It is not possible for a single application to generate core dump after
closing a window, and main window to remain functional.
You must have an application which consists of several processes.
This could make it easier for you to debug it.
The user is not getting this for
This is entirely possible, you could get it due to uninitialized memory,
which would behave differently every time. It could be a timing issue,
where program behaves differently on computers with different speed.
Sometimes small differences in the version of the underlying libraries
also cause difficult bugs to pop up.
So having bugs, which you can't reproduce, is nothing uncommon.
Your core file seems to come from a binary which was not compiled with
debug options. Your trace shows only calls to library functions.
I recommend to compile it again with -g option.
You can try to run it in DDD
if you manage to reproduce the core dump, or run it with Valgrind
if you can't find the bug.
Hope this helps,