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..22ae23f29af89eca8e1efb0d933e283fbaee5329 100644
--- a/third_party/WebKit/Source/core/page/scrolling/TopDocumentRootScrollerController.cpp
+++ b/third_party/WebKit/Source/core/page/scrolling/TopDocumentRootScrollerController.cpp
@@ -12,6 +12,7 @@
 #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 +83,19 @@ 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 +104,25 @@ 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();

[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();
 }
 
@@ -148,4 +173,9 @@ GraphicsLayer* TopDocumentRootScrollerController::rootScrollerLayer()
     return graphicsLayer;
 }
 
+Element* TopDocumentRootScrollerController::globalRootScroller() const
+{
+    return m_globalRootScroller.get();
+}
+
 } // namespace blink