You must be using KDE, Gnome, or some other Linux window manager that does not properly respect X11 protocol hints. You can partially fix this by setting the Unix environment variable AFNI_ENFORCE_ASPECT to YES. Details can be found in the file [
afni.nimh.nih.gov], including various instructions on how to set environment variables so that AFNI will see them.
In general, this file (README.environment) is a very useful file for users to peruse. You can affect the appearance and operation of AFNI in many ways by setting environment variables.
bob cox