1510369 - Add a pref to control dynamic reflow roots

Status: RESOLVED FIXED

(Core :: Layout, enhancement, P3)

Description

[Daniel Holbert [:dholbert]]

I think we should add an about:config pref to control whether we allow frames to become Dynamic Reflow Roots (basically just to control the flag-setting in the final patch of bug 1159042).

This will let us:
 - quickly triage regressions to determine whether or not Dynamic Reflow Roots are the cause.
 - easily turn off the feature if it's got late-breaking regressions that turn up  that we can't fix before it hits release.
 - (more importantly) turn it off without backing out the rest of the code in bug 1159042, which will mean we can still potentially ship the optimization for 'contain:layout+size' in bug 1497414 even if we've got some edge cases to sort out around dynamic reflow roots.

I suggest we name the pref "layout.dynamic-reflow-roots.enabled", and we can probably have it default to true for the time being.

Comment 1

[Daniel Holbert [:dholbert]]

Attachment: Bug 1510369 part 1: Make IsFixedPosContainingBlock assert that all fixed-pos CBs must also be abs-pos CBs. r?dbaron

Comment 2

[Daniel Holbert [:dholbert]]

Attachment: Bug 1510369 part 2: Adjust logic in InitDynamicReflowRoot() to stop checking conditions once we've determined we can't be a dynamic reflow root. r?dbaron

This patch does not change behavior - it's purely an optimization to some
existing logic.

Depends on D13131

Comment 3

[Daniel Holbert [:dholbert]]

Attachment: Bug 1510369 part 3: Add about:config pref 'layout.dynamic-reflow-roots.enabled' (enabled by default). r?dbaron

Depends on D13132

Comment 4

[Daniel Holbert [:dholbert]]

Thanks for the review!

Obviously, this layers on top of the patches from bug 1159042, so I'll land this after that bugs' patches have stuck.

Also FWIW, I verified that this patch does let me flip the pref to fix the testcase in bug 1510360.  (i.e. I verified that the pref does indeed allow you to turn off dynamic reflow roots.)

Comment 5

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/1be689a1973e
part 1: Make IsFixedPosContainingBlock assert that all fixed-pos CBs must also be abs-pos CBs. r=dbaron
https://hg.mozilla.org/integration/autoland/rev/e5ea5b3e4d0d
part 2: Adjust logic in InitDynamicReflowRoot() to stop checking conditions once we've determined we can't be a dynamic reflow root. r=dbaron
https://hg.mozilla.org/integration/autoland/rev/563996e91860
part 3: Add about:config pref 'layout.dynamic-reflow-roots.enabled' (enabled by default). r=dbaron

Comment 6

[Bogdan Tara[:bogdan_tara | bogdant]]

https://hg.mozilla.org/mozilla-central/rev/1be689a1973e
https://hg.mozilla.org/mozilla-central/rev/e5ea5b3e4d0d
https://hg.mozilla.org/mozilla-central/rev/563996e91860