test: compile each Tailwind render afresh, and fail on a compile error

[YevheniiKotyrlo]

Problem

src/__tests__/vendor/tailwind/_tailwind.tsx compiles every render through @tailwindcss/postcss with from: __dirname and base: Date.now().toString(). The plugin keeps a compiler per from and base (getContextFromCache) and reuses it until a file it read changes, unless no file is at from (L182–L196); from is the test directory, which does not change. So two renders that read the same millisecond share a compiler, and the second compiles the first one's classes:

jest.spyOn(Date, "now").mockReturnValue(0);
await render(<View className="left-0" />); // stylesheet: left-0
await render(<View className="left-1" />); // stylesheet: left-0, and no style

A vendor case can then assert on another case's output. On main, auto-cols-max and object-cover failed in one of six runs of src/__tests__/vendor/tailwind and passed in the next; a shared compiler gives that symptom, though no run captured the collision itself.

A compile that fails is only logged: the plugin prints the error and returns an empty stylesheet (L334–L352), so the render registers nothing, and a case that expects no style passes. And the class collection reads every child as an element, so a render whose tree holds text throws Cannot read properties of undefined (reading 'children').

Solution

render compiles its CSS with compile from @tailwindcss/node, the string form of the compileAst the plugin calls, which keeps no compiler between calls. Each render builds its own classes, and a CSS that fails to compile rejects the render with Tailwind's error. The plugin's cache serves watch mode, and a from that names no file would not rebuild for every render: once one render's CSS uses no Tailwind feature, the plugin keeps that compiler and returns before it looks for the file, so a later render's CSS comes back uncompiled (L158–L160). @tailwindcss/node joins the dev dependencies at the range the other Tailwind packages use, locked to the 4.1.13 the plugin already resolves.

The class collection skips text and empty children. render.debug takes the harness's options, as renderSimple.debug does, and render's options no longer list plugin and className, which it never read.

Tests

src/__tests__/vendor/tailwind/harness.test.tsx:

• freezes Date.now and renders left-0, then left-1: each render's stylesheet holds only its own class, and each view gets its own style;
• imports a missing file and expects the render to reject;
• renders after a render whose CSS uses no Tailwind feature;
• renders beside a caller's own console.error spy and expects it left in place;
• renders a tree with text and empty children.

The first, second and last fail with the harness from main.

Verification

Every ci.yml job passes on this head merged into main: lint, typecheck, the tests (1344 passed), both builds, the web export, nothing unstaged, commitlint, and the iOS dev build on macOS 27.0.1 with Xcode 27.0, where CI pins 26.6.

Run in one process, src/__tests__/vendor/tailwind ends at 388–390 MB of heap against 1589 MB on main; with four workers it takes 7.86 s against 8.13 s, inside this PR's spread. The method and every run are in this comment.

No existing issue tracks this. I searched for flaky, tailwind test cache, Date.now and cache bust.

[YevheniiKotyrlo]

Heap and time — main against this PR

Heap. Every file of src/__tests__/vendor/tailwind in one process, two passes a side; jest reads the heap after a forced GC at the end of each file:

node --expose-gc node_modules/jest/bin/jest.js src/__tests__/vendor/tailwind --runInBand --logHeapUsage

	after the first file	after the last file	growth per file, on average
main (eda2b286)	275 MB	1589 MB	69 MB
this PR, merged into main	108 MB	388–390 MB	14 MB

For scale, main's src/__tests__/native, which compiles no Tailwind, grows by 7 MB a file the same way.

Time. jest src/__tests__/vendor/tailwind --maxWorkers=4, one warm-up and five measured runs a side, alternating:

	runs	median
main (eda2b286)	8.18, 8.13, 8.15, 8.06, 7.86 s	8.13 s
this PR, merged into main	8.44, 7.86, 7.43, 7.42, 7.88 s	7.86 s

The difference sits inside this PR's spread.

Windows 11 Pro 10.0.26200, AMD Ryzen 9 9950X3D, Node 26.8.1, jest 29.7.0; @tailwindcss/node 4.1.13 here, @tailwindcss/postcss 4.1.13 on main.