Make RootScroller set the outer viewport scroll layer in the compositor

third_party/WebKit/Source/core/page/scrolling/RootScrollerController.cpp

Index: third_party/WebKit/Source/core/page/scrolling/RootScrollerController.cpp
diff --git a/third_party/WebKit/Source/core/page/scrolling/RootScrollerController.cpp b/third_party/WebKit/Source/core/page/scrolling/RootScrollerController.cpp
index 3822b25a1edc66b69aaa002e0c44ec1a3e61c19e..ef09315de54f4a6743a688fbee2f27d38514884f 100644
--- a/third_party/WebKit/Source/core/page/scrolling/RootScrollerController.cpp
+++ b/third_party/WebKit/Source/core/page/scrolling/RootScrollerController.cpp
@@ -9,11 +9,14 @@
 #include "core/frame/FrameHost.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/TopControls.h"
+#include "core/frame/VisualViewport.h"
 #include "core/layout/LayoutBox.h"
+#include "core/page/ChromeClient.h"
 #include "core/page/Page.h"
 #include "core/page/scrolling/OverscrollController.h"
 #include "core/page/scrolling/ViewportScrollCallback.h"
 #include "core/paint/PaintLayerScrollableArea.h"
+#include "platform/graphics/GraphicsLayer.h"
 #include "platform/scroll/ScrollableArea.h"
 
 namespace blink {
@@ -82,6 +85,7 @@ ViewportScrollCallback* RootScrollerController::createViewportApplyScroll(
 RootScrollerController::RootScrollerController(Document& document, ViewportScrollCallback* applyScrollCallback)
     : m_document(&document)
     , m_viewportApplyScroll(applyScrollCallback)
+    , m_changedSinceLastCompositingUpdate(false)
 {
 }
 
@@ -128,6 +132,8 @@ void RootScrollerController::updateEffectiveRootScroller()
 
     moveViewportApplyScroll(newEffectiveRootScroller);
     m_effectiveRootScroller = newEffectiveRootScroller;
+
+    m_changedSinceLastCompositingUpdate = true;
 }
 
 void RootScrollerController::moveViewportApplyScroll(Element* target)
@@ -156,6 +162,44 @@ void RootScrollerController::moveViewportApplyScroll(Element* target)
     m_viewportApplyScroll->setScroller(targetScroller);
 }
 
+void RootScrollerController::didUpdateCompositing()
+{
+    if (!m_changedSinceLastCompositingUpdate
+        || !m_document
+        || !m_document->frameHost()
+        || !m_effectiveRootScroller)
+        return;
+
+    FrameHost& frameHost = *m_document->frameHost();
+
+    WebLayer* newRootScrollerLayer = nullptr;
+
+    if (m_effectiveRootScroller->layoutObject()
+        && m_effectiveRootScroller->layoutObject()->isBox()) {
+
+        LayoutBox* box = toLayoutBox(m_effectiveRootScroller->layoutObject());
+        GraphicsLayer* graphicsLayer = nullptr;
+
+        if (box->isDocumentElement())
+            graphicsLayer = frameHost.visualViewport().layerForScrolling();
+        else
+            graphicsLayer = box->getScrollableArea()->layerForScrolling();
+
+        // TODO(bokan): We should assert graphicsLayer here and
+        // RootScrollerController should do whatever needs to happen to ensure
+        // the root scroller gets composited.
+
+        if (graphicsLayer)
+            newRootScrollerLayer = graphicsLayer->platformLayer();
+    }
+
+    m_changedSinceLastCompositingUpdate = false;
+
+    // Let the compositor-side counterpart know about this change.
+    if (m_document->isInMainFrame())
+        frameHost.chromeClient().didUpdateRootScrollerLayer(newRootScrollerLayer);
+}
+
 Element* RootScrollerController::defaultEffectiveRootScroller()
 {
     DCHECK(m_document);