bug#52542: 29.0.50; Emoji makes lines taller

bug#52542: 29.0.50; Emoji makes lines taller

[Rudolf Adamkovič via Bug reports for GNU Emacs, the Swiss army knife of text editors]

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On macOS 11/12, in Emacs 28/29, any emoji (e.g. alien) makes the line with it taller than the lines with no emojis, which breaks various things across Emacs, such as avatars in telega.el (see the attachment).

To reproduce:

1. start 'emacs -Q'

2. type

> foo
> 👽 bar
> baz

3. delete the alien

Actual: the line with "bar" becomes less tall

Expected: all lines always have constant height


[-- Attachment #2: Broken avatars in Telega --]
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In GNU Emacs 29.0.50 (build 10, x86_64-apple-darwin21.1.0, NS appkit-2113.00 Version 12.0.1 (Build 21A559))
 of 2021-12-15 built on Workstation.local
Repository revision: 5a057f6716e6991a60968e48f849da57a0339735
Repository branch: master
Windowing system distributor 'Apple', version 10.3.2113
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-- 
"'Contrariwise,' continued Tweedledee, 'if it was so, it might be; and if it were so, it would be; but as it isn't, it ain't. That's logic.'" -- Lewis Carroll, Through the Looking Glass

Rudolf Adamkovič <salutis@me.com> [he/him]
Studenohorská 25
84103 Bratislava
Slovakia

bug#52542: 29.0.50; Emoji makes lines taller

[Eli Zaretskii]

> Date: Thu, 16 Dec 2021 09:28:14 +0100
> From:  Rudolf Adamkovič via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> On macOS 11/12, in Emacs 28/29, any emoji (e.g. alien) makes the line with it taller than the lines with no emojis, which breaks various things across Emacs, such as avatars in telega.el (see the attachment).
> 
> To reproduce:
> 
> 1. start 'emacs -Q'
> 
> 2. type
> 
> > foo
> > 👽 bar
> > baz

Emacs displays the line as tall as needed for the tallest character
shown on that line.  This is normal, and not a bug.

> 3. delete the alien
> 
> Actual: the line with "bar" becomes less tall
> 
> Expected: all lines always have constant height

Your expectations are unfounded, IMO.  Emacs cannot possibly change
the fonts you have on your system, and cannot truncate glyphs of the
Emoji characters if they are taller than the font used for the default
face.  So it has no alternative but to make the line's height larger.

The only solution to that could be if you configure the fonts
installed on your system to have a font which supports Emoji and whose
height is not larger than that of the font used for the default face.

Bottom line is: I don't see how this is an Emacs bug nor how to solve
this in Emacs.

Thanks.

bug#52542: 29.0.50; Emoji makes lines taller

[Rudolf Adamkovič via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Eli Zaretskii <eliz@gnu.org> writes:

> Emacs displays the line as tall as needed for the tallest character
> shown on that line.  This is normal, and not a bug.
>
>> 3. delete the alien
>> 
>> Actual: the line with "bar" becomes less tall
>> 
>> Expected: all lines always have constant height
>
> Your expectations are unfounded, IMO.  Emacs cannot possibly change
> the fonts you have on your system, and cannot truncate glyphs of the
> Emoji characters if they are taller than the font used for the default
> face.  So it has no alternative but to make the line's height larger.

Interesting.  I based my expectation on the built-in Terminal
application on macOS.  It uses the same default font as Emacs, yet
emojis do not change the height of its lines.

> The only solution to that could be if you configure the fonts
> installed on your system to have a font which supports Emoji and whose
> height is not larger than that of the font used for the default face.

Cannot Emacs, when defaulting to "Apple Color Emoji", make the emojis a
bit smaller?  Terminal seems to do that.

> Bottom line is: I don't see how this is an Emacs bug nor how to solve
> this in Emacs.

As Apple often says in their keynotes, "Only Apple can do that." :)

Rudy

-- 
"Logic is a science of the necessary laws of thought, without which no
employment of the understanding and the reason takes place." -- Immanuel
Kant, 1785

Rudolf Adamkovič <salutis@me.com> [he/him]
Studenohorská 25
84103 Bratislava
Slovakia

bug#52542: 29.0.50; Emoji makes lines taller

[Eli Zaretskii]

> From: Rudolf Adamkovič <salutis@me.com>
> Cc: 52542@debbugs.gnu.org
> Date: Fri, 17 Dec 2021 22:57:54 +0100
> 
> > Your expectations are unfounded, IMO.  Emacs cannot possibly change
> > the fonts you have on your system, and cannot truncate glyphs of the
> > Emoji characters if they are taller than the font used for the default
> > face.  So it has no alternative but to make the line's height larger.
> 
> Interesting.  I based my expectation on the built-in Terminal
> application on macOS.  It uses the same default font as Emacs, yet
> emojis do not change the height of its lines.

If you can look into its sources and tell what is it doing, we could
perhaps use some of those ideas, if they fit how Emacs works with
fonts.  The main issue here is not the default font, it's the font
used for Emoji; I believe it's a different font in both cases.

> Cannot Emacs, when defaulting to "Apple Color Emoji", make the emojis a
> bit smaller?  Terminal seems to do that.

The Emoji come out as small or as large as the font tells us: the
metrics of each glyph are dictated by the font.  Emacs selects an
Emoji (and any other) font's size to match as well as possible the
size of the default face's font, so Emacs attempts to make the size
the same up front.  It is possible that we somehow miscalculate the
size, or fail to find the right match, or something like that.  But
once the font matching the default face's size is selected, Emacs
never changes the metrics of the font glyphs as provided by the font.