Enable will-change property for servo

[yezhizhen]

Needed for servo/servo#35470

Servo PR: servo/servo#35787

cc @xiaochengh

[Loirooriol]

LGTM, but this will need a Servo PR with the implementation before landing.

[yezhizhen]

I added a function, fn contains_critical_properties. I believe it is needed, to check if certain values are contained in will-change

I am also referring to https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_positioned_layout/Understanding_z-index/Stacking_context to check the list of properties whose non-initial value would create a stacking context.

[yezhizhen]

I am almost done. I have one question before Servo PR tho: change_bits_for_longhand for servo simply returns empty WillChangeBits. In that case, STACKING_CONTEXT_UNCONDITIONAL is never enabled in servo, even with filter/backdrop-filter.

stylo/style/values/specified/box.rs

Lines 1023 to 1034 in 8c57436

	#[cfg(feature="servo")]
	fn change_bits_for_longhand(_longhand: LonghandId) -> WillChangeBits { WillChangeBits::empty() }
	#[cfg(feature = "gecko")]
	fn change_bits_for_longhand(longhand: LonghandId) -> WillChangeBits {
	match longhand {
	LonghandId::Opacity => WillChangeBits::OPACITY,
	LonghandId::Contain => WillChangeBits::CONTAIN,
	LonghandId::Perspective => WillChangeBits::PERSPECTIVE,
	LonghandId::Position => {
	WillChangeBits::STACKING_CONTEXT_UNCONDITIONAL | WillChangeBits::POSITION
	},

I tried to enable it for servo. But would instead get errors like:

error[E0599]: no variant or associated item named `Contain` found for enum `generated::LonghandId` in the current scope
     --> D:\stylo\style\values\specified\box.rs:1026:21
      |
1026  |         LonghandId::Contain => WillChangeBits::CONTAIN,
      |                     ^^^^^^^ variant or associated item not found in `LonghandId`

Do you think I should enable these? If so, how?

[Loirooriol]

You can enable these properties with engines="gecko servo" and then disable them at runtime with servo_pref="layout.unimplemented".

Or merge both change_bits_for_longhand definitions with a bunch of #[cfg(feature = "gecko")].

Or copy the gecko definition for servo, just removing the properties that are disabled.

Maybe the 1st option is better because otherwise we may forget to update change_bits_for_longhand when we enable them in the future. But no strong opinion.

[yezhizhen]

I merged with latest main Stylo branch.