When a mouse button is given a window binding (via "Mouse <N> W ..."), the event passed to the window (as seen by xev) changes from:
"ButtonPress, ButtonRelease" to "KeymapNotify, LeaveNotify, EnterNotify, ButtonPress, ButtonRelease" I have at least one application that is confused by this and no longer recognises clicks within its double-click time. Is this an fvwm or application issue? duncan