bug#57813: Icon images are non-functional

[Juri Linkov]

> It seems in some cases this can cause an overlap between the heading and
> the image, like on the screenshot:

It's hard to guess whether the outlines are using buttons or margins.
Probably buttons, because margins are not displayed in *Completions*.
Actually the same problem exists for both of them.  By default the
button/margin face is inherited, because in most modes inheriting
fontification from the outline heading lines provides a nicer look,
but your example demonstrates that in some cases that face needs to be
overridden.  Then there are several variants what face to use instead:
1. the default face; 2. the outline faces outline-1, outline-2, ...
corresponding to the outline level.  So it's not clear what to prefer here.

But there is a simpler way to avoid such a problem: it's possible
to customize the face 'completions-group-separator' to remove
the face attribute 'strike-through'.  Then you can also use:
(setq-local outline-minor-mode-highlight 'override)
that will use the outline faces on the group headings,
and the button/margin will inherit it.