bug#43405: Tool bar item doesn't align to the right edge

[Eli Zaretskii]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: 43405@debbugs.gnu.org,  juri@linkov.net
> Date: Fri, 18 Sep 2020 10:38:18 +0200
> 
>     Eli> Some attribute of the binding, similar to :image and :vert-only, I
>     Eli> guess?
> 
>     >> I mean, I have no idea if the macOS or MS-Windows tool bar have a
>     >> similar feature.
> 
>     Eli> I don't know about macOS, but MS-Windows uses the native tool bar
>     Eli> produced by our own code, i.e. it displays a Lisp string in a special
>     Eli> window.
> 
> OK, so I took a look, and Iʼm not sure itʼs possible with the native
> tool bar. We have '(space :align-to right)', but that just inserts
> space up to a specified location, everything subsequent is
> appended. In order to calculate the correct location, Iʼd need to know
> the width of everything that came after the space, which only
> redisplay can tell us, unless thereʼs a function Iʼve missed?

The support for doing this with the native tool bar must be in C, and
should indeed be part of the display engine.  So everything redisplay
knows should be at your fingertips.