bug#43506: 26.1; line-height sometimes has no effect on the line height

[Eli Zaretskii]

> From: Markus Triska <triska@metalevel.at>
> Cc: 43506@debbugs.gnu.org
> Date: Sat, 19 Sep 2020 12:07:24 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >      There are several ways to explicitly specify a larger line height,
> >   either by specifying an absolute height for the display line, or by
> >   specifying vertical space.  However, no matter what you specify, the
> >   actual line height can never be less than the default.
> 
> I find that this is not the case: For example, if I change "3" to "t" in
> the snippet I posted, then I get "0" in the minibuffer, indicating that
> the line height can become as low as 0 by using this property.

The value t is not a valid value for the line-height property.  So you
are invoking "unspecified behavior" here by using it.

> > IOW, this property can only enlarge the line's height, which is
> > confirmed by the fact that using 300 in your example does work as
> > expected.
> 
> Please see above: The property seems to be usable to get very small line
> heights too.

That the invalid value t produces a zero-height screen line might be a
separate bug in the display engine, but AFAICT it's harmless: the
cursor is displayed normally, and a zero-height screen line is useless
anyway, because you cannot show anything in that line.

> However, it does not seem to work for integers.

As documented.

> > The documentation already mentions the limitations, see above.
> 
> The limitations seem not to be correctly documented at the moment.

The limitations are documented; what happens when you use invalid
values isn't (and doesn't have to be).