[Menu] In a huge menu, the upward scrolling arrow stops working (regression)

Component: JavaFXCompatibility manual
Build: javafx8u20b14
PlatForm: widows7 x64

Steps(the test case is graphics/api/control/menu/ScrollableMenuTestTest.java):
1.Click on 'File' menu. It contains a down arrow in the botton, it is OK.
2.Using the mouse hovering of down arrow, the items in the top are not visible, then the up arrow will be shown. It is OK.
3.Keeping the mouse hovering of the down arrow, untill the last item is visible, the down arrow will not show. It is OK.
4.But, at this time, if we move the mouse hovering of the up arrow, the items can not move, it is the bug. Please refer to ScrollableMenuItem.png.

Previous result:
javafx8u11b04 + windows7 x64 + jdk8u11b04 x64: PASS