Initial pop-ups and login UI elements might block the screen, might shown as black screen on initial launch from steamOS mode. This issue can be mitigated by running in desktop mode.

there's no option for scaling resolution in fullscreen mode

There's a white pop-up that like to cover the screen. It has no content and only have two options "accept" and "cancel" in chinese. They both doesn't seems to to do anything when you click on it beside repeatedly reopen. However, there's a black pop-up box that cover the screen after you finish logging in that impossible to remove on steamOS mode. This does not seems to be an issue on desktop mode. I suspect it might be related to the initial pop-up