[css-inline-3] Canvas 2D's textBaseline = "middle" neccessitates an em-middle baseline?

[annevk]

It's described as "The middle of the em square".

This is a follow-up to #5312 that only looked at the measureText() API which doesn't expose this.

cc @yiyix @kojiishi @macnmm @litherum

[annevk]

Maybe an alternative is that I describe it as the middle of the em-over and em-under baselines, but we might as well have a term for it?

[fantasai]

@annevk I think you should be able to use the central baseline definition for this, no? Note that vertical-align: middle is defined as using the "ex-middle" baseline, so there will be some inconsistency in terminology between Canvas and CSS.

[annevk]

What is ex-middle? I don't see it in https://drafts.csswg.org/css-inline/#baseline-types.

And are we sure the middle of em-over and em-under is equal to the middle of ideographic-under and ideographic-over? If so, that's probably worth noting in the specification, but it seems unlikely that would be the case?

[fantasai]

@annevk sorry, “x-middle”. You're right, it could differ in the case where the ideographic baselines are not defined by the font and the ascent + descent != 1em, we don't currently define how to find the central baseline in that case. In that case I'd say you should just define it as halfway between em-over and em-under, assuming that's the relationship you want to encode there.

[annevk]

Yeah, though it seems a bit weird that we would sometimes give that an explicit baseline definition (e.g., as for central) and sometimes not.

[fantasai]

@annevk Yeah, but it's a simple relationship between two existing terms (whereas defining what each of these terms are is not). I'll note CSS doesn't use these at all so defining any of them here is a bit weird to begin with :)

[annevk]

We ended up defining this as:

Halfway between the em-over baseline and the em-under baseline