Hello! I've made custom blocks with JavaScript that let you lock the mouse cursor and use the mouse's movement deltas instead of its exact position for whatever you want. I think these are quite cool and wanted to share them here :)
Here's an example script that you can drag into your project to use the blocks (remember to turn on javascript extensions!):
People who want to use the Mouse Lock block would then have to make a Start button in the game so that the mouse can't interfere with any other element outside of the activity frame (viewport).