I have been informed about the existence of this bug by Akio Burns.
Unfortunately, he has not reacted so far to my request to file a bug
report so I'm sloppily doing this here instead:

With emacs -Q type C-x 2 C-x 2 C-x b RET

This should put you in the *Messages* buffer.  Now do

M-: (setq window-size-fixed t)

followed by

M-: (balance-windows)

Window sizes remain unchanged.  The attached patch should fix that.  I
would like to apply it to the release branch.  Objections?

Thanks, martin