Eli Zaretskii <eliz@gnu.org> writes:

> Note a few subtle issues:
>
>  . I've limited the feature to the mode line; programs that set
>    header-line and tab-line either don't want this, or should format
>    those lines to not waste screen space to begin with;

Yup.

>  . I've refrained from squeezing spaces if they have non-default
>    faces, on the assumption that those spaces are significant -- the
>    result is that the trailing spaces of the buffer name are not
>    squeezed, but I think we have no choice here, as down that path
>    lies madness (think a mode line that plays some fancy games with
>    font sizes).

Right, so with the patch, I get these two mode lines (with and without
compaction):