bug#56820: outline-minor-mode replacing the first character with an arrow

[Eli Zaretskii]

> Cc: Lars Ingebrigtsen <larsi@gnus.org>, 56820@debbugs.gnu.org
> From: Juri Linkov <juri@linkov.net>
> Date: Sun, 04 Sep 2022 20:02:48 +0300
> 
> > This patch could help to alleviate the problem by keeping
> > the first character displayed on the outline button:
> 
> Pushed.

This doesn't work well:

  . moving the mouse pointer on and off the button causes horizontal
    movement of both the arrow and the following character, the one
    that was added to the overlay's 'display' string (I guess this
    depends on the font used for the arrow characters?);
  . one cannot put the cursor on the first character that's displayed
    on the outline button

I very much hope we can improve the visuals here, because otherwise
the feature looks unfinished at best.

Some related questions:

  . do we really need to hide the first character of the line by the
    overlay? doesn't before-string work?
  . wouldn't it be better if the arrow buttons were displayed in the
    window's margin, and would thus avoid indenting the characters on
    that line wrt the rest of the code?

Thanks.