[Last modified: Feb 98]

Answer:

When a popup menu is created as the child of a widget the menu system installs
a translation on the parent of the popup and descendants with an action which:
(1) when 3-rd button (the default for the menuPost resource) is pressed the
cursor changes and the mouse is grabbed for 3 seconds; (2) disables event
handlers on the descendants and the handlers are never called; (3) an event
handler installed on the parent works fine.

It is done so that the correct event handler will (in fact) be called.  There
is a grab with owner_events true.  The grab is released by a timer, but
normally the posted menu shell puts up it's own grab.

If you only have widgets then you can use the subwindow field in the event to
identify the original widget.  If you have gadgets or other data that you want
to change the menu for (or use a specific menu for) then you must do a walk of
the parent's children to find the best match.

One thing to beware of is that even with the grab,  because the menu system
does a grab with owner events true, you must either have an event handler, or
nothing that will use the event on each widget in the hierarchy of the menu's
parent.  If a child widget has another event handler for button down, it may
swallow the event and do something else.