Simple as reportbug

Initial report

Unless you know which package to report the bug against, you can report the bug against the xorg metapackage:

reportbug xorg

Like most packages related to the X server, reporting a bug against this package triggers a bug script which is going to collect X-related information, be it installed packages, running kernel, X configuration, X log, and so on.

Note

In case this metapackage wasn’t used to install your X stack, report a bug against xserver-xorg instead. And if that one isn’t installed either, go for xserver-xorg-core.

If a backtrace shows up in the X log, it’s much appreciated to try and get a full backtrace using gdb.

Follow-up with more info

If you reported a bug against another package and if that bug was reassigned to an X-related package, we might need more information. You can run the bug script manually and attach its output to your mail to the bug report:

/usr/share/bug/xserver-xorg-core/script 3>/tmp/script.log
Note

Make sure there’s no space between 3 and >, that’s a shell redirection.