1840944 - Implement `break-before: column` and `break-after: column`

Status: NEW

(Core :: Layout: Columns, enhancement)

Description

[Daniel Holbert [:dholbert]]

Attachment: testcase 1 for break-before/break-after:column

This bug tracks implementation of break-before:column and break-after:column, as well as the associated avoid-column values.

Spec: https://drafts.csswg.org/css-break/#column-break-values

STR:

1. Load testcase.

EXPECTED RESULTS:
There should be a column-break before every italic character, and a column-break after every bold character. Rendering should look like:

a  c  d  e  g
b        f  h
            i
            j

ACTUAL RESULTS:
We render all of the characters in the first column.

Chrome and WebKit produce expected results.

Note: I think this is the reason we fail these three WPT tests, which are testing baseline-alignment of multicol flex items, with forced column-breaks involved:
https://wpt.fyi/results/css/css-flexbox/alignment/flex-align-baseline-multicol-001.html
https://wpt.fyi/results/css/css-flexbox/alignment/flex-align-baseline-multicol-002.html
https://wpt.fyi/results/css/css-flexbox/alignment/flex-align-baseline-multicol-003.html

Comment 1

[Daniel Holbert [:dholbert]]

TYLin, I hope you don't mind if I assign this to you, to look at when cycles are available. Seems to be in your area of expertise, spanning multicol / fragmentation and flexbox. :)

Comment 2

[Daniel Holbert [:dholbert]]

(In reply to Daniel Holbert [:dholbert] from comment #0)

Note: I think this is the reason we fail these three WPT tests, which are testing baseline-alignment of multicol flex items, with forced column-breaks involved:
https://wpt.fyi/results/css/css-flexbox/alignment/flex-align-baseline-multicol-001.html
https://wpt.fyi/results/css/css-flexbox/alignment/flex-align-baseline-multicol-002.html
https://wpt.fyi/results/css/css-flexbox/alignment/flex-align-baseline-multicol-003.html

We fail some analogous grid tests, for what looks like the same reason -- there are forced column-breaks, which we're ignoring, which make us fail these tests:
https://wpt.fyi/results/css/css-grid/alignment/grid-align-baseline-multicol-001.html
https://wpt.fyi/results/css/css-grid/alignment/grid-align-baseline-multicol-002.html
https://wpt.fyi/results/css/css-grid/alignment/grid-align-baseline-multicol-003.html