Blitz tracking PR

[nicoburns]

Putting this up to track the Stylo changes we have made for Blitz and that we would like to see upstreamed into Servo's version in this repo. Servo will also want these in it's own right as it comes to implement a full implementation of the CSS Grid, gap, and alignment styles.

Not necessarily expecting this to merged as-is (UPDATE: given improvements to the feature flagging, I feel like this probably could be merged / upstreamed now).

Corresponding Servo PR: https://github.com/servo/servo/pull/32686

Upstream submissions

• https://bugzilla.mozilla.org/show_bug.cgi?id=1906395
• https://bugzilla.mozilla.org/show_bug.cgi?id=1906994
• https://bugzilla.mozilla.org/show_bug.cgi?id=1906995

Changes made

• Built on top of https://github.com/servo/stylo/pull/51
• Cherry-picked https://github.com/servo/stylo/commit/ddb7af7127e6c6c64e6202fab00d42aa4b33a5c3 from upstream
• Remove alignment style adjustment function (which should be implemented in layout anyway - implemented in https://github.com/servo/servo/pull/32686)
• Add / make public a couple of helper methods to AlignFlags
• Switch to Gecko's representation of alignment styles
• Enable row-gap and the shorthand gap (behind flexbox flag)
• Switch column-gap to being behind the flexbox flag rather than the multi-column layout flag
• Enable CSS Grid styles (styles shared by flexbox and grid are behind the flexbox flag, so you need to enable both for grid support)
• Enable aspect_ratio in layout_2020 (this is now behind the flexbox flag rather than the legacy-layout flag)

Notes

• I think the "use gecko alignment implementation" changes probably could be upstreamed as-is (but this will require a corresponding servo PR)
• The grid-only styles are also quite straightforward because they're behind a flag.

• The case where it's more tricky is the styles that are shared by Flexbox and Grid because Stylo currently only allows styles to be gated behind a single pref. There are a few options here:

  1. We could just make them enabled unconditionally on the basis that implementation are coming soon.
  2. We could update stylo to accept a list of prefs. This would be quite easy to implement but would have a small additional runtime cost.
  3. We could add separate "alignment" and "gap" preferences that need to be enabled seperately. This would be simple but would be cumbersome when trying to actually use them.
  4. We could do the separate prefs as above + add some mechanism to automatically enable prefs

  I'd probably favour (ii).

  Currently based on: 2024-05-31

[nicoburns]

Upgraded to 2024-05-31

[nicoburns]

I've cherry-picked ddb7af7127e6c6c64e6202fab00d42aa4b33a5c3 from upstream, which along with https://github.com/servo/stylo/pull/51 makes the diff (both against main and upstream a lot cleaner).