Selecting tooltip frames considered harmful

[martin rudalics]

Emacs tooltip frames are not "first-class" frames.  They don't have
minibuffer windows which means that calling 'message' when a tooltip
frame is selected will crash Emacs.  For example, with emacs -Q run
the following function and move the mouse cursor over the mode line
until a tooltip pops up:

(defun foo ()
  (interactive)
  (track-mouse
    (while (progn
             (setq event (read-key))
             (or (mouse-movement-p event)
                 (memq (car event) '(select-window switch-frame))))
      (dolist (frame (visible-frame-list))
        (when (frame-parameter frame 'tooltip)
          (select-frame frame)
          (message "..."))))))

Note also the following excerpt from choose_minibuf_frame

      /* I don't think that any frames may validly have a null minibuffer
         window anymore.  */
      if (NILP (sf->minibuffer_window))
        emacs_abort ();

so the following function will crash us in a similar way:

(defun bar ()
  (interactive)
  (track-mouse
    (while (progn
             (setq event (read-key))
             (or (mouse-movement-p event)
                 (memq (car event) '(select-window switch-frame))))
      (dolist (frame (visible-frame-list))
        (when (frame-parameter frame 'tooltip)
          (select-frame frame)
          (read-from-minibuffer "..."))))))

You won't observe these crashes with GTK+ tooltips since these are not
Emacs frames.  Customize 'x-gtk-use-system-tooltips' to nil in order
to see them with GTK builds.

There seem to be various ways to fix this.  For the 'message' case we
could:

(1) Ignore the message in message3_nolog (the ideal solution but maybe
    too simplistic).

(2) Send the message to stderr instead (confusing for plain messages
    and hardly suited for 'read-from-minibuffer').

(3) Give tooltip frames a minibuffer window of some other frame (with
    the usual consequences when trying to delete that frame).

(4) Try to find a frame with a minibuffer and put the message there
    (it might be tricky to find the most suited minibuffer here).

(5) Avoid that a tooltip frame gets selected (hardly feasible).

A similar solution should be found when reading from the minibuffer.

Suggestions welcome, martin