Does this fix #3727 as well? In particular, do all three of the following lines display identically?

#let test = context math.class("relation", "--")
$ a test b $

$ a class("relation", "--") #metadata("hi") b $

$ a class("relation", "--") b $

They all look identical, but I'm not sure if it's correct as it doesn't look centered.

Edit: Ok, it's just the "--" not being measured correctly, it looks good with other characters.
Edit 2: It already looks like that on main too, so nothing changed with this PR

Why another field instead of just setting spaced to false for ignorant/invisible?

Because then it wouldn't be truly ignorant but just like a normal class frame which influences the spacing between neighboring frames, e.g. vary class operators would become binary in the test cases

// Operator spacing
$#counter("test").update(3)   b$ \
$#place(dx: 5em)[a]   b$

Okay.

I have to admit that I don't really like the growing list of fields on the FrameFragment struct but it seemed to be the easiest way.

Me neither, but I guess one more field won't change anything.

If you could fix the conflict, I'll merge.

Thanks!