Disable overlay scrollbars in Blink when hidden by the compositor.

third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  * Copyright (C) 2008, 2011 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 32 matching lines @@
 static const float kMinFractionToStepWhenPaging = 0.875f;
 
 namespace blink {
 
 struct SameSizeAsScrollableArea {
   virtual ~SameSizeAsScrollableArea();
 #if ENABLE(ASSERT)
   VerifyEagerFinalization verifyEager;
 #endif
   Member<void*> pointer[2];
-  unsigned bitfields : 16;
+  unsigned bitfields : 17;
   IntPoint origin;
 };
 
 static_assert(sizeof(ScrollableArea) == sizeof(SameSizeAsScrollableArea),
               "ScrollableArea should stay small");
 
 int ScrollableArea::pixelsPerLineStep(HostWindow* host) {
   if (!host)
     return kPixelsPerLineStep;
   return host->windowToViewportScalar(kPixelsPerLineStep);
 }
 
 float ScrollableArea::minFractionToStepWhenPaging() {
   return kMinFractionToStepWhenPaging;
 }
 
 int ScrollableArea::maxOverlapBetweenPages() {
   static int maxOverlapBetweenPages =
       ScrollbarTheme::theme().maxOverlapBetweenPages();
   return maxOverlapBetweenPages;
 }
 
 ScrollableArea::ScrollableArea()
     : m_scrollbarOverlayColorTheme(ScrollbarOverlayColorThemeDark),
       m_scrollOriginChanged(false),
       m_horizontalScrollbarNeedsPaintInvalidation(false),
       m_verticalScrollbarNeedsPaintInvalidation(false),
-      m_scrollCornerNeedsPaintInvalidation(false) {}
+      m_scrollCornerNeedsPaintInvalidation(false),
+      m_scrollbarsHidden(false) {}
 
 ScrollableArea::~ScrollableArea() {}
 
 void ScrollableArea::clearScrollAnimators() {
 #if OS(MACOSX)
   if (m_scrollAnimator)
     m_scrollAnimator->dispose();
 #endif
   m_scrollAnimator.clear();
   m_programmaticScrollAnimator.clear();
@@ skipping 431 matching lines @@
     // scheduled on the compositor.
     // TODO(ymalik): We have a non-transient "main thread scrolling reason"
     // that doesn't actually cause shouldScrollOnMainThread() to be true.
     // This is confusing and should be cleaned up.
     return !!(reasons &
               ~MainThreadScrollingReason::kHandlingScrollFromMainThread);
   }
   return true;
 }
 
+bool ScrollableArea::scrollbarsHidden() const {
+  return hasOverlayScrollbars() && m_scrollbarsHidden;
+}
+
+void ScrollableArea::setScrollbarsHidden(bool hidden) {
+  if (m_scrollbarsHidden == static_cast<unsigned>(hidden))
+    return;
+  m_scrollbarsHidden = hidden;
+  didChangeScrollbarsHidden();
+}
+
 IntRect ScrollableArea::visibleContentRect(
     IncludeScrollbarsInRect scrollbarInclusion) const {
   int scrollbarWidth =
       scrollbarInclusion == IncludeScrollbars ? verticalScrollbarWidth() : 0;
   int scrollbarHeight =
       scrollbarInclusion == IncludeScrollbars ? horizontalScrollbarHeight() : 0;
 
   return enclosingIntRect(
       IntRect(scrollOffset().width(), scrollOffset().height(),
               std::max(0, visibleWidth() + scrollbarWidth),
@@ skipping 49 matching lines @@
   return IntSize(std::max(0, size.width() - verticalScrollbarWidth()),
                  std::max(0, size.height() - horizontalScrollbarHeight()));
 }
 
 DEFINE_TRACE(ScrollableArea) {
   visitor->trace(m_scrollAnimator);
   visitor->trace(m_programmaticScrollAnimator);
 }
 
 }  // namespace blink