Under Cygwin/X11, using the window manager close widget
to exit the PDL::Graphics::TriD window results in the
perldl shell exiting. From reading the PDL Begginnings
book I see that others have observed the same problem
(i.e. it is not specific to the Cygwin porting effort).
I took a look at the glpXNextEvent code in TriD::OpenGL
and observed that it only handles a few of the standard
X11 events. Specifically, it does *not* handle the
DestroyNotify event. I believe that adding a condition
for this error type and the corresponding code in the
higher level perl code would fix this problem.