bug#21710: 25.0.50; self-insert-command before a folded outline inserts after the outline

[Oleh Krehel <ohwoeowho@gmail.com>]

Eli Zaretskii <eliz@gnu.org> writes:

> The reason for this is that on all header lines but the last inserting
> the first 't' makes it invisible.

This isn't a good behavior. `self-insert-command' should not put text
into an invisible region. Instead it should put it ahead of it.

"C-e" should complement this behavior, starting from:

* |foo...
* bar...

pressing "C-e" should result in:

* foo|...
* bar...

And from the state above, pressing "C-d" should result in:
(user-error "Overlay read-only"), instead of deleting hidden characters.

Actually, the points above are often a source of confusion and
frustration. I wouldn't be surprised that this bug was reported before.
Currently, I work around this problem either by unfolding all outlines
while I'm editing, or transforming "foo|" into "foot|" with "C-b ot C-d"
instead of a simple "t".

> Then point adjustment kicks in, and moves point out of the
> invisible region, so the second 't' is inserted at the end of the
> sub-level text.

Can you point me to a place where I can read up on this point
adjustment? I'd like to help in fixing this bug, if it's not too
complicated. So far, I'm looking at this code:

(defun outline-flag-region (from to flag)
  "Hide or show lines from FROM to TO, according to FLAG.
If FLAG is nil then text is shown, while if FLAG is t the text is hidden."
  (remove-overlays from to 'invisible 'outline)
  (when flag
    ;; We use `front-advance' here because the invisible text begins at the
    ;; very end of the heading, before the newline, so text inserted at FROM
    ;; belongs to the heading rather than to the entry.
    (let ((o (make-overlay from to nil 'front-advance)))
      (overlay-put o 'evaporate t)
      (overlay-put o 'invisible 'outline)
      (overlay-put o 'isearch-open-invisible
		   (or outline-isearch-open-invisible-function
		       'outline-isearch-open-invisible))))
  ;; Seems only used by lazy-lock.  I.e. obsolete.
  (run-hooks 'outline-view-change-hook))

The comment about `front-advance' implies that what I proposed above was
the intended behavior, which is broken currently.