Fix division by zero in polar color space unpremultiply when alpha is 0

[queelius]

Summary

• Fix division by zero in polar_premultiply macro's unpremultiply method when alpha == 0.0
• Add && self.alpha != 0.0 guard to match the existing rectangular_premultiply behavior
• Add tests for color-mix() with zero-alpha colors in all four polar color spaces (HSL, HWB, LCH, OKLCH)

Problem

When using color-mix() to interpolate colors with zero alpha in polar color spaces (HSL, HWB, LCH, OKLCH), the unpremultiply step divides by alpha (which is 0), producing NaN values. These NaN values then serialize as none in the CSS output.

For example:

/* Input */
.foo { color: color-mix(in oklch, oklch(0.5 0.2 120 / 0), oklch(0.7 0.1 240 / 0)); }
/* Before fix: */ .foo{color:oklch(none none 180/0)}
/* After fix:  */ .foo{color:oklch(0% 0 180/0)}

The rectangular_premultiply macro already guards against this with self.alpha != 0.0, but polar_premultiply was missing the check.

Test plan

• Added test cases for color-mix() with zero-alpha in HSL, HWB, LCH, and OKLCH
• All existing tests pass (cargo test - 117 tests + doc tests)

Fixes #1194

Generated with Claude Code

[Copilot]

Pull request overview

Fixes a division-by-zero bug in polar color-space interpolation when alpha == 0.0, preventing NaN components from leaking into serialized CSS (e.g. none in oklch() / lch() output) during color-mix().

Changes:

• Add an alpha != 0.0 guard in the polar_premultiply macro’s unpremultiply method (matching rectangular_premultiply behavior).
• Add regression tests covering color-mix() with zero-alpha inputs in HSL, HWB, LCH, and OKLCH.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/values/color.rs	Prevents division by zero during unpremultiply for polar color spaces when alpha is zero.
src/lib.rs	Adds regression tests to ensure zero-alpha color-mix() in polar spaces doesn’t serialize NaN/none.

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[queelius]

Closing this to keep your queue clean. Happy to reopen if useful, or feel free to pick up any of it. Thanks!