`import.meta.env.ENTRYPOINT` is `undefined` for the side panel entrypoint

Describe the bug

Like the title suggests, import.meta.env.ENTRYPOINT is undefined for the side panel entrypoint, when I was expecting it to be set to "sidepanel".

Ideally, import.meta.env.ENTRYPOINT should never be undefined and instead always be set to the current entrypoint's name, even for unlisted entrypoints. For example, even in scripts run in entrypoints/some-unlisted-entrypoint/... I'd expect it to be set to "some-unlisted-entrypoint".

Reproduction

wxt-env-entrypoint-sidepanel-repro.zip

Steps to reproduce

1. Run npm install followed by npm run dev
2. Open the side panel by clicking the extension's action
3. Inspect side panel logs

System Info

System:
  OS: macOS 15.4.1
  CPU: (12) arm64 Apple M2 Pro
  Memory: 112.61 MB / 32.00 GB
  Shell: 5.9 - /bin/zsh
Binaries:
  Node: 22.14.0 - ~/.nvm/versions/node/v22.14.0/bin/node
  npm: 11.3.0 - ~/.nvm/versions/node/v22.14.0/bin/npm
  pnpm: 10.7.0 - ~/.nvm/versions/node/v22.14.0/bin/pnpm
Browsers:
  Chrome: 135.0.7049.97
  Safari: 18.4

Used Package Manager

npm

Validations

• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

Start with the attached reproduction by running npm install and npm run dev, then inspect the side panel logs and the build handling for import.meta.env.ENTRYPOINT. Done means the side panel reports "sidepanel" and unlisted entrypoint directories receive their own entrypoint name instead of undefined.