(construct_mouse_click, construct_menu_click):

(construct_mouse_click, construct_menu_click):
Don't update x_mouse_grabbed, Vmouse_depressed, last_mouse_frame here.
(XTread_socket): Update them here, so we do it without fail.
(XTmouse_position, XTread_socket case MotionNotify):
Check last_mouse_frame is nonzero before using it.