bug#41250: 28.0.50; Dired displays unconditionally ls-switches on modeline

[Drew Adams <drew.adams@oracle.com>]

> > an integer - show first N chars of switches
> 
> I don't think this is a useful value: the user will rarely know how
> much space is available on the mode line.

A user may know how much space they're willing
to give to this, as a general preference/rule.

Mode-line data can vary considerably, depending
on one's setup, the buffer's mode, and other
things.  And the effective available space
depends on window width.

So of course no particular truncation constant
length will fit all contexts.  Such truncation
is of limited utility, IMO, but I thought that's
what was requested.

Sure, truncation could instead be relative (%).
In that case what it's relative to needs to be
considered.

This is why, in the general case, a function
value is there.  You'll recall that in an earlier
mail I said that truncation can just be done by
such a function.  (Well, at that point I said a
`format' string - that too can truncate.)

What I wrote up is just a simple truncation.  If
you have a better one you want to suggest, fine.

> Also, truncating without showing ellipsis or some
> other sign of truncation is IMO a sub-optimal UI.

Arguable - mode-line space is limited.  But maybe.

I imagine you're suggesting appending a char such
as `.' to whatever truncation is used.  That's
fine by me (though I'm not crazy about that char,
which I find generally illegible).  An alternative
(more readable, but wastes 2 more chars, is `...'.
Another alternative is to surround the set of
switches with delimiters, e.g. "" or '' or [] ...