Bug 312606 – [threaded-animations] additive transform animations use matrix interpolation which does not account for additivity

• Component: Animations
• Status: RESOLVED FIXED

[Antoine Quint]

Created attachment 479156 [details]
Test

Consider this simple animation:

    @keyframes scale {
        0%, 100% { transform: scale(2) }
    }

… and this additional style:

    div {
        transform: translateX(100px);
        animation: scale 1s infinite;
        animation-composition: add;
    }

This should yield a computed value of `transform: translateX(100px) scale(2)` but we end up instead in `void TransformationMatrix::blend(const TransformationMatrix&, double, CompositeOperation)` which has no idea about how to account for additivity.

[Radar WebKit Bug Importer]

<rdar://problem/175039068>

[Antoine Quint]

Pull request: https://github.com/WebKit/WebKit/pull/63008

[Antoine Quint]

Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/59327

[Antoine Quint]

Committed 311516@main (31a9b52): https://commits.webkit.org/311516@main