Web-Viewer: Don't auto-connect to wss://hostname when an ?url= is missing

[jprochazk]

What

• Closes app.rerun.io should show the welcome screen #3276
• Closes app.rerun.io should default to a blank viewer with the Welcome Screen #3196

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested demo.rerun.io (if applicable)

• PR Build Summary
• Docs preview
• Examples preview
• Recent benchmark results
• Wasm size tracking

[emilk]

I'm not sure this is 100% the right approach.

The fallback_to_websockets: bool seems to me quite a hack, especially as it is controlled from JavaScript. It is, as far as I can tell, trying to subvert the fallback in get_url for what URL to connect to. Do we need this fallback at all though? All the places where we open the web-viewer for the user we already add the ?url=… query parameter for them.

For instance, cargo r -p raw_mesh -- --serve will print this:

Listening for websocket traffic on ws://localhost:9877. Connect with a Rerun Web Viewer.
Started web server on http://localhost:9090
Web server is running - view it at http://localhost:9090?url=ws://localhost:9877

and then it opens that last URL.

I think it would make more sense to remove the fallback from get_url and handle no URL by simply adding no receiver to the application. This will make the web-viewer without an url= query act similar to just running rerun, i.e. make it more similar to the native viewer

[jprochazk]

I removed the fallback_to_ws path, and changed it to only add the receiver if url is present.

[emilk]

Should we also remove this dropdown in this PR then?

The demo link in the PR description https://demo.rerun.io/pr/3345 does not show the welcome page

[jprochazk]

That's because demo.rerun.io gets the URL implicitly, like app.rerun.io did before this PR. I think getting rid of demo.rerun.io is a separate step, as is cleaning up all the related scripts and CI to not do redundant work anymore, setting up app.rerun.io/pr/<pr_number> so we can use that instead of demo.rerun.io, redirecting demo.rerun.io to app.rerun.io, etc.