bug#46476: Feature request: Right-aligning part of the modeline

[Eli Zaretskii <eliz@gnu.org>]

> From: Paweł Kraśnicki
>  <paul.krasnicki@gmail.com>
> Date: Fri, 12 Feb 2021 23:59:26 +0100
> 
> Many people who use Emacs with significant customization have their modeline
> split into left and right aligned parts. I've also seen the Lisp code for this
> in several packages.

Can you describe how is this done in Lisp?

> I think it would be useful to have this feature as a
> %-construct so that the users who want it wouldn't have to install a package or
> write custom Lisp that uses the `display' text property.
> 
> Everything after this construct would be aligned to the right. For example,
> assuming the construct was %=, the mode-line-format
> 
> "l%=r"
> 
> would cause the modeline in a 20 character wide window to look like
> 
> l                  r
> 
> It could also be useful to generalize this to centering, though I've seen this
> used less often. Then, the mode-line-format
> 
> "lllll%=c%=r"
> 
> would cause the modeline to look like
> 
> lllll     c        r

The immediate question about this I have is what to do when the
mode-line string is too long for the window's width?  Right now, we
simply chop the stuff on the right that doesn't fit, but if the
mode-line string has 2 or 3 different part, that should be revised,
right?