Disable clipping on root scroller's ancestors.

third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.h

Index: third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.h
diff --git a/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.h b/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.h
index fbe3af31fe120941880ea97fa1b543c2c864fc25..16f3ec2e106406975e93120a4ad0e513de3c93f1 100644
--- a/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.h
+++ b/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.h
@@ -133,6 +133,12 @@ public:
     void updateRootLayerAttachment();
     void updateRootLayerPosition();
 
+    // If the root scroller isn't the root layer then the PaintLayerCompositor
+    // must disable clipping on its layers so that the root scroller can
+    // expand/shrink its clipping layer in response to top controls and have
+    // the result be visible.
+    void setClippingOnRootScrollerAncestors();

[chrishtr, 2016/09/01 23:42:57]

Name it disableClippingOnRootScrollerAncestors instead?

[bokan, 2016/09/09 00:19:02]

It can be used to enable clipping as well if the root scroller is moved. It's
also now used to set clipping to the default values instead of in layer creation
(see reply to your comment in this function's implementation). I've renamed to
`updateClippingOnCompositorLayers` which I think describes it better.

On a side note, do we make a distinction between `masking` and `clipping`? I've
been using the term interchangeably but don't know if that's true.

[chrishtr, 2016/09/09 18:21:15]

Masking and clipping are kind of the same thing. Most people say "mask" when
they mean something other than a rectangular clip due to overflow:<hidden or
scroll> or their equivalent for frames. In the cases in this CL it's mostly the
latter...

+
     void setIsInWindow(bool);
 
     static PaintLayerCompositor* frameContentsCompositor(LayoutPart*);