Disable scrollbars on the root scroller when using visual viewport scrollbars.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
index f4d15b2eedadc3050bed99b495cc9f3bc1f32b17..742a651142649cf011455bdb5dbd3840902678ca 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -71,7 +71,9 @@
 #include "core/page/FocusController.h"
 #include "core/page/Page.h"
 #include "core/page/scrolling/RootScrollerController.h"
+#include "core/page/scrolling/RootScrollerUtil.h"
 #include "core/page/scrolling/ScrollingCoordinator.h"
+#include "core/page/scrolling/TopDocumentRootScrollerController.h"
 #include "core/paint/PaintLayerFragment.h"
 #include "platform/PlatformGestureEvent.h"
 #include "platform/PlatformMouseEvent.h"
@@ -805,6 +807,22 @@ void PaintLayerScrollableArea::clampScrollOffsetsAfterLayout() {
   m_scrollbarManager.destroyDetachedScrollbars();
 }
 
+void PaintLayerScrollableArea::didChangeGlobalRootScroller() {
+  // On Android, where the VisualViewport supplies scrollbars, we need to
+  // remove the PLSA's scrollbars. In general, this would be problematic as
+  // that can cause layout but this should only ever apply with overlay
+  // scrollbars.
+  if (!box().frame()->settings() ||
+      !box().frame()->settings()->viewportEnabled())
+    return;
+
+  bool needsHorizontalScrollbar;
+  bool needsVerticalScrollbar;
+  computeScrollbarExistence(needsHorizontalScrollbar, needsVerticalScrollbar);
+  setHasHorizontalScrollbar(needsHorizontalScrollbar);
+  setHasVerticalScrollbar(needsVerticalScrollbar);
+}
+
 bool PaintLayerScrollableArea::shouldPerformScrollAnchoring() const {
   return RuntimeEnabledFeatures::scrollAnchoringEnabled() &&
          m_scrollAnchor.hasScroller() &&
@@ -1728,14 +1746,22 @@ void PaintLayerScrollableArea::setTopmostScrollChild(PaintLayer* scrollChild) {
 }
 
 bool PaintLayerScrollableArea::visualViewportSuppliesScrollbars() const {
-  if (!layer()->isRootLayer())
+  LocalFrame* frame = box().frame();
+  if (!frame || !frame->settings())
     return false;
 
-  LocalFrame* frame = box().frame();
-  if (!frame || !frame->isMainFrame() || !frame->settings())
+  // On desktop, we always use the layout viewport's scrollbars.
+  if (!frame->settings()->viewportEnabled())
+    return false;
+
+  const TopDocumentRootScrollerController& controller =
+      layoutBox()->document().frameHost()->globalRootScrollerController();
+
+  if (!controller.globalRootScroller())
     return false;
 
-  return frame->settings()->viewportEnabled();
+  return RootScrollerUtil::scrollableAreaFor(
+             *controller.globalRootScroller()) == this;
 }
 
 Widget* PaintLayerScrollableArea::getWidget() {