1645642 - Dynamic reflow roots don't handle things properly when they themselves need to be fragmented

Status: NEW

(Core :: Layout, defect, P3)

Description

[Daniel Holbert [:dholbert]]

This is a followup to bug 1644819.

STR:

1. Load https://jsfiddle.net/dholbert/e1cp9a63/ in Nightly (or in release with layout.dynamic-reflow-roots.enabled = true)
2. Click the button in the testcase.

EXPECTED RESULTS: The added teal box should fragment between the columns.

ACTUAL RESULTS: It doesn't fragment. It just overflows.

If you remove display:flow-root or will-change:transform, then you get EXPECTED RESULTS.
Similarly, if you disable dynamic reflow roots, then you get EXPECTED RESULTS.

This indicates that we're breaking the layout by promoting #dynamicRRCandidate to be a dynamic reflow root.

See bug 1644819 comment 11 & bug 1644819 comment 12 for more thoughts on this. The patch here may want to partly or entirely back out the "part 2" patch from that bug, because the fix here may need to be something more severe than that bug's fix (in a way that makes bug 1644819 part 2 unnecessary)

Comment 1

[Ting-Yu Lin [:TYLin] (PST, UTC-8)]

Daniel, could you update the testcase so that #dynamicRRCandidate is display:flow-root to match the description in comment 0?

Comment 2

[Daniel Holbert [:dholbert]]

Oops, sorry about that -- fixed (and used jsfiddle "set as base" to update the original jsfiddle at https://jsfiddle.net/dholbert/e1cp9a63/ )

(apparently I had modified the testcase to double-check the non-dynamic-reflow-root behavior, and I had posted that version instead. :))

Comment 3

[Daniel Holbert [:dholbert]]

Attachment: testcase 1 (automatic, no interaction needed)

Comment 4

[Daniel Holbert [:dholbert]]

Attachment: reference case 1 (same as testcase, just with the "will-change: transform" commented out)