Perfherder has detected a browsertime performance regression from push 661802e1edce3d2a0e966f9f9e21838f50658b7e. As author of one of the patches included in that push, we need your help to address this regression.

Please acknowledge, and begin investigating this alert within 3 business days, or the patch(es) may be backed out in accordance with our regression policy.

Support & Actionable Steps

For questions, please contact aesanu@mozilla.com. You can also find help on Slack (#perf-help) or Matrix (#perftest).

Action guide details:

• See the guide to handling regression bugs for investigation steps.
• Review the alert summary for affected tests, graphs, and comparisons.
• Check the PerfCompare results.
• Tests on Try: Run all tests using ./mach try perf --alert 50685 (See documentation link for details).
• Profiling jobs: Trigger jobs from Treeherder job view or request them from aesanu@mozilla.com.

Set release status flags based on info from the regressing bug 2041551

When layout.abspos.fragment-aware-inline-cb.enabled=true, nsBlockFrame needs to recursively traverse all inline descendants to reflow abspos descendants [1]. If the block subtree does not contain any abspos descendants, this traversal is not needed. I'm investigating if we can set a bit indicating "this block has abspos descendants" to avoid that traversal.

[1] https://searchfox.org/firefox-main/rev/8e16ea8e5df0354144902ae0911a5609344bdb6d/layout/generic/nsBlockFrame.cpp#1802-1807

ReflowAbsoluteFramesInInlineFrame() always returns Some() if any abspos is
reflowed. This patch makes WalkInlineDescendantsToReflowAbsoluteFrames()
behave the same.

This prepares for the next part, where we can examine the returned value to
decide if any abspos element is reflowed.

This is a speed optimization, and does not change the behavior.

https://hg.mozilla.org/mozilla-central/rev/5c809ce03bc3
https://hg.mozilla.org/mozilla-central/rev/45cb5c8c5f2d
https://hg.mozilla.org/mozilla-central/rev/92c2cb7f67cf