OK, this is a bit of a problem.
For the GL renderer, we don’t have a choice about which thread we use - the code that renders the GUI must run on the GL thread because that’s how GL does things. What we do to make things thread-safe is to suspend the message thread while it’s active, and that works just fine.
However, this does trigger the new message-thread checker, and AFAICT there’s no way for us to dynamically disable it for the bits we know are safe, the only way to stop it complaining is to turn it off in your project.
On the plus side, it does only seem to trigger a warning once per location in the code, and only happens in a couple of places, so you can just ignore it and continue, and your app should run.
Honestly don’t think there’s much else we could do here as a workaround! Ideas welcome, but I think that for people doing GL GUIs it’ll just have to be something you turn off.