Try to play a game on the web browser. After starting a game the stadia menu bar would not disappear.
@Tim4ever Hey try this! It's a solution that was found from a different area
"start Stadia after you enable guided access, then click power button three times to enter guided access mode and three times again when you are done with it."
More info on guided access
Here's how to get it working. Hit Share and then select the Add to Home Screen option and then use that new icon to open it and you won't have the menu bar issue.
You can also disable the tab bar in horizontal mode in the iOS Settings
I figured out how to remove the Stadia menu bar. I had to go into Safaris settings and disable request desktop view for the Stadia url.