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

third_party/WebKit/Source/core/frame/FrameView.cpp

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index 216b78f2a864c8875e49195dd53f7aa3c8213749..3e44b959c9218a1aaae2a6823088c12f2f40ba9e 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -89,6 +89,7 @@
 #include "core/page/FocusController.h"
 #include "core/page/FrameTree.h"
 #include "core/page/Page.h"
+#include "core/page/scrolling/RootScrollerUtil.h"
 #include "core/page/scrolling/ScrollingCoordinator.h"
 #include "core/page/scrolling/TopDocumentRootScrollerController.h"
 #include "core/paint/FramePainter.h"
@@ -2673,6 +2674,19 @@ FrameView* FrameView::parentFrameView() const {
   return nullptr;
 }
 
+void FrameView::didChangeGlobalRootScroller() {
+  if (!ScrollbarTheme::theme().usesOverlayScrollbars())
+    return;
+
+  // Avoid drawing two sets of scrollbars when visual viewport is enabled.
+  bool shouldHaveHorizontalScrollbar = false;
+  bool shouldHaveVerticalScrollbar = false;
+  computeScrollbarExistence(shouldHaveHorizontalScrollbar,
+                            shouldHaveVerticalScrollbar, contentsSize());
+  m_scrollbarManager.setHasHorizontalScrollbar(shouldHaveHorizontalScrollbar);
+  m_scrollbarManager.setHasVerticalScrollbar(shouldHaveVerticalScrollbar);
+}
+
 void FrameView::updateWidgetGeometriesIfNeeded() {
   if (!m_needsUpdateWidgetGeometries)
     return;
@@ -3497,9 +3511,21 @@ void FrameView::removeChild(Widget* child) {
   m_children.remove(child);
 }
 
-bool FrameView::visualViewportSuppliesScrollbars() const {
-  return m_frame->isMainFrame() && m_frame->settings() &&
-         m_frame->settings()->viewportEnabled();
+bool FrameView::visualViewportSuppliesScrollbars() {
+  // On desktop, we always use the layout viewport's scrollbars.
+  if (!m_frame->settings() || !m_frame->settings()->viewportEnabled() ||
+      !m_frame->document() || !m_frame->document()->frameHost())
+    return false;
+
+  const TopDocumentRootScrollerController& controller =
+      m_frame->document()->frameHost()->globalRootScrollerController();
+
+  if (!controller.globalRootScroller())
+    return false;
+
+  return RootScrollerUtil::scrollableAreaFor(
+             *controller.globalRootScroller()) ==
+         layoutViewportScrollableArea();
 }
 
 AXObjectCache* FrameView::axObjectCache() const {
@@ -3756,8 +3782,9 @@ void FrameView::computeScrollbarExistence(
     bool& newHasHorizontalScrollbar,
     bool& newHasVerticalScrollbar,
     const IntSize& docSize,
-    ComputeScrollbarExistenceOption option) const {
-  if (m_frame->settings() && m_frame->settings()->hideScrollbars()) {
+    ComputeScrollbarExistenceOption option) {
+  if ((m_frame->settings() && m_frame->settings()->hideScrollbars()) ||
+      visualViewportSuppliesScrollbars()) {
     newHasHorizontalScrollbar = false;
     newHasVerticalScrollbar = false;
     return;