bug#22870: 25.0.91; Mouse-clicks fail on button overlays with display properties

[Artur Malabarba]

1. Visit the scratch buffer.

2. Evaluate this code:

(defun my-action (&rest _)
  (interactive)
  (message "WORKED!"))

(make-button (point-min) (+ 30 (point-min))
             'display "..."
             'action #'my-action
             'follow-link t)

3. As expected, the first 30 chars of the buffer will be hidden under a
button that displays as “...”.  Hitting RET on that buffer works as
expected (you see the message “Worked!” on the echo-area).

4. Try clicking on the button with the mouse. You'll get the following error:

Debugger entered--Lisp error: (void-function nil)
  nil(("..." . 1))
  button-activate(("..." . 1) t)
  push-button((mouse-2 (#<window 3 on *scratch*> 1 (14 . 2) 15461622 ("..." . 
1) 1 (1 . 0) nil (5 . 2) (9 . 18))))
  funcall-interactively(push-button (mouse-2 (#<window 3 on *scratch*> 1 (14 . 
2) 15461622 ("..." . 1) 1 (1 . 0) nil (5 . 2) (9 . 18))))
  call-interactively(push-button nil nil)
  command-execute(push-button)

5. Adding a `mouse-action' property makes no difference.

6. Without the `display' property, everything works as expected.



Looks like this happens because the first argument given to
`button-activate' is `("..." . 1)', while it should be the overlay.