Looks good! I think it makes sense to add a brief What's new for this to make users aware that it is added.

Also, I think it may be enough to have a "lightweight" test for this as it doesn't seem very dependent on the font:

@anntzer I think you as our TeX expert should review this.

Sure, l'll need a couple of days though.

This seems reasonable, but 1) do you know why the overline() implementation is so different, using a fill instead? (which seems inconsistent with the TeXbook); 2) the TeXbook (rules G9/G10 p443) add an extra thickness of whitespace clearance over the overline / under the underline; do you know why they are not needed here (if they indeed aren't)?

the TeXbook (rules G9/G10 p443) add an extra thickness of whitespace clearance over the overline / under the underline

I think that extra space is used to set the minimum leading.

If the leading is high, the vertical distance between the tops of the Ts and the underscores on the previous lines remains unchanged even if the underscores are underlined. (As if the underline wasn't there at all.)

Upon reducing the leading, the Ts remain some distance away from the underline. It's like the underline forces the next line to go lower if it is too close.

I have not looked through Mathtext's leading rules, but it certainly looks like it's better to follow what TeX does. I will update this. Getting second thoughts about this now. Should I add that space at the bottom, or leave this as it is?

do you know why the overline() implementation is so different, using a fill instead

That's a good question, one which I wondered myself, but couldn't figure out. In fact, I haven't completely understood how glue works in Mathtext.