>     Jesse> diff --git a/doc/lispref/frames.texi b/doc/lispref/frames.texi
>     Jesse> index 618ea16fcf..75a6725f03 100644
>     Jesse> --- a/doc/lispref/frames.texi
>     Jesse> +++ b/doc/lispref/frames.texi
>     Jesse> @@ -1857,6 +1857,11 @@ Layout Parameters
>     Jesse>  Additional space to leave below each text line, in pixels (a positive
>     Jesse>  integer).  @xref{Line Height}, for more information.
>
>     Jesse> +@vindex line-spacing-vertical-center@r{, a frame parameter}
>     Jesse> +@item line-spacing-vertical-center
>     Jesse> +If non-nil, centers the line content vertically when using
>     Jesse> +using the @code{line-spacing} variable.
>     Jesse> +
>
> I donʼt think you implemented this as a frame parameter, which means the
> documentation should go in "@node Line Height" in display.texi, or you
> could implement the frame parameter as well. [1]
>
>     Jesse>    if (extra_line_spacing > 0)
>     Jesse>      {
>     Jesse> -      it->descent += extra_line_spacing;
>     Jesse> +      if (! BVAR (XBUFFER (it->w->contents), line_spacing_vertical_center))
>
> I think this currently works because Qnil == 0, for now, but you should
> probably use NILP.

I've worked the on these issues you raised. Hopefully it's all ok now.