bug#59719: 29.0.60; emacs-news-mode does not adjust the width of the fringe to accommodate the outline icons

[Juri Linkov <juri@linkov.net>]

>> set-window-margins specifies the number of characters.
>
> Yes.  But that's not a problem: you can ask for 2 or 3 characters, as
> needed.
>
>> Maybe need to set the margin width to more characters if string-width
>> returns more than 1 character?
>
> Why not use string-pixel-width, and divide by what default-font-width
> returns?

This means to iterate over all icons and get their maximal width.

>> But the core problem is that when font size is increased in the buffer,
>> the margin pixel size doesn't increase proportionally.
>
> This is how the margins were designed: they never resize automatically.  It
> is up to the Lisp program to do that.

There is one thing that I don't understand about margins: could you clarify
how the default width of a character cell is calculated?  Is it based
on the default font size?