With Emacs 26 and a minibuffer-only frame, *Completions* may be
displayed in the minibuffer window.  To reproduce with emacs -Q do
(make-frame '((minibuffer . only))) and in that minibuffer-only frame
type C-h f followed by TAB.  This will display *Completions* in the
minibuffer window and it can become quite complicated to get rid of it
again.  Hence I would like to apply the attached trivial patch to the
release branch.  Any objections?

Thanks, martin