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

[Eli Zaretskii]

> Cc: larsi@gnus.org, yilkalargawworkneh@gmail.com, 56820@debbugs.gnu.org
> Date: Sun, 04 Sep 2022 21:09:14 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> 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

Some more problems:

  . how does one turn the buttons on and off?  changing the value of
    outline-minor-mode-use-buttons seems to work only in one direction
    -- to turn them on, and even for that, I need to click "Show All"
    first?  I cannot seem to be able to turn the buttons off
    afterwards: even turning off outline-minor-mode doesn't remove
    them from display
  . it seems to be impossible to force Emacs to use emoji for buttons
    if the U+1F7E0 character doesn't have a font -- this is okay as
    the default, but if the user insists on using emoji and only
    emoji, why not let them?  (I wanted to try emoji because
    outline-minor-mode doesn't define the image alternative, and I
    don't like the way the symbol alternative looks on display)
  . the buttons have a dark gray background that doesn't look good.
    what's worse, they have a darker (black?) 2-pixel margins on left
    and right, which disappear when the mouse pointer hovers above the
    button and the button is shown in mouse-face.  the fact that these
    "margins" disappear explains why the arrow symbol moves
    horizontally when mouse pointer is moved across the button