Correct overlay scrollbar check FrameView::scrollbarExistenceDidChange

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

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 2103 matching lines @@
 
 void FrameView::scrollbarExistenceDidChange() {
   // We check to make sure the view is attached to a frame() as this method can
   // be triggered before the view is attached by LocalFrame::createView(...)
   // setting various values such as setScrollBarModes(...) for example.  An
   // ASSERT is triggered when a view is layout before being attached to a
   // frame().
   if (!frame().view())
     return;
 
-  bool usesOverlayScrollbars = ScrollbarTheme::theme().usesOverlayScrollbars();
+  Element* customScrollbarElement = nullptr;
+
+  bool usesOverlayScrollbars =
+      ScrollbarTheme::theme().usesOverlayScrollbars() &&
+      !shouldUseCustomScrollbars(customScrollbarElement);
 
   // FIXME: this call to layout() could be called within FrameView::layout(),
   // but before performLayout(), causing double-layout. See also
   // crbug.com/429242.
   if (!usesOverlayScrollbars && needsLayout())
     layout();
 
   if (!layoutViewItem().isNull() && layoutViewItem().usesCompositing()) {
     layoutViewItem().compositor()->frameViewScrollbarsExistenceDidChange();
 
@@ skipping 3125 matching lines @@
 void FrameView::setAnimationHost(
     std::unique_ptr<CompositorAnimationHost> host) {
   m_animationHost = std::move(host);
 }
 
 LayoutUnit FrameView::caretWidth() const {
   return LayoutUnit(getHostWindow()->windowToViewportScalar(1));
 }
 
 }  // namespace blink