Lars Ingebrigtsen <larsi@gnus.org> writes:

> I think we wanted to introduce mode line constructs to ensure a minimum
> width to elements (like "L13") for those that want that, and somehow
> have variable pitch fonts that have numbers of unequal width (uncommon
> as those fonts are).

How about installing the attached for the `header-line' face while we
work on sorting out all that?

AFAICT, tabulation already works there (e.g. `tabulated-list-mode'), and
it would allow us to give this some testing before Emacs 28.