reattach it.
    if (needsOverflowControlsReattached)
        parent->addChild(overflowControlsContainer);

The "various graphics layers" you refer to are the children of
overflowControlsContainer.

 
         if (shouldAppendLayer(layer))
             info.childLayersOfEnclosingCompositedLayer->append(currentCompositedLayerMapping->childForSuperlayers());
     }
 
     if (layer.scrollParent()
         && layer.scrollParent()->hasCompositedLayerMapping()
         && layer.scrollParent()->compositedLayerMapping()->needsToReparentOverflowControls()
         && layer.scrollParent()->scrollableArea()->topmostScrollChild() == &layer)
         info.childLayersOfEnclosingCompositedLayer->append(layer.scrollParent()->compositedLayerMapping()->detachLayerForOverflowControls(*info.enclosingCompositedLayer));
 }
 
 }