1402217 - stylo: custom property referring variable with cycle dependency is not marked invalid correctly

Status: RESOLVED FIXED

(Core :: CSS Parsing and Computation, defect, P3)

Description

[Xidorn Quan [:xidorn] UTC+10]

Attachment: testcase

See the testcase.

There are three custom properties in the target element:
> --varA: var(--varB);
> --varB: var(--varA);
> --varC: var(--varB);

apparently, all of the three should be invalid, because --varA and --varB forms a dependency cycle, and --varC refers to this cycle.

However, stylo fails to mark --varC invalid in this case, but instead inherits it from the parent.

Both Gecko and Blink agree on the correct behavior (that --varC computes to its initial value rather than inheriting).

Web platform test css/css-variables-1/variable-substitution-variable-declaration.html checks this behavior.

Comment 1

[Xidorn Quan [:xidorn] UTC+10]

I wouldn't say this is super important, but it is probably something we want to fix for shipping stylo.

Comment 2

[Xidorn Quan [:xidorn] UTC+10]

Attachment: another testcase

The current algorithm is actually very unreliable.

For a testcase like this, the cycle removal can ignore certain items depending on the order of traversal. Moving declarations around within #outer rule can lead to different result.

Comment 3

[Xidorn Quan [:xidorn] UTC+10]

Actually, comment 2 and comment 0 are different issues. There is no test for the issue of comment 2 at the moment.

I'll file a new bug for that one.

Comment 4

[Xidorn Quan [:xidorn] UTC+10]

Attachment: Servo PR

Comment 5

[Xidorn Quan [:xidorn] UTC+10]

https://hg.mozilla.org/integration/autoland/rev/8dad665a3b1a
https://hg.mozilla.org/integration/autoland/rev/d2b23d922b73

Comment 6

[Bobby Holley (:bholley)]

Please either request uplift or set 57 status to wontfix.

Comment 7

[Xidorn Quan [:xidorn] UTC+10]

I don't think it's important to uplift this. It is a behavior change, but mostly for handling error case.