Emacs: TODO indicator on left fringe has a strange side-effect - deleting characters

Question

I just read Emacs :TODO indicator at left side, and tried it out. It seems intriguing. The little indicator triangles appear, but I'm getting a weird side effect: the text itself is being altered. Characters are being deleted.

Before:

After:

The mode-line does indicate that the buffer has been altered after running annotate-todo. What explains this?

(I'm using emacs 22.2.1 on Windows)

Answer 1

Ahhh... I see the error of my ways earlier. Here's a new version.

(defun annotate-todo ()
  "put fringe marker on TODO: lines in the curent buffer"
  (interactive)
  (save-excursion
    (goto-char (point-min))
    (while (re-search-forward "TODO:" nil t)
      (let ((overlay (make-overlay (- (point) 5) (point))))
        (overlay-put overlay 'before-string (propertize (format "A")
                                                        'display '(left-fringe right-triangle)))))))

The first solution used a the 'display text property, which changes how the specified text is displayed, in this case it was replaced by the triangle in the left fringe. What I needed to do was to use a 'before-string overlay instead. Which doesn't change the string being displayed.

Another advantage, the cut/paste of the code annotated by this does not carry the markup.

I've updated the code in the original question to reflect this change as well.