Sorry, I think I've just found a new bug :)
I've tried to change the avatar Y starting position, and while it works in the box, the script preview doesn't refresh. After saving the script and exporting the game, it seems nothing had changed in the script. So I restarted the application then tried to change the script directly, which, oddly enough, seems only possible using the right click and copy/paste. I can't type anything in the preview at the bottom of the screen, but I can copy and paste stuff. Yet saving the script after modifying it this way doesn't seem to actually save it either.