Are you playing with a gamepad or your keyboard? If it is with a gamepad, can you go to https://html5gamepad.com/ press some buttons (it should react by showing numbers), and see if it is stable with this site. If it is unstable you may have a problem with your gamepad. Also, when it happens, can you move with the keyboard (still assuming you are using a gamepad)
For the window to appear, you may have to click the address bar of your browser to avoid the game capturing your keyboard input.
Also which browser are you using? Like Firefox, Chrome, Edge, Safari, something else?
Finally, you may have less problems by downloading the version for your computer.
Thanks for taking time to help me investigate. Hope we'll find a solution.