[Exim] Problem with eximon under Linux

I have a (probably naive) build of Exim 4.22, and in particular eximon,
under RedHat Linux 7.1. The build uses the default (all commented-out)
Local/eximon.conf and the default values of X11, XINCLUDE, XLFLAGS and
X11_LD_LIB. eximon work fine, except that it quits with the message

Error: Shell widget menu has zero width and/or height

if you right-click in the bottom pane anywhere _other_ than on a message.
This does not happen for a very similar build under Solaris. Does anyone
recognize this?

Jon.

--
Jon Warbrick
Web/News Development, Computing Service, University of Cambridge