1703213 - Firefox layout is different from other browsers if there are IDEOGRAPHIC SPACE (U+3000)

Status: VERIFIED FIXED

(Core :: Layout: Text and Fonts, defect)

Description

[Alice0775 White]

Attachment: IDEOGRAPHIC_SPACE.html

Steps to reproduce:

1. Open https://www.city.daito.lg.jp/site/covid-19/23963.html
   or attached testcase

Actual results:
Firefox layout is different from other browsers

Regression window:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=4e3362864fbd&tochange=264f0a7a878c

Suspect: Bug 765166

Comment 1

[Alice0775 White]

Attachment: screenshot

Comment 2

[Jonathan Kew [:jfkthame]]

I think the primary issue here is that Gecko treats U+3000 like a generic ideographic character, allowing a line-break before or after it, whereas the other browsers only allow a break after it (and not within a sequence of multiple U+3000 characters).

A reduced testcase that shows this, even for single U+3000 characters:

data:text/html;charset=utf-8,<div style="background:yellow;width: 3.5em">%E3%80%87%E3%80%80%E3%80%87%E3%80%80%E3%80%87%E3%80%80%E3%80%87

This renders differently for me in each of Safari, Chrome and Firefox.

There was some discussion related to this in https://github.com/w3c/csswg-drafts/issues/2500.

Comment 3

[Jonathan Kew [:jfkthame]]

This should be superseded by the Unified Segmenter work and migration to a more UAX14-based line-breaker in the fairly near future. Still, I think it's worth applying a simple fix for now in our existing line-break implementation, to improve compatibility here (and it turns out to fix a whole bunch of currently-failing WPT tests).

Comment 4

[Jonathan Kew [:jfkthame]]

Attachment: Bug 1703213 - Disallow soft line break between adjacent IDEOGRAPHIC SPACE characters. r=m_kato

Comment 5

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/dab1af643107
Disallow soft line break between adjacent IDEOGRAPHIC SPACE characters. r=m_kato

Comment 6

[Alexandru Michis [:malexandru]]

https://hg.mozilla.org/mozilla-central/rev/dab1af643107

Comment 7

[Alice0775 White]

I verified fix in Nightly89.0a1(20210408215439).