Vanilla (Parcel): Default Open File is Unexpected / Unreliable

🐛 bug report

Description of the problem

Opening a Vanilla (Parcel) sandbox, it is not clear which file will open.

I have been unable to find why this is in my quick investigation of the behavior, but starting from a fresh sandbox, its behavior can be characterized as the following through these reproduction steps:

1. A new Vanilla (Parcel) sandbox will open src/index.js, even though it is not listed under the main field in package.json
2. If this sandbox is forked by a user and this file removed (either with or without the reference in index.html), then the default file that will open will now be index.html
3. If the src/styles.css file is renamed to src/index.css then this file will now be the default file to open.
4. If the index.html file is deleted and recreated, this will again become the default file to open 😳

It seems like there should be a more clear, reliable list of default files. My suggestion would be:

1. Optional: The last files open, if any (see #1456 below)
2. Initial opened file setting (see #514 below)
3. The main field in package.json

If these three configuration options are not set, I would suggest not opening any file initially.

This may also apply to other templates, potentially in different ways.

How has this issue affected you? What are you trying to accomplish?

I would like my sandboxes to have a reliable default initial file, based on clear rules.

Link to sandbox: link (optional)

Your Environment

Related

Allow setting the initial opened file in sandbox.config.js: https://github.com/codesandbox/codesandbox-client/issues/514

Add sandbox configuration to change default opened file: https://github.com/codesandbox/codesandbox-client/issues/1254

Other related features:

Array syntax for main field in package.json: https://github.com/codesandbox/codesandbox-client/issues/1739

Open previously-opened files on refresh: https://github.com/codesandbox/codesandbox-client/issues/1456

Partially related:

Vanilla (Parcel) Unreliable Behavior When index.html Under Other Path (for example, in src)
https://github.com/codesandbox/codesandbox-client/issues/2240

Trace the Vanilla (Parcel) sandbox's default-file selection using package.json, index.html, src/index.js, and src/styles.css. Reproduce the four cases, then verify that the documented precedence is applied consistently and that no file opens when no setting or main entry exists.