Bug 251911 – [web-animations] line-height should not transition from default value to a number

• Component: Animations
• Status: RESOLVED FIXED

[Antoine Quint]

Created attachment 464906 [details]
Reduction

We incorrectly interpolate the "line-height" property when it animates from its initial value (normal) to a number. This was reported in https://stackoverflow.com/questions/75151629/css-transitions-in-safari/.

[Antoine Quint]

rdar://104346766

[Antoine Quint]

The problem is that BuilderConverter::convertLineHeight() loses the `<number>` type here:

    if (primitiveValue.isNumber())
        return Length(primitiveValue.doubleValue() * 100.0, LengthType::Percent);

Additionally, the `normal` value is also converted to a percent value here:

    if (valueID == CSSValueNormal)
        return RenderStyle::initialLineHeight();

Where RenderStyle::initialLineHeight() builds this value:

    // Returning -100% percent here means the line-height is not set.
    static Length initialLineHeight() { return Length(-100.0f, LengthType::Percent); }

This means that when we get to deciding whether it's OK to interpolate between `normal` and `0`, which is what the original Stack Overflow example does, we consider two percent values and decide it's OK to interpolate.

We are going to need to:

1. identify the `normal` value with special logic
2. preserve the `number` type somehow

Finally, looking at the spec for `line-height` I see it takes in `<length-percentage>` as a value and yet the animation wrapper does not specify this. So there's likely another bug here where we would fail to yield a transition between "line-height: 10px" and "line-height: 10%" I expect.

[Antoine Quint]

Actually, a transition between "line-height: 10px" and "line-height: 10%" works because we convert the "10%" to a Fixed value as well so we end up having the same type. This code is pretty weird :)

[Antoine Quint]

Alright, so all we need to do is to have a custom `canInterpolate()` override for `line-height` and return false if any value maps to RenderStyle::initialLineHeight(). The default LengthPropertyWrapper logic will otherwise apply since <number> values map to LengthType::Percent and <length-percentage> values map to LengthType::Fixed.

[Antoine Quint]

Pull request: https://github.com/WebKit/WebKit/pull/9807

[Antoine Quint]

Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/38416

[EWS]

Committed 260028@main (221eb1d58b67): <https://commits.webkit.org/260028@main>

Reviewed commits have been landed. Closing PR #9807 and removing active labels.