Mouse up updates scrollbar hovered part if needed.

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

Index: third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp b/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
index a50f1a0593a3824166d8b6404b2e7eeab40ce59e..d0b52964c01ba46db4d0db75eaf7ec6b6791ce11 100644
--- a/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
+++ b/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
@@ -472,12 +472,11 @@ void Scrollbar::mouseUp(const PlatformMouseEvent& mouseEvent) {
     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);
+    }
   }
 }