I don't see anything inside the Flexbox spec that says we should honor these properties unless the flexbox container is inside a fragmentainer. We're essentially treating the flexbox container itself as a fragmentainer which seems wrong to me. Specifically, 9.3.5 says "(or until a forced break is encountered, see §10 Fragmenting Flex Layout)": https://drafts.csswg.org/css-flexbox/#main-sizing But all of §10 only applies on the condition there is a fragmentainer. https://drafts.csswg.org/css-flexbox/#pagination so the "until a forced break is encountered" only applies on condition there is a fragmentainer. Neither css-flexbox nor css-break mentions that a flexbox container can act as a fragmentainer, AFAICT. https://drafts.csswg.org/css-break

I can sympathize with the use case though - forcing the item to start a new row. I think we should ask the CSSWG for a separate feature to support this use case, and possibly make it work for Grid item placement and block line breaking too.

Yeah, looks like this spec-text has changed. When I implemented this behavior, the spec said: > Otherwise, starting from the first uncollected item, > collect consecutive items one by one until [...], > or until a forced break is encountered. [...] > A break is forced wherever the CSS2.1 > page-break-before/page-break-after [CSS21] or the > CSS3 break-before/break-after [CSS3-BREAK] properties > specify a fragmentation break. Here's a snapshot of the spec from back then https://www.w3.org/TR/2014/WD-css-flexbox-1-20140325/#algo-line-break

(I'm mostly out today, but I'll take a closer look at this over the weekend or next week, and perhaps email the editors to clarify whether they think forced line-breaking should be possible.)

Pushed by aethanyc@gmail.com: https://hg.mozilla.org/integration/autoland/rev/c8cc6f915008 Don't honor forced break properties when generating flex lines. r=dholbert