[css-text-3] line breaks and ideographic space

[frivoal]

(reference: https://bugzilla.mozilla.org/show_bug.cgi?id=1450228)

Browsers differ in how they handle line breaks for IDEOGRAPHIC SPACE (U+3000):

• Firefox allows break before and after
• Chrome and Safari forbid a break before, and allow a break after
• Edge forbids a break before and allow a break after, but also allows hanging overflow

try it here

Quoting @MurakamiShinyu

This is important because IDEOGRAPHIC SPACE is normally used after "！" or "？" in the middle of a paragraph to keep 1em space after such punctuation marks as explained in JLREQ 3.1.6 Positioning of Dividing Punctuation Marks (Question Mark and Exclamation Mark) and Hyphens

That logic would favor Edge's behavior.

UAX-14 says that IDEOGRAPHIC SPACE (U+3000) has class BA (for break after), which would support the behavior of Chrome/Safari/Edge over Firefox's, but:

• I cannot I cannot find any reference in css-text-3 to UAX-14's BA class
• The line-break property does not mention IDEOGRAPHIC SPACE (U+3000)

Should we reference the BA class somehow?
Should we directly list DEOGRAPHIC SPACE (U+3000) in some level of line-break?
Should we say something about allowing/requiring it to hang?

[fantasai]

CSS Text Level 3 does not normatively define the line breaking behavior of every character. Nor does it normatively reference UAX14's tables, because while they are a good start, they do not represent the ideal breaking behavior of every language.

We could add specific rules for ideographic space. Allowing it to hang seems reasonable: if @MurakamiShinyu agrees we should specify that I'm happy to do it. Note that if it hangs, it effectively is BA because it's effectively zero-width when occurring at the end of the line. ;)

[MurakamiShinyu]

Yes, I agree the behavior allowing hanging is preferable.

[frivoal]

I wonder what we should do about runs of IDEOGRAPHIC SPACEs should the whole thing hang? Only the last one? Hopefully we can just treat that as an edge case that's not really how people should write web pages, and spec whatever is simplest.

[frivoal]

@MurakamiShinyu Do you have a suggestion about whether to hang multiple ideographic spaces? Or maybe we should hang one and wrap the rest?

[frivoal]

f55dc74 hangs the whole sequence of ideographic spaces at the end of the line, let us know if that's fine.