bug#19905: Provide support for truncation in column data

[Stefan Kangas]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

> There should be a `display' property (call it :truncate-at) to make it
> possible to indicate that the covered text should be truncated (with
> the rest replaced by an ellipsis or something like that) so as not to go
> past some particular position.
>
> This could then be used in things like tabulated-list-mode to get
> properly aligned columns even when using proportional fonts.
>
> Most/all the uses I can think of would combine this :truncate-at with
> an :align-to, so maybe a combined :truncate-or-align would be even
> better, tho usually the two positions aren't equal (we usually want to
> preserve a bit of space between the columns).

See also my recently reported Bug#44315:

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=44315

My proposal is similar, but different.  I'm not sure which would produce
the best results in practice for tabulated-list-mode.