> Uwe Brauer <oub@mat.ucm.es> writes:

> That kind of error would happen when you are in the minibuffer when
> trying to split a window.  Are you sure that clicking on a menu entry
> happens while point is still in the main window and the minibuffer is
> still inactive?

Yes it happens even then. 


Meanwhile I think I found the culprit. It is a specific 21.4.22 bug (I
personally switched to 21.5.X years ago and never encountered this bug).

The whole regions in minibuf.el reads like

,----
| 	    ;; #### should be 2 not 3, but that causes
| 	    ;; "window too small to split" errors for some
| 	    ;; people (but not for me ...) There's a more
| 	    ;; fundamental bug somewhere.
`----


This does not sound too good. [1]


I found a kludge to solve the bug I encountered. I already contacted the
last remaining maintainer(s), Aidan, and will wait for his comment.
Maybe you could keep the bug open till I receive some feedback from
Aidan.

Regards




(Maybe I should also file a bug report to Debian/Ubuntu maintainers, but
I don't make myself to many illusions whether they are willing to apply
the patch).

Footnotes:
[1]  Strangely enough these comments are
deleted in the xemacs 21.5 branch, but I have trouble to find the
relevant changelog entries. Right now I am not even sure where the
source code of 21.4.22 is, because I can't find it in bitbucket.