Create Pinch Virtual Viewport scrollbar layers in Blink.

Source/web/WebViewImpl.cpp

Index: Source/web/WebViewImpl.cpp
diff --git a/Source/web/WebViewImpl.cpp b/Source/web/WebViewImpl.cpp
index 1cdb62d6aa28f411514f57116393205350dad0ec..26a09b16e69f04ac2fb81b06534c395b10842e11 100644
--- a/Source/web/WebViewImpl.cpp
+++ b/Source/web/WebViewImpl.cpp
@@ -31,6 +31,7 @@
 #include "config.h"
 #include "WebViewImpl.h"
 
+#include "core/rendering/RenderLayerCompositor.h"
 #include "public/platform/Platform.h"
 #include "public/platform/WebDragData.h"
 #include "public/platform/WebFloatPoint.h"
@@ -3803,11 +3804,9 @@ void WebViewImpl::setRootGraphicsLayer(GraphicsLayer* layer)
         if (layer) {
             m_rootGraphicsLayer = m_pinchViewports->rootGraphicsLayer();
             m_rootLayer = m_pinchViewports->rootGraphicsLayer()->platformLayer();
-            m_pinchViewports->registerViewportLayersWithTreeView(m_layerTreeView);
         } else {
             m_rootGraphicsLayer = 0;
             m_rootLayer = 0;
-            m_pinchViewports->clearViewportLayersForTreeView(m_layerTreeView);
         }
     } else {
         m_rootGraphicsLayer = layer;
@@ -3817,10 +3816,24 @@ void WebViewImpl::setRootGraphicsLayer(GraphicsLayer* layer)
     setIsAcceleratedCompositingActive(layer);
 
     if (m_layerTreeView) {
-        if (m_rootLayer)
+        if (m_rootLayer) {
             m_layerTreeView->setRootLayer(*m_rootLayer);
-        else
+            // We register viewport layers here since there may not be a layer
+            // tree view prior to this point.
+            if (m_pinchViewports) {
+                m_pinchViewports->registerViewportLayersWithTreeView(m_layerTreeView);
+            } else {
+                GraphicsLayer* rootScrollLayer = compositor()->scrollLayer();
+                rootScrollLayer->platformLayer()->setScrollClipLayer(m_rootLayer);

[enne (OOO), 2013/09/12 18:51:21]

I don't think the root layer is the clip layer for the root scroll layer.  The
clip layer is one of the children, right?  Can you do this where the clip layer
/ root scroll layer is created rather than here?  The same comment applies to
PinchViewports::registerViewportLayersWithTreeView.

(I also think it's a little bit indirect to store the clip layer on a scroll
layer since it's really information for the scrollbars that just happen to be on
that scrollbar.  I think this is workable though; it's just some design
awkwardness, I think.)

[wjmaclean, 2013/09/12 19:01:56]

Oops, you're right ... I'll fix this.
This happens in RenderLayerCompositor, which doesn't (at present) ever seem to
deal with the WebLayer interface ... is it OK to add it there?
This is doable.
 
I thought if I gave the clip layer info to the scroll layer, then the scrollbar
could just accept values computed by the scroll layer, and never have to worry
about where they come from. The scroll layer can do one set of computations for
one or two scrollbars in the same call. I thought this seemed a bit cleaner, but
if it becomes unworkable it can change. :-)

+                m_layerTreeView->registerViewportLayers(m_rootLayer, rootScrollLayer->platformLayer(), 0);
+            }
+        } else {
             m_layerTreeView->clearRootLayer();
+            if (m_pinchViewports)
+                m_pinchViewports->clearViewportLayersForTreeView(m_layerTreeView);
+            else
+                m_layerTreeView->clearViewportLayers();
+        }
     }
 
     suppressInvalidations(false);