Disable Game Mode in Roblox (quick)
Launch Roblox and make it fullscreen.
Click the controller icon in the Apple menu and disable Game Mode.
Sometimes it turns on by itself, so check regularly to make sure it's off. If you are unable to turn it off, try restarting your Mac.
If Game Mode Won't Turn Off (pause/auto-reenable issues)
Follow these steps to kill the gamepolicyd process in Activity Monitor:
Open Activity Monitor (Applications > Finder).
Press the magnifying-glass icon and search:
Copy:
Double-click the process (the only one shown) and ensure the process window appears as below:
Press the Quit button and choose Force Quit.
Alternatively:
Back on the search results, select gamepolicyd (click ONCE) and press the "x" in a hexagon in the upper-left to quit.
Note: gamepolicyd will start to back up after being force-quit (this is normal).
After force-quitting, you should be able to turn off Game Mode. Go to the upper-right and press the game controller icon.