Hi AbsenteeGamer, I am sorry you're experiencing that! It's very strange. I can't reproduce the error personally, but that is just one machine. I have two questions for you if you don't mind:
1) Do you have a game controller of some sort plugged in while playing the game? This is just a hunch, I am not sure if that would have any effect on the game.
2) Do you mind sharing some of the specs of your machine? Operating system and brand would be a good starting point, and then I can look into whether other people have had problems with the Ren'py engine and full screen.
Just on a cursory (pun not intended but funny anyway!) glance, I found some steps for other people who have had issues with cursors disappearing in full screen games in general:
Mouse cursor invisible when playing any full screen game (Windows 10)
- Press Windows and X keys together and select Control Panel.
- Click on Mouse.
- Navigate to pointer options tab.
- Under Visibility, uncheck the box which says “display pointer trails”.
- Click on OK to close the window.
Or this Method:
Follow the steps below to change the display text size to 100 % and check if it resolves the issue:
- Right Click on the desktop screen and select Personalize.
- Click on Display.
- Check the box that says “let me choose one scaling level for all my display”. Choose Smaller (100%)-default.
- Click on Apply. Restart the game and check if the point becomes invisible again.
I hope that helps, otherwise I can dig a little deeper.