The problem is that many post-1997 X11 window managers, including Apple's quartz-wm, do not implement the standard way to enforce aspect that X11 requires. AFNI calls this standard method to constrain the aspect ratio, but quartz-wm, KDE, and GNOME all ignore it. I'm not sure how to fix this; when you set AFNI_ENFORCE_ASPECT to YES, AFNI will try to detect window resizes and set the aspect ratio properly, but that doesn't always work for reasons I don't understand (esp. when shrinking a window, vs. growing it).
As a partial fixup, if you Button-1 click (left-click) in the vertical grayscale bar just to the right of the image sub-window, the image window should re-aspect itself. So it should be easy to fix.
bob cox