fix: resolve single box-shadow from runtime CSS variables

[YevheniiKotyrlo]

Summary

Fix box-shadow shorthand resolution when CSS variables are resolved at runtime (not inlined at compile time). Currently, a single shadow from a runtime variable produces boxShadow: [] instead of the expected shadow object.

Problem

When a CSS variable holding a single box-shadow value is resolved at runtime, the shorthand handler receives a flat array of tokens like [0, 4, 6, -1, "#000"]. The handler incorrectly iterates each primitive individually — passing 0, 4, 6, etc. to the pattern matcher one at a time. No single primitive matches any shadow pattern, so all return undefined and get filtered out.

The runtime path is triggered when variables are:

• Defined multiple times (e.g., :root + .dark for theme switching)
• Not inlined (inlineVariables: false)

The compile-time path (single-definition variables that get inlined) is not affected — lightningcss parses the full shadow value correctly.

Reproduction

:root { --themed-shadow: 0 4px 6px -1px #000; }
.dark { --themed-shadow: 0 4px 6px -1px #fff; }
.test { box-shadow: var(--themed-shadow); }

Expected: boxShadow: [{ offsetX: 0, offsetY: 4, blurRadius: 6, spreadDistance: -1, color: "#000" }]
Actual: boxShadow: []

Solution

Detect whether the resolved args array is flat (single shadow) or nested (multiple shadows) before entering the multi-shadow handler:

• Flat [0, 4, 6, -1, "#000"] → first element is a primitive → pass the entire array to the pattern handler as a single shadow
• Nested [[0, 4, 6, -1, "#000"], [0, 1, 2, 0, "#333"]] → first element is an array → use existing flatMap logic for multiple shadows

The existing multi-shadow path (nested arrays from comma-separated box-shadow values) is unchanged.

Verification

• yarn typecheck — pass
• yarn lint — pass
• yarn build — pass
• yarn test — 971 passed, 3 failed (pre-existing babel plugin tests), 22 skipped (pre-existing)

Related

• Follow-up to fix: box shadow from css variables #145 which fixed nested variable shadows but missed this flat single-shadow case