[BCC'd to open a new issue on gnome.gitlab.org. Re."That old GTK bug"]

Attached is a reduced test case which was requested in the now closed
issue #221 - quickly sloppily hacked up from simple.c from gtk+-demos.

gtk_main is replaced by a loop which calls g_main_context_iteration.
X errors are handled via XSetIOErrorHandler and XSetErrorHandler to
handle a closed display and continue with the next
g_main_context_iteration.

To Run the test case:
gcc simple.c -Wno-deprecated -Wno-deprecated-declarations -g3 $(pkg-config gtk+-3.0 --cflags --libs ) -lX11
$ DISPLAY=:0 Xephyr :1
$ DISPLAY=:1 G_DEBUG=fatal-warnings ./a.out
kill Xephyr

The stack trace which you get from killing Xephyr is attached.

[Also featured in the code but not relevant to this bug report is the
"closed" signal which is expected from when GdkDisplay connection is
lost. This signal does not seem to fire]