If you're using a combo like Pico System and 8x zoom, the simulator part will cover everything from the screen, including the UI used to change the device and the zoom.
Exiting the project and reopening it unfortunately keeps the zoom level.
Fortunately this is fixed by a browser refresh so it's really a minor issue.
What I call the Device/Zoom/Run/Build UI:
If you're using a combo like Pico System and 8x zoom, the simulator part will cover everything from the screen, including the UI used to change the device and the zoom.