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before-string overlay and show-paren-mode

From: Stephen Berman
Subject: before-string overlay and show-paren-mode
Date: Sat, 31 Oct 2009 01:32:19 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1.50 (gnu/linux)

Start Emacs with -Q, insert the following text into a buffer:


then eval the following:

(defun my-test ()
  (goto-char (point-min))
  (let ((begin (search-forward "begin")))
    (search-forward "end")
    (narrow-to-region (1+ begin) (line-beginning-position))
    (goto-char (point-min)))
  (unless (let ((ovlist (overlays-in (1- (point)) (1+ (point)))))
            (when ovlist (overlay-get (car ovlist) 'before-string)))
    (let ((ov (make-overlay (point) (point) nil t t)))
      (overlay-put ov 'before-string "* "))))

(global-set-key "\C-cy" 'my-test)

and now type `C-c y' in the buffer with the inserted text.  The buffer
nows displays this:

* (test)

As expected, now repeating `C-c y' does not alter the display, i.e. does
not add more overlays.  Now enable show-paren-mode (e.g. by checking
"Paren Match Highlighting" in the Options menu).  Now repeating `C-c y'
display one additional "* " for each repetition (provided point remains
on `(').  Surprisingly, however, typing `M-x my-test' repeatedly does
not add any "* ".  In addition, typing a self-inserting character and
then deleting it has the effect that typing `C-c y' no longer adds "* ";
except after widening and removing the overlays: then repeating `C-c y'
adds "* " once only, as when show-paren-mode is disabled.

Is the described behavior after enabling show-paren-mode expected?  If
so, can anyone explain it?

Steve Berman

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