Slate legacy JSX fixture bindings should stay in the Bun preload loader

Problem

The full slate fixture harness was red even though the product-code tests were already green.

The failures were not coming from Slate behavior. They were happening before the tests even ran:

• React is not defined
• jsx is not defined

The obvious trap after that was to patch the spec harness instead of fixing the fixture loader. That creates a second source of truth for legacy JSX handling and leaks fake globals into a lane that should just import fixtures and run assertions.

Symptoms

• bun test ./test/index.spec.ts failed across large swaths of interfaces/**, operations/**, normalization/**, and transforms/** before any assertion ran.
• The current Bun preload already had a legacy-hsx-fixtures loader targeting packages/slate, packages/slate-history, and packages/slate-hyperscript fixture trees.
• packages/slate/test/index.spec.ts still grew an extra helper that wrapped every fixture import with temporary globalThis.jsx and globalThis.React mutation.

What Didn't Work

• Treating the remaining failures as product regressions first.
• Splitting the test stack into more runtime lanes just to paper over a legacy fixture family.
• Keeping a spec-local helper that mutates globals around every dynamic import.

Solution

Keep one owner for legacy fixture binding: the Bun preload loader in config/bun-test-setup.ts.

That loader already:

• matches the legacy fixture filename family
• runs those files through a classic JSX transform with jsxFactory: 'jsx'
• injects a jsx import from slate-test-jsx.js when the fixture does not declare one itself

Once that loader is in place, the spec harness should stay boring:

• dynamic-import the fixture module
• run the fixture
• do not patch globalThis.jsx
• do not replace globalThis.React with a fake hyperscript object

That let packages/slate/test/index.spec.ts drop the withLegacyClassicJsxGlobals(...) wrapper entirely.

Why This Works

The classic Slate fixture corpus is not normal React test code. It is a legacy hyperscript-shaped file family.

That means the fix belongs at module-load time, not in the spec body.

Once the preload loader owns that translation, every matching fixture module gets the same runtime treatment before evaluation, and the spec files stop carrying duplicate compatibility logic.

Prevention

• When a large legacy fixture family suddenly fails with React is not defined or jsx is not defined, inspect the same-path fixture imports before touching product code.
• Prefer one filename-pattern preload transform over per-spec global mutation.
• Do not add another Bun lane just because one fixture family needs custom JSX handling. Keep the loader scoped to the right file family instead.
• If the preload loader already matches the failing fixture path, remove spec wrappers before adding more runtime indirection.