Um, this now accepts a 3d line and a 2d point? I'm not clear what the semantics here is or whether this is desired. At the very least we should document the expected input dimensions for the function.

This is an internal helper that I think we pulled out.

Given that @scottshambaugh was seeing this warning we are apparently passing in 3D points in practice.

While this is an odd internal wart, I do not think it is worth the effort we have collectively thrown at this function recently!

My point is that we are doing some ill-defined things in the function. Note that this function is only used in format_coord and I wouldn't be surprised if it's the cause of #22775.

The earlier work #22628 was defensive in only bailing out when both line-defining points were equal in 3d. That does not alter the behavior - it's not making anything worse at least.

I don't know what the correct behavior here should be, but I believe that neglecting the z-direction will add another quirk to the coord determination.
Edit: Rethinking this, since we are anyway only measuring the distance in x, y. This check is not making it worse either. - As little as that is worth.

More generally, I support @anntzer's view that we should stop displaying 3D coords, because we can't get them from a 2d pointer consistently.