Have you ever noticed that when you change the size or zoom level of your window while a Scratch project is in fullscreen, it doesn't update the stage to fit the window? This project fixes that bug! Try adjusting the "Fit to window" slider, which toggles on/off the fitting script in the project. This just so you can see that there is a difference.
It works by spawning a UI element (the ask and wait input box) that forces Scratch to check the window size, causing the project to fit the window.