bug#55684: 29.0.50; wrong-type-argument in minibuffer-only frames

[Alan Mackenzie]

Hello, Stefan.

On Fri, May 27, 2022 at 16:09:13 -0400, Stefan Monnier wrote:
> Package: Emacs
> Version: 29.0.50


>     src/emacs -Q --eval '(setq default-frame-alist `((minibuffer . nil)))'

> Stays "frozen" because of a wrong-type-argument error in
> `Fset_frame_selected_window` on `CHECK_LIVE_WINDOW (window);`.

Yes.  This is unfortunate.  Sorry.

> The patch below seems to "fix" it, but I must admit that I don't really
> understand this code (see for example the FIXME comment in the patch).
> Does someone know what is the user-level behavior we're trying to
> obtain here?

When the mini-window in frame F1 is the selected window, and we switch
to a different frame F2, F1 remembers the mini-window as its selected
window.  When we later switch back to F1, we check whether there is
still a minibuffer in the mini-window, and if not select a different
window.

There are several reasons why there might not be a minibuffer on
returning to F1.  The MB in F1 might have moved to F2 on the frame
switch, and been terminated there, for example.

> It seems that this comes from Alan's commit dfa3e6f424b20fe27d904.


>         Stefan


> diff --git a/src/frame.c b/src/frame.c
> index 252dc591bfa..dc55004b193 100644
> --- a/src/frame.c
> +++ b/src/frame.c
> @@ -1576,8 +1576,14 @@ do_switch_frame (Lisp_Object frame, int track, int 
> for_deletion, Lisp_Object nor
>       to a different window, the most recently used one, unless there is a
>       valid active minibuffer in the mini-window.  */
>    if (EQ (f->selected_window, f->minibuffer_window)
> +         /* FIXME: Can this test ever fail?  I.e. can 'minibuffer_window'
> +            ever contain a non-mini-buffer (and if so, should we care here)? 
> */

Yes, the test can fail.  The buffer in the mini-window might be (the
non-active) minibuffer 0.  The meaning of the argument Qt is to return
non-nil only for active minibuffers.

>        && NILP (Fminibufferp (XWINDOW (f->minibuffer_window)->contents, Qt)))
> -    Fset_frame_selected_window (frame, call1 (Qget_mru_window, frame), Qnil);
> +    {
> +      Lisp_Object w = call1 (Qget_mru_window, frame);
> +      if (!NILP (w))            /* Can be nil in minibuffer-only frames.  */
> +        Fset_frame_selected_window (frame, w, Qnil);
> +    }
 
>    Fselect_window (f->selected_window, norecord);

I think the patch looks OK.  I'm going to comment on Eli's suggestion.

-- 
Alan Mackenzie (Nuremberg, Germany).