Minibuffer command un-maximises frame on W2k

XEmacs 21.5.16, built on W2k under Cygwin. 1. With the current frame maximised, typing just about any command in the mini buffer will un-maximise the frame. F.e. C-x C-g will do it. 2. I would have used the bug report facility in xemacs but that reports "Symbol's function definition is void: nil" after entering the subject. It is possible that mail is not configured fully on this particular xemacs installation, but the error message gives no clue to that. -- Manfred