When I start Emby while offline (WiFi off or WiFi disconnected), Emby is very flaky and slow.
If you press Home and then select Settings, none of those buttons work. You have to actually connect WiFi to be able to enter the individual settings screens to change such items as Playback->Enable External Video Players
To reproduce on your Pixelbook:
- Turn WiFi off like you're on a bus w/o WiFi
- Start Emby
- Press Home
- Press Hamburger->Settings
- Select Playback
- Wait until time ends for the spinning disc.