Disable clipping on root scroller's ancestors.

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

Index: third_party/WebKit/Source/core/page/scrolling/TopDocumentRootScrollerController.cpp
diff --git a/third_party/WebKit/Source/core/page/scrolling/TopDocumentRootScrollerController.cpp b/third_party/WebKit/Source/core/page/scrolling/TopDocumentRootScrollerController.cpp
index b23bf7cfd2002b6d4170398fc7fb00fdb6675afa..46839e5ca4c819321b0ea0ec8bf04f21c04f0153 100644
--- a/third_party/WebKit/Source/core/page/scrolling/TopDocumentRootScrollerController.cpp
+++ b/third_party/WebKit/Source/core/page/scrolling/TopDocumentRootScrollerController.cpp
@@ -9,9 +9,12 @@
 #include "core/frame/FrameHost.h"
 #include "core/frame/FrameView.h"
 #include "core/html/HTMLFrameOwnerElement.h"
+#include "core/layout/LayoutView.h"
+#include "core/layout/compositing/PaintLayerCompositor.h"
 #include "core/page/ChromeClient.h"
 #include "core/page/scrolling/OverscrollController.h"
 #include "core/page/scrolling/ViewportScrollCallback.h"
+#include "core/paint/PaintLayer.h"
 #include "platform/scroll/ScrollableArea.h"
 
 namespace blink {
@@ -82,8 +85,19 @@ void TopDocumentRootScrollerController::updateGlobalRootScroller()
     // scrolling the element so it will apply scroll to the element itself.
     target->setApplyScroll(m_viewportApplyScroll, "disable-native-scroll");
 
+    // A change in global root scroller requires a compositing inputs update to
+    // the new and old global root scroller since it might change how the
+    // ancestor layers are clipped. e.g. An iframe that's the global root
+    // scroller clips its layers like the root frame.  Normally this is set
+    // when the local effective root scroller changes but the global root
+    // scroller can change because the parent's effective root scroller
+    // changes.
+    setNeedsCompositingInputsUpdateOnGlobalRootScroller();
+
     m_globalRootScroller = target;
 
+    setNeedsCompositingInputsUpdateOnGlobalRootScroller();
+
     // Ideally, scroll customization would pass the current element to scroll to
     // the apply scroll callback but this doesn't happen today so we set it
     // through a back door here. This is also needed by the
@@ -92,12 +106,30 @@ void TopDocumentRootScrollerController::updateGlobalRootScroller()
     m_viewportApplyScroll->setScroller(targetScroller);
 }
 
+void TopDocumentRootScrollerController
+    ::setNeedsCompositingInputsUpdateOnGlobalRootScroller()
+{
+    if (!m_globalRootScroller)
+        return;
+
+    PaintLayer* layer = m_globalRootScroller->document()
+        .rootScrollerController()->rootScrollerPaintLayer();
+
+    if (layer)
+        layer->setNeedsCompositingInputsUpdate();
+
+    if (LayoutView* view = m_globalRootScroller->document().layoutView()) {
+        view->compositor()
+            ->setNeedsCompositingUpdate(CompositingUpdateRebuildTree);
+    }
+}
+
 void TopDocumentRootScrollerController::didUpdateCompositing()
 {
-    FrameHost* frameHost = m_document->frameHost();
+    RootScrollerController::didUpdateCompositing();
 
     // Let the compositor-side counterpart know about this change.
-    if (frameHost)
+    if (FrameHost* frameHost = m_document->frameHost())
         frameHost->chromeClient().registerViewportLayers();
 }
 
@@ -148,4 +180,9 @@ GraphicsLayer* TopDocumentRootScrollerController::rootScrollerLayer()
     return graphicsLayer;
 }
 
+Element* TopDocumentRootScrollerController::globalRootScroller() const
+{
+    return m_globalRootScroller.get();
+}
+
 } // namespace blink