the yes-no-cancel dialog which appears at appearing at closing changed-but-not-saved document, allows focusing on its label (e.g. using [tab])
Created attachment 3825 Prevents tabstops on the info area of GtkMessageDialogs in mousepad
Mousepad uses GtkMessageDialog for the yes-no-cancel popups. The GtkMessageDialog sets its info are to sensitive by default what allows tab stops on it. The attached patch removes the GTK_SENSITIVE flags from the GtkMessageDialogs. The user still can operate with the contents of the message area with the mouse (e.g. to copy the contents).
I confirm this bug and the patch provided by Dmitri can't be applied in the current master, but after some tweaking it's proven to correctly fix the bug.
I want to see if it can be fixed at the GTK+ level before we add a workaround. I filed a bug about it: https://bugzilla.gnome.org/show_bug.cgi?id=710941
This does not seem to affect the GTK3 build. Closing the report.