When you load into a world or join a server, you've probably noticed that screens show up there that you can't really customize, because they get closed automatically.
To edit them, you need to open an instance of them that will stay open as long as you want, which is possible by opening a "dummy screen instance" of them.
The "Dummy Screen Instances" are a feature of FancyMenu that makes it possible to open SOME of the screens that vanish too fast to customize them.
To see all screens that can be opened via the Dummy Screen Instance feature, click on menu bar -> Tools -> Dummy Screen Instances. The menu that opens shows all screens you can open as a dummy screen instance. Left-clicking the screens in the menu will open a dummy instance of them for you to customize them in peace.
For example, when joining a world or server, there are at least two screens that will show up during loading:
Important: The "World Loading Screen" is not the same as the "Game Loading Screen" (aka. Splash Screen or Resource Reloading Screen). The "Game Loading Screen" can be customized with Drippy Loading Screen, which is an addon for FancyMenu.