> Shouldn't it be something like this?
 >
 > +  :type '(choice (repeat (symbol :tag "Major mode")) boolean)
 > +  :version "31.1"
 > +  :group 'windows)
 >
 > Otherwise it would give a warning due type.

Right.

 >> +  (let (kill-from-mode)
 >> +    (with-current-buffer (window-buffer (window-normalize-window window))
 >> +      ;; Run the hook from the buffer implied to get any buffer-local
 >> +      ;; values.
 >> +      (run-hooks 'quit-window-hook)
 >> +
 >> +      (setq kill-from-mode
 >> +	    (or (eq quit-window-kill-buffer-modes t)
 >> +		(and (listp quit-window-kill-buffer-modes)
 >> +		     (derived-mode-p quit-window-kill-buffer-modes)))))
 >
 > Wouldn't it be more convenient to put this into the let varlist?

It would be more convenient ...

 > +  (let ((kill-from-mode
 > +         (or (eq quit-window-kill-buffer-modes t)
 > +		      (and (listp quit-window-kill-buffer-modes)
 > +		           (derived-mode-p quit-window-kill-buffer-modes)))))
 >
 >
 > ... or is there something i'm missing?

... but 'derived-mode-p' says to

  "Return non-nil if the current major mode is derived from one of MODES.

so I have to make the buffer current first via 'with-current-buffer' and
only then I can call 'derived-mode-p'.

martin