Bug 239670 – [cssom] Some longhands indexed in computed style are not serialized

• Component: CSS
• Status: RESOLVED FIXED

[Oriol Brufau]

A computed style only indexes declarations for longhand properties:

    var cs = getComputedStyle(document.body);
    var longhands = new Set(cs);
    longhands.has("perspective-origin-x"); // true
    longhands.has("perspective-origin-y"); // true
    longhands.has("perspective-origin"); // false

However, then it fails to serialize some longhands:

    cs.getPropertyValue("perspective-origin-x"); // ""
    cs.getPropertyValue("perspective-origin-y"); // ""
    cs.getPropertyValue("perspective-origin"); // "960px 123.9375px"

If only the shorthand can be serialized, either index the shorthand instead of the longhands (perspective-origin-x/y are actually not standard, it may be OK to pretend they don't exist), or let the longhands serialize properly.

[Radar WebKit Bug Importer]

<rdar://problem/92539244>

[Oriol Brufau]

Created attachment 458684 [details]
Patch

[EWS Watchlist]

This patch modifies the imported WPT tests. Please ensure that any changes on the tests (not coming from a WPT import) are exported to WPT. Please see https://trac.webkit.org/wiki/WPTExportProcess

[Oriol Brufau]

Created attachment 458700 [details]
Patch

[Oriol Brufau]

Is this good, or should I instead exclude these longhands from being indexed in the computed style?

[Tim Nguyen (:ntim)]

Comment on attachment 458700 [details]
Patch

My issue with this is that perspective-origin-x/y and transform-origin-x/y/z aren't standard. Can we try to unship those (at least the unprefixed versions)?

[Oriol Brufau]

I'm afraid of compat risks of completely unshipping them.
But the option of not indexing them in computed styles would be closer to that.

[Tim Nguyen (:ntim)]

(In reply to Oriol Brufau from comment #7)
> I'm afraid of compat risks of completely unshipping them.
> But the option of not indexing them in computed styles would be closer to
> that.

Chrome ships the prefixed versions, but not the unprefixed ones. Seems like we could do a similar thing?

For this bug specifically, perspective-origin should be indexed, but not the other 2 longhands.

[Oriol Brufau]

Created attachment 458713 [details]
Patch

[Oriol Brufau]

(In reply to Tim Nguyen (:ntim) from comment #8)
> (In reply to Oriol Brufau from comment #7)
> > I'm afraid of compat risks of completely unshipping them.
> > But the option of not indexing them in computed styles would be closer to
> > that.
> 
> Chrome ships the prefixed versions, but not the unprefixed ones. Seems like
> we could do a similar thing?

Chrome treats both perspective-origin and -webkit-perspective-origin-{x,y} as longhands that share a computed value. But that's so broken for revert-layer: https://bugs.chromium.org/p/chromium/issues/detail?id=1319414

So I prefer not to do that.

> For this bug specifically, perspective-origin should be indexed, but not the
> other 2 longhands.

I have attached a new patch that does that, without unshipping the non-standard longhands.

[Darin Adler]

I don’t mean to override Tim’s judgement here. May want to hear from him before landing.

[Oriol Brufau]

To summarize the problem with unshipping perspective-origin-x/y while keeping -webkit-perspective-origin-x/y:

 a) If perspective-origin becomes a longhand, we have a problem with revert-layer (same as Blink).
 b) If perspective-origin becomes a shorthand of -webkit-perspective-origin-x/y, then we have the same problem as now:
    i) If -webkit-perspective-origin-x/y are indexed in computed styles instead of perspective-origin, then they should serialize.
    ii) Otherwise -webkit-perspective-origin-x/y should be skipped, and index perspective-origin instead.


Option a) seems undesirable. And option b) seems orthogonal: whatever needs to be done with -webkit-perspective-origin-x/y when unshipping perspective-origin-x/y is the same that needs to be done to perspective-origin-x/y when not unshipping them.

So I would land the patch as-is. Option b) can be considered in another bug.

[Tim Nguyen (:ntim)]

(In reply to Darin Adler from comment #11)
> I don’t mean to override Tim’s judgement here. May want to hear from him
> before landing.

The current patch is fine as it is. I'll file a new bug to unship the unprefixed non-standard longhands.

[EWS]

Committed r294002 (250438@main): <https://commits.webkit.org/250438@main>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 458713 [details].