Mouse up updates scrollbar hovered part if needed.

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

 /*
  * Copyright (C) 2004, 2006, 2008 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 454 matching lines @@
   bool isCaptured = m_pressedPart == ThumbPart;
   setPressedPart(NoPart);
   m_pressedPos = 0;
   m_draggingDocument = false;
   stopTimerIfNeeded();
 
   if (m_scrollableArea) {
     if (isCaptured)
       m_scrollableArea->mouseReleasedScrollbar();
 
-    // m_hoveredPart won't be updated until the next mouseMoved or mouseDown, so
-    // we have to hit test to really know if the mouse has exited the scrollbar
-    // on a mouseUp.
     ScrollbarPart part = theme().hitTest(*this, mouseEvent.position());
-    if (part == NoPart)
+    if (part == NoPart) {
+      setHoveredPart(NoPart);
       m_scrollableArea->mouseExitedScrollbar(*this);
+    }
   }
 }
 
 void Scrollbar::mouseDown(const PlatformMouseEvent& evt) {
   // Early exit for right click
   if (evt.pointerProperties().button == WebPointerProperties::Button::Right)
     return;
 
   setPressedPart(theme().hitTest(*this, evt.position()));
   int pressedPos = orientation() == HorizontalScrollbar
@@ skipping 132 matching lines @@
     invalidParts = AllParts;
   if (invalidParts & ~ThumbPart)
     m_trackNeedsRepaint = true;
   if (invalidParts & ThumbPart)
     m_thumbNeedsRepaint = true;
   if (m_scrollableArea)
     m_scrollableArea->setScrollbarNeedsPaintInvalidation(orientation());
 }
 
 }  // namespace blink