On bug 827976, we lazy-fied the hidden window creation, and moved it to happen after first paint (on _delayedStartup). We want to make it to not be used at all on a normal browsing session, but there's still some remaining work to do, which will be tracked here.
The hidden window will remain supported in the platform, because:
- it is necessary on macOS
- we may still allow it to be used on some random features that would be harder to port.