bug#73752: 29.4; Ligatures are randomly rendered with extra spaces

[Eli Zaretskii <eliz@gnu.org>]

> From: Visuwesh <visuweshm@gmail.com>
> Cc: luangruo@yahoo.com,  73752@debbugs.gnu.org,  xuan@xlk.me
> Date: Mon, 28 Oct 2024 09:56:13 +0530
> 
> Real-life Lisp programs certainly do not change face font attributes so
> often but I believe the script does it so to reproduce the issue
> quickly.  In a regular Emacs session, it is enough for the same text to
> be shown in different font sizes (as a consequence of using C-x C-+) and
> font weights to eventually exhibit this misalignment IME.

You need to catch this situation in some reproducible recipe.  Because
up front I don't understand how is this possible: we cache each
composition with its font object, which includes the font size (and
also slant, weight, etc.), so a different variant of the same font
ought to match only cache entries that use the exact same font.  Or
maybe I don't understand well enough what
composition_gstring_put_cache does to compute the hash of a
glyph-string header (which is the hash key for a composition)?

> I do not understand the technical details but the width of the glyph
> used to draw it is not the one that should be used for the underlying
> font (weight, size, etc. included) which leads to this misalignment.

Which seems to indicate that we somehow use a different font's metric.

> To make it more clear, let's say that =:= is shaped for a font X with a
> specific weight, size, etc.  At a later point in time, the width of the
> glyph corresponding to X is used to draw =:= with font Y of same family.
> This leads to the observed misalignment AFAIU.

But how can this happen?  Without a reproducible recipe, which can be
reproduced without waiting for too long, it is very hard to
investigate this.