Disable clipping on root scroller's ancestors.

third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp

Index: third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp b/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
index 6ec56ae219e45641cc2236ae067ceed775daf0b3..57c10fd8d9f9c884360f031eda8b1ea6695ecb49 100644
--- a/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
@@ -51,6 +51,7 @@
 #include "core/loader/FrameLoaderClient.h"
 #include "core/page/ChromeClient.h"
 #include "core/page/Page.h"
+#include "core/page/scrolling/RootScrollerController.h"
 #include "core/page/scrolling/ScrollingCoordinator.h"
 #include "core/paint/FramePainter.h"
 #include "core/paint/ObjectPaintInvalidator.h"
@@ -252,6 +253,7 @@ void PaintLayerCompositor::updateIfNeededRecursiveInternal()
             scrollableArea->updateCompositorScrollAnimations();
     }
 
+    m_layoutView.document().rootScrollerController()->didUpdateCompositing();
 #if ENABLE(ASSERT)
     ASSERT(lifecycle().state() == DocumentLifecycle::CompositingClean);
     assertNoUnresolvedDirtyBits();
@@ -419,6 +421,8 @@ void PaintLayerCompositor::updateIfNeeded()
             }
         }
 
+        setClippingOnRootScrollerAncestors();
+
         GraphicsLayerUpdater updater;
         updater.update(*updateRoot, layersNeedingPaintInvalidation);
 
@@ -461,6 +465,28 @@ void PaintLayerCompositor::updateIfNeeded()
         InspectorInstrumentation::layerTreeDidChange(m_layoutView.frame());
 }
 
+void PaintLayerCompositor::setClippingOnRootScrollerAncestors()
+{
+    bool shouldClip = !rootLayer()->hasRootScrollerAsDescendant();
+    if (m_rootContentLayer)
+        m_rootContentLayer->setMasksToBounds(shouldClip);
+
+    RootScrollerController* topRootScrollerController =
+        m_layoutView.document().topDocument().rootScrollerController();
+    bool frameIsRootScroller =
+        topRootScrollerController->rootScrollerLayer() == scrollLayer();
+
+    // We normally clip iframes' (but not the root frame) overflow controls
+    // host and container layers but if the root scroller is the iframe itself
+    // we want it to behave like the root frame.
+    shouldClip &= !frameIsRootScroller && !m_layoutView.frame()->isLocalRoot();
+
+    if (m_containerLayer)
+        m_containerLayer->setMasksToBounds(shouldClip);

[chrishtr, 2016/09/01 23:42:57]

Could you try moving these to the sites where we call setMasksToBounds on each
of these layers, and
generalize the logic there?

[bokan, 2016/09/09 00:19:02]

Currently, setMasksToBounds gets called only when these layers get created but
this needs to get called from compositing updates, so I don't think I can move
this logic there. On the flip side, I think I can just remove the
setMasksToBounds calls in the layer creation code (in ensureRootLayer) and rely
on this code. The latest patch does this. 

+    if (m_overflowControlsHostLayer)
+        m_overflowControlsHostLayer->setMasksToBounds(shouldClip);
+}
+
 static void restartAnimationOnCompositor(const LayoutObject& layoutObject)
 {
     Node* node = layoutObject.node();