I have a laptop with no trackpad, just a pen and pointer stick. The pointer is very hard to use for most people, so I let them use the stylus instead. However, when using the stylus, the cursor will immediately jump to the lower right-hand corner. I suspect this is an issue with relative versus absolute cursor movement, and may be only an issue in fullscreen.
What looks to be happening is that while normally, a mouse/trackpad will make small increments to the mouse's position, a tablet pen will just give the position the cursor should be at. The game is taking that info, and adding it to the current position like it would relative movement.
This isn't a serious issue, but a bug that should definitely be noted.
https://github.com/kyung01/xo-fractions-game/issues/20