Description

Fixes #22033.

When a builder (or plugin) reassigns the whole build.rolldownOptions object on a resolved environment config, the resulting build ignores the new input and falls back to index.html, producing a chunk named index-*.js instead of entry-*.js.

Root cause. resolveBuildEnvironmentOptions in packages/vite/src/node/build.ts installs a getter/setter proxy via setupRollupOptionCompat(merged, 'build') so that rollupOptions and rolldownOptions stay in sync. The next step builds resolved with { ...merged, ... }. The spread invokes the getter and copies the current value as a plain data property, so resolved.rollupOptions and resolved.rolldownOptions become two independent properties that happen to point at the same object. Once a consumer replaces the whole rolldownOptions, rollupOptions keeps the stale reference; resolveRolldownOptions then reads options.rollupOptions.input (still undefined) and falls back to index.html.

This is why earlier attempts (#22034, #22051) didn't catch it — they didn't identify the proxy-breaking spread.

Fix. Re-install the proxy on resolved after the spread by calling setupRollupOptionCompat(resolved, 'build') again. Six lines of change including a comment that points at #22033.

Alternatives explored

• Replacing the spread with manual property assignment: more invasive and easy to regress on future refactors.
• Eagerly reading both fields before the spread and copying both: doesn't fix the underlying problem (later reassignment of one field still desyncs the other).
• Re-installing the proxy after the spread (this PR): minimal and matches the existing setup pattern.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

Testing

• Added regression test in packages/vite/src/node/__tests__/build.spec.ts that:

  1. Resolves the client environment config,
  2. Reassigns the whole rolldownOptions to { input: '/entry.js' },
  3. Asserts the proxy mirrors the change to rollupOptions.input,
  4. Runs the build and asserts the output chunk filename is entry-*.js (not index-*.js, which would mean a fallback to index.html).

  Verified failing before the fix and passing after.

• pnpm run test-unit — full suite passes (780 passed, 4 skipped, includes the new test).

• pnpm run typecheck — passes.

• pnpm run build — passes.

• pnpm run lint — clean apart from a pre-existing parse error in playground/preserve-symlinks/module-a/linked.js, which is unrelated to this change.