bug#29078: 25.2; font issue with FreeType 2.8; should not use the rounded ascender and descender

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Eli Zaretskii <eliz@gnu.org>,  29078@debbugs.gnu.org
> Date: Mon, 18 Nov 2019 21:31:43 +0100
> 
> Vincent Lefevre <vincent@vinc17.net> writes:
> 
> > On 2019-11-18 19:32:41 +0100, Lars Ingebrigtsen wrote:
> >> Using
> >> 
> >>     ftcrhb:-PfEd-DejaVu Sans
> >> Mono-normal-normal-normal-*-25-*-*-*-m-0-iso10646-1 (#x957)
> >
> > Under Debian/unstable, I don't get it either. Neither with
> >
> >   ftcrhb:-PfEd-DejaVu Sans 
> > Mono-normal-normal-normal-*-15-*-*-*-m-0-iso10646-1
> >
> > But the bug occurs with
> >
> >   ftcrhb:-PfEd-DejaVu Sans 
> > Mono-normal-normal-normal-*-18-*-*-*-m-0-iso10646-1
> >
> > That's for the rendering. I don't know whether the cell height is
> > honored in all these cases.
> 
> Ah, yes.  If I say
> 
> emacs -Q -fn "DejaVu Sans Mono:size=30"
> 
> and vary the size, I get the gaps in the lines for some sizes (like 23)
> but not others (like 25).

Strangely, I see small (1- or 2-pixel) gaps for Courier New on
MS-Windows, and only for a couple of size values.  Moreover, with
DejaVu Sans Mono, I don't see any gaps for all sizes between 9 and 30.

FWIW, the font-related APIs on MS-Windows report glyph metrics as
integer values, so Emacs doesn't round anywhere.  So either MS-Windows
does a better job with that font (unlikely) or there's something else
at work here.

I guess we will have to wait until someone tells us what to do here.