fix(serve): serve index.html from public subdirectories

[desty]

Description

Fixes #6714

When index.html lives under a public/ subdirectory (e.g. public/foo/bar/index.html), navigating to /foo/bar/ now serves that file — matching /foo/bar/index.html and typical static-server directory index behavior.

Also aligns extensionless public HTML resolution between dev and preview:

URL	resolves to (when present)
/foo/bar/	public/foo/bar/index.html
/foo	public/foo.html

Approach

Update servePublicMiddleware to mirror htmlFallbackMiddleware for public files only:

• rewrite only for GET/HEAD with Accept containing text/html or */*
• require the rewritten path to exist in the in-memory publicFiles set
• then serve with the existing sirv instance (extensions: [])

This deliberately does not use sirv's extensions: ['html'], which would also map /foo → /foo/index.html (preview does not do that).

Tests

Playground html coverage (serve + build):

• /subdir/ and /subdir/index.html → public/subdir/index.html
• /public-file and /public-file.html → public-file.html

Review follow-ups addressed

• Run the new test in build as well (not serve-only) — @sapphi-red
• Make extensionless public HTML behavior consistent between dev and preview — @sapphi-red
• Avoid broad sirv extensions: ['html'] / document intentional /file → /file.html — @avinashkamat48

Verification

pnpm test-serve html   # 65 passed
# new case also verified under:
VITE_TEST_BUILD=1 vitest run -c vitest.config.e2e.ts html -t "should serve index.html from public subdirectories"

Rebased onto latest main.

[sapphi-red]

We should run this test in build as well so that it's consistent.

[desty]

Addressed in f1caef5 — removed runIf(isServe) so this test now runs in build as well (the extensionless-alias assertions stay dev-only since the fallback middleware only exists on the dev server).

Verified locally:

• pnpm test-serve html → 65 passed
• pnpm test-build html → ✓ should serve index.html from public subdirectories

[sapphi-red]

the extensionless-alias assertions stay dev-only since the fallback middleware only exists on the dev server

we should make this behavior consistent in dev and build.

[desty]

Done in 472b50b. Dev now mirrors the preview server's html fallback for public files: /foo resolves to /foo.html and /foo/ to /foo/index.html, using the same method/Accept conditions as htmlFallbackMiddleware (instead of sirv's extensions option, which would also resolve /foo → /foo/index.html — something the preview server does not do).

The test assertions now run unconditionally in both serve and build:

• pnpm test-serve html → 65 passed
• pnpm test-build html → 55 passed (✓ should serve index.html from public subdirectories in both)
• assets / fs-serve playgrounds pass in serve as regression checks

[avinashkamat48]

Adding extensions: ["html"] to the public sirv middleware appears to broaden behavior beyond directory indexes: an explicit request for /foo can now resolve to public/foo.html if that file exists. The PR title/test only covers /subdir/ -> /subdir/index.html, so this extensionless-file fallback may be an unintended public serving change. Please add a test for /file vs /file.html (or avoid the global extensions option and handle only trailing-slash index.html) so the new behavior is intentional.

[desty]

Rebased onto latest main and updated the PR description to match the final approach (472b50b / current tip).

Review feedback is addressed as follows:

• test runs in serve and build
• public HTML fallback mirrors htmlFallbackMiddleware so dev matches preview (/foo/ → index.html, /foo → .html)
• no sirv extensions: ['html'] (avoids /foo → /foo/index.html)

@sapphi-red ready for another look when you have time.

[desty]

Rebased onto latest main.

Review feedback from June/July is still in this patch:

• the new test runs in serve and build
• public HTML fallback mirrors htmlFallbackMiddleware (/foo/ → index.html, /foo → .html)
• no sirv extensions: ['html'] (avoids /foo → /foo/index.html, which preview does not do)

The previous Build&Test: node-24, macos-latest failure was playground/wasm/__tests__/wasm.spec.ts > should work when inlined — unrelated to this change.

@sapphi-red ready for another look when you have time.