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 63956fde1c12d32cd755584d38f2a55bc3fadd64..8b9914626f65fd36b44d47d3c578db36150ff0e3 100644
--- a/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
@@ -52,6 +52,7 @@
 #include "core/page/ChromeClient.h"
 #include "core/page/Page.h"
 #include "core/page/scrolling/ScrollingCoordinator.h"
+#include "core/page/scrolling/TopDocumentRootScrollerController.h"
 #include "core/paint/FramePainter.h"
 #include "core/paint/ObjectPaintInvalidator.h"
 #include "core/paint/TransformRecorder.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()
             }
         }
 
+        updateClippingOnCompositorLayers();
+
         GraphicsLayerUpdater updater;
         updater.update(*updateRoot, layersNeedingPaintInvalidation);
 
@@ -461,6 +465,37 @@ void PaintLayerCompositor::updateIfNeeded()
         InspectorInstrumentation::layerTreeDidChange(m_layoutView.frame());
 }
 
+void PaintLayerCompositor::updateClippingOnCompositorLayers()

[chrishtr, 2016/09/09 18:21:16]

Ditto for unittesting of this method.

[bokan, 2016/09/10 00:05:54]

I couldn't find unit tests for PaintLayer or PaintLayerCompositor so I added it
to my existing RootScrollerTest unit tests.

+{
+    bool shouldClip = !rootLayer()->hasRootScrollerAsDescendant();
+    if (m_rootContentLayer) {
+        // FIXME: with rootLayerScrolls, we probably don't even need
+        // m_rootContentLayer?
+        m_rootContentLayer->setMasksToBounds(
+            !RuntimeEnabledFeatures::rootLayerScrollingEnabled() && shouldClip);
+    }
+
+    // TODO(bokan): Temporary hack-cast to TDRSC until the follow-up patch
+    // lands and we can get a pointer to a TopDocumentRootScrollerController
+    // legitimately.
+    TopDocumentRootScrollerController* globalRootScrollerController =
+        (TopDocumentRootScrollerController*)m_layoutView.document().topDocument().rootScrollerController();
+
+    Element* documentElement = m_layoutView.document().documentElement();
+    bool frameIsRootScroller = documentElement && documentElement->isSameNode(
+        globalRootScrollerController->globalRootScroller());
+
+    // 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);
+    if (m_overflowControlsHostLayer)
+        m_overflowControlsHostLayer->setMasksToBounds(shouldClip);
+}
+
 static void restartAnimationOnCompositor(const LayoutObject& layoutObject)
 {
     Node* node = layoutObject.node();
@@ -1024,28 +1059,17 @@ void PaintLayerCompositor::ensureRootLayer()
         m_rootContentLayer->setSize(FloatSize(overflowRect.maxX(), overflowRect.maxY()));
         m_rootContentLayer->setPosition(FloatPoint());
         m_rootContentLayer->setOwnerNodeId(DOMNodeIds::idForNode(m_layoutView.node()));
-
-        // FIXME: with rootLayerScrolls, we probably don't even need m_rootContentLayer?
-        if (!RuntimeEnabledFeatures::rootLayerScrollingEnabled()) {
-            // Need to clip to prevent transformed content showing outside this frame
-            m_rootContentLayer->setMasksToBounds(true);
-        }
     }
 
     if (!m_overflowControlsHostLayer) {
         ASSERT(!m_scrollLayer);
         ASSERT(!m_containerLayer);
 
-        // Create a layer to host the clipping layer and the overflow controls layers.
+        // Create a layer to host the clipping layer and the overflow controls
+        // layers.  Whether these layers mask the content below is determined
+        // in updateClippingOnCompositorLayers.
         m_overflowControlsHostLayer = GraphicsLayer::create(this);
-
-        // Clip iframe's overflow controls layer.
-        bool containerMasksToBounds = !m_layoutView.frame()->isLocalRoot();
-        m_overflowControlsHostLayer->setMasksToBounds(containerMasksToBounds);
-
-        // Create a clipping layer if this is an iframe or settings require to clip.
         m_containerLayer = GraphicsLayer::create(this);
-        m_containerLayer->setMasksToBounds(containerMasksToBounds);
 
         m_scrollLayer = GraphicsLayer::create(this);
         if (ScrollingCoordinator* scrollingCoordinator = this->scrollingCoordinator())