Please describe the problem:
Normally, when you zoom in on an image, The GIMP will resize the window if you have 
that feature turned on (File -> Preferences -> Image Windows -> "[ ] Resize window on 
zoom").  Normally, if the image is zoomed to a point that it won't fit on screen, The 
GIMP will resize the window to preset dimensions if this feature is enabled.

However, if you are zoomed in to this point where the window size is in that "preset 
dimensions" range, and you then manually resize the window to something smaller than 
the default size, The GIMP will forceably resize the window back to the default when 
the window gains focus again.

Steps to reproduce:
1. Open two images of any type.
2. Turn on the "Resize window on zoom" feature in Preferences.
3. Using the =/+ key, zoom the image such that it's too big to fit on the screen.  
Notice that the window eventually gets resized to a fixed "default" size.
4. Using the window borders or your favorite window manager shortcuts, manually 
resize that window so that it is smaller than the default size.
5. Give focus to the other image window.
6. Give focus to the resized window.  Notice that it suddenly resizes back to the 
default size it went to when you first zoomed in.


Actual results:
The above, exactly as described.

Expected results:
The window dimensions should stay exactly as I have set them, unless I explicitly 
change them or change the zoom level.

Does this happen every time?
Yes, it is consistant

Other information:
The window manager I use is Afterstep, with focus-follows-mouse mode enabled.