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

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

Index: third_party/WebKit/Source/core/layout/compositing/GraphicsLayerTreeBuilder.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/GraphicsLayerTreeBuilder.cpp b/third_party/WebKit/Source/core/layout/compositing/GraphicsLayerTreeBuilder.cpp
index 4f2c16d8db14990328fa04fd28dc44dfda081ac8..c4cc864a78511a741494e7158a8036b5dd124ed9 100644
--- a/third_party/WebKit/Source/core/layout/compositing/GraphicsLayerTreeBuilder.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/GraphicsLayerTreeBuilder.cpp
@@ -100,27 +100,7 @@ void GraphicsLayerTreeBuilder::rebuild(PaintLayer& layer, AncestorInfo info)
             parented = PaintLayerCompositor::parentFrameContentLayers(toLayoutPart(layer.layoutObject()));
 
         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()) {

[chrishtr, 2015/11/19 17:29:52]

Where has this logic gone?

[skobes, 2015/11/19 18:30:07]

This code was restoring the overflow controls in the case where they were
disconnected from the tree by parentForSublayers()->setChildren or by
parentFrameContentLayers.  But it did so by separating them from
m_overflowControlsHostLayer, which is no longer safe.

The overflow controls are now restored by CompositedLayerMapping::setSublayers,
which keeps them under m_overflowControlsHostLayer.

-            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);
 
         if (shouldAppendLayer(layer))
             info.childLayersOfEnclosingCompositedLayer->append(currentCompositedLayerMapping->childForSuperlayers());