bug#11840: 24.1.50; nil keymap element on high priority overlay should fallthrough to low priority overlay

[Christopher Monsanto]

One more symptom, and I doubt this could be anything but a bug: if you
delete the high priority overlay, we *still* don't use the lower
priority overlay's keymap!

Try this:

(setq m (make-sparse-keymap))
(define-key m "k" '(lambda nil (interactive) (message "overlay 1")))

(setq m2 (make-sparse-keymap))
(define-key m2 "k" nil)

(let ((o (make-overlay 0 5))
      (o2 (make-overlay 0 5)))
  (overlay-put o 'keymap m)
  (overlay-put o 'priority 100)
  (overlay-put o2 'keymap m2)
  (overlay-put o2 'priority 9999)
  (delete-overlay o2))

eval-buffer again, and k self inserts. If we never create the second
overlay, it prints the msg.