fix(html): handle trailing slash in transformIndexHtml pre-transform

[XploitMonk0x01]

Fixes #18964
Normalizes relative URL pre-transform path resolution when htmlPath ends with /
Adds regression tests in packages/vite/src/node/server/middlewares/tests/indexHtml.spec.ts

[travisbreaks]

Review

Verdict: The fix is correct and well-scoped. A few observations and one suggestion.

The bug

path.posix.dirname('/example/dir/') returns /example (not /example/dir/), so when htmlPath ends with / (common in SSR with appType: 'custom' and req.originalUrl), relative script references like ./filename.js resolve to the wrong parent directory. The pre-transform request then 404s.

The fix

The getHtmlDirnameForRelativeUrl function is the right approach: if the path already ends with /, it IS the directory, so return it unchanged. Otherwise, delegate to dirname() as before.

The function is pure, exported, and unit-tested separately from the middleware. Good.

Edge cases

• Root path /: Both branches produce / (dirname of / is /, and / ends with /). No regression.
• path.posix.join('/', '/example/dir/', './filename.js') correctly yields /example/dir/filename.js. Verified.
• ../ references: path.posix.join('/', '/example/dir/', '../other.js') correctly yields /example/other.js. Works as expected.
• Non-trailing-slash paths (e.g., /example/dir/index.html): Fall through to dirname() as before. No behavior change.

Suggestion for test coverage

The tests are solid for the extracted function. One additional case worth considering:

test('handles parent-relative resolution from trailing slash directory', () => {
  const preTransformUrl = path.posix.join(
    '/',
    getHtmlDirnameForRelativeUrl('/example/dir/'),
    '../other.js',
  )
  expect(preTransformUrl).toBe('/example/other.js')
})

This would cover the ../ case, which is common in real HTML and exercises different path.posix.join behavior.

One note on scope

The first replacer block (lines ~142-157, non-pre-transform URL rewriting) does not use dirname(htmlPath) for relative URLs, so it's unaffected by this bug and correctly left untouched.

Minor: query strings in htmlPath

If htmlPath arrives with a query string (e.g., /example/dir/?t=123), endsWith('/') would be false and the bug would still trigger. In the current Vite codebase, htmlPath appears to come in clean (no query/hash), so this is not a practical concern today, but worth noting for robustness.

Overall: clean, minimal, correct. Nice contribution.