When I load up 21w19a, and it prompts me to create a new world, I usually use ctrl + a to highlight all of the text (New World) to delete it faster. When I do that, the screen starts flashing black. I'll still be able to play the game (???), for example, if I click on the spot where the "cancel" button would be, it would send me back to the main menu, but the screen wouldn't update.