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

[Eli Zaretskii <eliz@gnu.org>]

> From: Oleh Krehel <ohwoeowho@gmail.com>
> Date: Mon, 19 Oct 2015 17:16:11 +0200
> 
> To reproduce with "emacs -Q", create a file ~/outline-test with the
> following contents:
> 
> * foo
> some text
> * bar
> some text
> 
> Open the file, move to start, "M-x" `outline-minor-mode',
> "M-x" `outline-hide-sublevels'.
> 
> Move to the end of bar and insert "tt" - it works correctly.
> 
> But move to the end of foo and insert "tt": here's what it looks like
> ("|" is the point):
> 
> * foo...t|
> * bartt...
> 
> Here's the full text after "M-x" `outline-show-all':
> 
> * foot
> some textt
> * bartt
> some text
> 
> As you see, the second "t" was inserted after the outline.

The reason for this is that on all header lines but the last inserting
the first 't' makes it invisible.  On the last line, 't' stays
visible.  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.

This hints at stickiness, but I cannot find anything in the
documentation about stickiness of overlays.