Window scrolls while context menu is open

As discussed at http://musescore.org/en/node/45336#comment-226406, for some installations scrolling is not disabled when a context menu is open. This is only a minor bug, especially as it's not clear how many users are affected by it, but it would be nice if at some point in the future it could be fixed.

Video of the issue: http://musescore.org/sites/musescore.org/files/Screen_Capture.mov_.zip