entering? in mouse-event%
Returns #t if this event is for the mouse entering a window,
#f otherwise.
When the mouse button is up, an enter/leave event lets a window know
that it will start/stop receiving mouse events. When the mouse button
is down, however, the window receiving the mouse-down event receives
all mouse events until the button is released; enter/leave events are
not sent to other windows, and are not reliably delivered to the
click-handling window (since the window can detect movement out of
its region via
get-x and
get-y). See also Mouse and Keyboard Events.