[web-animations] additive/accumulative color blending fails to yield intermediary out-of-bounds values

[Antoine Quint]

We have 28 FAIL results across these WPTs all due to the same issue:

    - web-animations/animation-model/animation-types/accumulation-per-property-001.html
    - web-animations/animation-model/animation-types/accumulation-per-property-002.html
    - web-animations/animation-model/animation-types/addition-per-property-001.html
    - web-animations/animation-model/animation-types/addition-per-property-002.html

These run this subtest from web-animations/animation-model/animation-types/property-types.js for `add` and `accumulate` composite operations for a variety of color CSS properties:

    test(t => {
      const idlName = propertyToIDL(property);
      const target = createTestElement(t, setup);
      target.style[idlName] = 'rgb(128, 128, 128)';
      const animation = target.animate(
        {
          [idlName]: ['rgb(255, 0, 0)', 'rgb(0, 0, 255)'],
        },
        { duration: 1000, composite }
      );
      testAnimationSamples(animation, idlName,
                           [{ time: 0,   expected: 'rgb(255, 128, 128)' },
                            // The value at 50% is interpolated
                            // from 'rgb(128+255, 128, 128)'
                            // to   'rgb(128,     128, 128+255)'.
                            { time: 500, expected: 'rgb(255, 128, 255)' }]);
    }, `${property} supports animating as color of rgb() with overflowed `
       + ' from and to values');

The issue is that we always clamp values as we blend colors so we can't have the intermediary representations of rgb(128+255, 128, 128) and rgb(128, 128, 128+255) above for additive and accumulate.

Note that when we blend using `add` or `accumulate`, it will only ever be through some intermediary calls to `CSSPropertyAnimation::blendProperties()` made under `KeyframeEffect::setAnimatedPropertiesInStyle()` which will not produce web-facing values. Only at the end of that method do we call `CSSPropertyAnimation::blendProperties()` with a `replace` composite operation which would be expected to clamp.

[Antoine Quint]

Created attachment 460581 [details]
Test

[Radar WebKit Bug Importer]

<rdar://problem/96593559>

[Sam Weinig]

Pull request: https://github.com/WebKit/WebKit/pull/4674

[EWS]

Committed 254850@main (35a0ba08385c): <https://commits.webkit.org/254850@main>

Reviewed commits have been landed. Closing PR #4674 and removing active labels.