+(defvar mdw-fallback-window-alist nil
+ "Alist mapping frames to fallback windows.")
+
+(defun mdw-cleanup-fallback-window-alist ()
+ "Remove entries for dead frames and windows from the fallback alist."
+ (let ((prev nil)
+ (cursor mdw-fallback-window-alist))
+ (while cursor
+ (let* ((assoc (car cursor))
+ (tail (cdr cursor)))
+ (cond ((and (frame-live-p (car assoc))
+ (window-live-p (cdr assoc)))
+ (setq prev cursor))
+ ((null prev)
+ (setq mdw-fallback-window-alist tail))
+ (t
+ (setcdr prev tail)))
+ (setq cursor tail)))))
+
+(defun mdw-set-fallback-window (cancel)
+ "Prefer the selected window for pop-up buffers in this frame.
+With a prefix argument, clear the fallback window."
+ (interactive "P")
+ (let* ((frame (selected-frame)) (window (selected-window))
+ (assoc (assq (selected-frame) mdw-fallback-window-alist)))
+ (cond (cancel
+ (cond (assoc
+ (setcdr assoc nil)
+ (message "Fallback window cleared."))
+ (t
+ (message "No fallback window active in this frame."))))
+ ((window-dedicated-p window)
+ (error "Window is dedicated to its buffer."))
+ (t
+ (if assoc (setcdr assoc window)
+ (push (cons frame window) mdw-fallback-window-alist))
+ (message "Fallback window set.")))
+ (mdw-cleanup-fallback-window-alist)))
+