With sweep selection the actual drop is made on releasing the button,
and this event was not caught for button 2. Button-2 motion events were
also not caught by the sweep-selection handler.
<Btn1Up>: HandleUserMove(0) \n \
<Btn1Motion>: AnimateUserMove() \n \
<Btn3Motion>: HandlePV() \n \
+ <Btn2Motion>: HandlePV() \n \
<Btn3Up>: PieceMenuPopup(menuB) \n \
+ <Btn2Up>: PieceMenuPopup(menuB) \n \
Shift<Btn2Down>: XawPositionSimpleMenu(menuB) XawPositionSimpleMenu(menuD)\
PieceMenuPopup(menuB) \n \
Any<Btn2Down>: XawPositionSimpleMenu(menuW) XawPositionSimpleMenu(menuD) \