test: compile each Tailwind render afresh, and fail on a compile error - #477

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, 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.

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). A CSS that names a path to scan, through @source or a source() root, rejects too, since the harness builds only the classes of @source inline(); @source not scans nothing and compiles. @tailwindcss/node joins the dev dependencies at the range the other Tailwind packages use, locked to the 4.1.13 the plugin already resolves, and @tailwindcss/postcss and postcss leave them: the harness was their only use outside example/, whose own @tailwindcss/postcss brings postcss with it.

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;
• names a path to scan, through @source and through a source() root, and expects the render to reject;
• excludes a path with @source not and expects its classes.

With the harness from main, five of the six cases fail: all but the @source not render.

Verification

Every ci.yml job passes on this head merged into main: lint, typecheck, the tests (1347 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 MB of heap against 1587 MB on main; with four workers its runs take 6.22–8.01 s against 6.45–10.42 s on main, ranges that overlap. The method and every run, both heap passes and the warm-ups included, are in this comment.

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