[henryqdineen]

What changed / motivation ?

main is currently red: packages/style-value-parser fails border-radius.test.js › symmetric radii serialize without a /, because roundTrip('10px 20px') returns 10px 20px / 10px. I noticed it while rebasing #1700 onto main, which now has a failing tests / packages build even though that PR only touches babel-plugin.

Why it broke main: Without a /, the parser passes only the four horizontal radii to BorderRadiusShorthand, and the constructor filled in the missing vertical radii from verticalTopLeft rather than from their horizontal counterparts. So 10px 20px was stored as horizontal 10 20 10 20 / vertical 10 10 10 10. That internal state has always been wrong, but toString hid it: it built the vertical string (sStr) from the horizontal values, so both halves always matched and the / was dropped. #1759 fixed sStr to read the vertical values (and #1742 made the same change), which exposed the bad constructor defaults. #1759's own new round-trip test fails on its own commit, but the tests workflow never ran on that PR, so it merged without anyone seeing it fail.

Fix: When no vertical radii are passed, each vertical radius now mirrors its horizontal counterpart, per the CSS spec. When some vertical radii are passed, the existing CSS fill-in rules still apply to the rest (topRight falls back to topLeft, bottomRight to topLeft, bottomLeft to topRight).

Linked PR/Issues

Follow-up to #1759 and #1742. Unblocks #1700.

Additional Context

The existing Valid: border-radius: … parse tests compare parse(...) to new BorderRadiusShorthand(...), so both sides go through the same constructor defaults and couldn't catch this. The round-trip test from #1759 does catch it and now passes. npm run test:packages, flow check, and Prettier all pass locally.

This also fixes a Prettier failure on main: an assertion added in #1742 to border-radius.test.js wasn't formatted.

Pre-flight checklist

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1 Skipped Deployment

Project	Deployment	Actions	Updated
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[imdreamrunner]

I think this is the right fix. In CSS, when border-radius has no /, each corner's vertical radius equals its horizontal one. On main, the constructor sets all four vertical radii to top-left, so 10px 20px was stored with vertical radii 10px 10px 10px 10px. Nobody noticed until #1759 started writing out the vertical radii. Since then, 10px 20px comes back as 10px 20px / 10px, which is why main's packages job is red.

[henryqdineen]

Thanks @imdreamrunner. Do you have merge permissions?