Call ScrollableArea::ShowOverlayScrollbars for explicit scrolls only.

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "platform/scroll/ScrollbarThemeOverlay.h"
 
 #include "platform/scroll/ScrollbarTestSuite.h"
 #include "platform/testing/TestingPlatformSupport.h"
 
 namespace blink {
@@ skipping 97 matching lines @@
   mock_scrollable_area->ClearNeedsPaintInvalidationForScrollControls();
 
   // Move off should cause invalidation
   vertical_scrollbar->SetHoveredPart(kNoPart);
   EXPECT_TRUE(vertical_scrollbar->ThumbNeedsRepaint());
   EXPECT_TRUE(mock_scrollable_area->VerticalScrollbarNeedsPaintInvalidation());
 
   vertical_scrollbar->ClearThumbNeedsRepaint();
   mock_scrollable_area->ClearNeedsPaintInvalidationForScrollControls();
 
-  // Disabling the scrollbar is used to hide it so it should cause invalidation
-  // but only in the general sense since the compositor will have hidden it
-  // without a repaint.
+  // Hiding the scrollbar should invalidate the layer (SetNeedsDisplay) but not
+  // trigger repaint of the thumb resouce, since the compositor will give the
+  // entire layer opacity 0.
+  EXPECT_CALL(*mock_scrollable_area, ScrollbarsHidden()).WillOnce(Return(true));
   vertical_scrollbar->SetEnabled(false);
   EXPECT_FALSE(vertical_scrollbar->ThumbNeedsRepaint());
   EXPECT_TRUE(mock_scrollable_area->VerticalScrollbarNeedsPaintInvalidation());
 
   mock_scrollable_area->ClearNeedsPaintInvalidationForScrollControls();
 
+  // Showing the scrollbar needs to repaint the thumb resource, since it may
+  // have been repainted in the disabled state while hidden (e.g. from
+  // SetProportion on bounds changes).
+  EXPECT_CALL(*mock_scrollable_area, ScrollbarsHidden())
+      .WillOnce(Return(false));
   vertical_scrollbar->SetEnabled(true);
-  EXPECT_FALSE(vertical_scrollbar->ThumbNeedsRepaint());
+  EXPECT_TRUE(vertical_scrollbar->ThumbNeedsRepaint());
   EXPECT_TRUE(mock_scrollable_area->VerticalScrollbarNeedsPaintInvalidation());
 
   ThreadState::Current()->CollectAllGarbage();
 }
 
 }  // namespace blink