Disable clipping on root scroller's ancestors.

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/page/scrolling/TopDocumentRootScrollerController.h"
 
 #include "core/dom/Document.h"
 #include "core/dom/Element.h"
 #include "core/frame/FrameHost.h"
 #include "core/frame/FrameView.h"
 #include "core/html/HTMLFrameOwnerElement.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 {
 
 // static
 TopDocumentRootScrollerController* TopDocumentRootScrollerController::create(
     Document& document)
 {
     return new TopDocumentRootScrollerController(document);
 }
@@ skipping 50 matching lines @@
         return;
 
     if (m_globalRootScroller)
         m_globalRootScroller->removeApplyScroll();
 
     // Use disable-native-scroll since the ViewportScrollCallback needs to
     // apply scroll actions both before (TopControls) and after (overscroll)
     // 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
     // ViewportScrollCallback to swap the target into the layout viewport
     // in RootFrameViewport.
     m_viewportApplyScroll->setScroller(targetScroller);
 }
 
+void TopDocumentRootScrollerController
+    ::setNeedsCompositingInputsUpdateOnGlobalRootScroller()
+{
+    if (!m_globalRootScroller)
+        return;
+
+    PaintLayer* layer = m_globalRootScroller->document()
+        .rootScrollerController()->rootScrollerPaintLayer();
+
+    if (layer)
+        layer->setNeedsCompositingInputsUpdate();

[chrishtr, 2016/09/14 17:16:41]

I think you're going to need

compositor()->setNeedsCompositingUpdate(CompositingUpdateRebuildTree);

because the masking etc needs to be updated, right?

[bokan, 2016/09/14 17:25:46]

Ah, yes, you're right, thanks for catching that. Done.

+}
+
 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();
 }
 
 void TopDocumentRootScrollerController::didAttachDocument()
 {
     FrameHost* frameHost = m_document->frameHost();
     FrameView* frameView = m_document->view();
 
     if (!frameHost || !frameView)
         return;
@@ skipping 30 matching lines @@
 
     GraphicsLayer* graphicsLayer = area->layerForScrolling();
 
     // TODO(bokan): We should assert graphicsLayer here and
     // RootScrollerController should do whatever needs to happen to ensure
     // the root scroller gets composited.
 
     return graphicsLayer;
 }
 
+Element* TopDocumentRootScrollerController::globalRootScroller() const
+{
+    return m_globalRootScroller.get();
+}
+
 } // namespace blink