Problem

To match CSS, thegap property should not really have Auto or Undefined variants.

See: bevyengine/bevy#6743 (comment)

Proposed solution

A new LengthPercentage enum which does not include the Undefined or Auto variants

enum LengthPercentage {
    /// Abstract absolute units
    Points(f32),
    /// The dimension is stored in percentage relative to the parent item.
    Percent(f32),
}

Not 100% sure on the name, but LengthPercentage is what the CSS specification uses and is pretty descriptive.

Notes

This isn't really a problem in practice. We currently just treat Auto and Undefined as 0.0. But it is potentially confusing to users of taffy.