bug#20538: 24.4; Attempt to delete a surrogate minibuffer frame

[martin rudalics <rudalics@gmx.at>]

 > My usage scenario is the following:
 >
 > Normally I run emacs with always no more than two frames.  I create
 > a 2nd frame as needed, but I delete it when it is not needed anymore
 > so that it does not clutter the desktop.  An ediff session then adds
 > one more frame for the ediff control panel.  Yet during an ediff
 > session I can get side-tracked: the frame used for displaying the ediff
 > buffers A and B may get used for something else, then I create
 > another frame and finally I want to delete the frame that Ediff
 > wants to use as surrogate minibuffer frame.

Such things happen to me all the time.  That's why I prefer
`ediff-setup-windows-plain' (and `split-window-horizontally' as
`ediff-split-window-function').

 > I understand that the frame displaying the ediff control panel needs
 > *some* other frame to provide a minibuffer.  Is it necessary that a
 > particular frame serves this purpose?  Or would it be sufficient
 > that emacs made sure that always at least one frame has a proper
 > minibuffer?  (In the case of the ediff control panel, I believe it
 > is fair to assume that anyway this frame rarely requires a proper
 > minibuffer. For me, the control panel is effectively the minibuffer
 > for an ediff session: commands are entered in that buffer; there is
 > no need for "a minibuffer for the ediff minibuffer".)

At any time every frame has a corresponding minibuffer window and that
window won't change for the frame's entire lifetime.

 > If nothing else, it would be good if the error message issued when
 > attempting to delete a surrogate minibuffer frame could be improved:
 > I got this error message at a point when I was not thinking about
 > ediff, but I just wanted to get back to my default "one frame
 > setup"; and it happened I wanted to delete the wrong frame to
 > achieve this goal.

The problem is that the entity that issues the message is not aware of
the intrinsics of ediff.

martin