While working on info.el I needed a function to return info windows on all
frames. I was a bit surprised to discover there is no built-in option in
window-list or elsewhere as far as I can see to get all windows for all frames,
or a filtered list containing interesting windows for all frames. I ended up
with this:

#+begin_src emacs-lisp
(defun window-list-by-mode (mode &optional all-frames)
  "Return list of windows displaying buffers whose major mode is MODE.

If ALL-FRAMES is non-nil, consider all frames, otherwise just selected
frame."
  (mapcar
   (lambda (window) (cons (prin1-to-string window) window))
   (cl-remove-if-not
    (lambda (window)
      (with-current-buffer (window-buffer window)
        (eq major-mode mode)))
    (if all-frames
        (apply #'nconc (mapcar (lambda (f) (window-list f)) (frame-list)))
      (window-list)))))
#+end_src

Anyway, looking at window.c, I can see that window_candidate_p actually does
consider windows on all frames, but 't option is never passed through from
window-list. To save gymnastics via apply and nconc and doing the double work, I
have patched window-list function to let 't option further and documented the
change. It seems to me that it works as intended. I have tested myself, I hope I
haven't missed some possible case, but I'm a little suspisious: what was the
reason why this wasn't documented and used? I mean, it is already there, but
cut-off for some reason?