Make RootScroller set the outer viewport scroll layer in the compositor

third_party/WebKit/Source/web/WebViewImpl.cpp

Index: third_party/WebKit/Source/web/WebViewImpl.cpp
diff --git a/third_party/WebKit/Source/web/WebViewImpl.cpp b/third_party/WebKit/Source/web/WebViewImpl.cpp
index 0003e29d42b451d8872648ca81a9b81b86e07a54..a993bf9a02b17686766e814eccee2ef48f6842b4 100644
--- a/third_party/WebKit/Source/web/WebViewImpl.cpp
+++ b/third_party/WebKit/Source/web/WebViewImpl.cpp
@@ -1872,6 +1872,17 @@ void WebViewImpl::didUpdateTopControls()
     }
 }
 
+void WebViewImpl::didUpdateRootScrollerLayer(WebLayer* rootScrollerLayer)
+{
+    if (!m_layerTreeView)
+        return;
+
+    if (!page()->mainFrame() || !page()->mainFrame()->isLocalFrame())

[dcheng, 2016/06/29 22:34:29]

I'm out of the loop on the root scroller work: is it possible to have a root
scroller in a non-main frame?

[bokan, 2016/06/29 22:37:02]

It is in the sense that an iframe can set document.rootScroller so the machinery
is still there to handle that but it has no visible effect right now so I chose
to simply not tell the compositor about it for remote frames.

+        return;
+
+    m_layerTreeView->setRootScrollerLayer(rootScrollerLayer);
+}
+
 TopControls& WebViewImpl::topControls()
 {
     return page()->frameHost().topControls();