> Given the name of the variable x-underline-at-descent-line, it'd

> be nice if the natural height of the font was used rather than the

> height + extra line spacing to determine where to place the underline.

> Does that make sense? Basically, it'd place the underline at the same

> place regardless of what line-spacing was set to (as if it were set to

> nil).

Something like that should already be possible.  Underline placement is

determined by the following rules, considered in this order:

1. If x-underline-at-descent-line is non-nil, underline at the descent line,

where the descent line is the bottom of the current glyph row, inclusive of

extra line spacing.

2. If x-use-underline-position-properties is non-nil and the font has underline

position info, use that.

3. If the current font can be determined, use half of the font's descent,

"descent" here meaning the maximum amount by which a character from the font

descends below the baseline.

4. Otherwise, use the default offset (1 point).

The only one of these possibilities that should be affected a change in line

spacing is 1.  Is that not what you see?