This is a modded version of the game 'Square' where you can hold down space to spawn the block you have selected to your mouse curser. You can also use the Replace <Block> with <Block> Block that I made in the blocks tab to then do some weird stuff... then to activate it, just press e! This is just a fun thing I thought of when I was looking at the original game's code.
Thank you to @StatfordJames for creating the original project, 'Square ◆ scrolling platformer creator #games.'. (It's an amazing game, so make sure to check it out!)