Like that you can create custom layouts for the settings screen for example.

You can create layouts for every Vanilla screen in the game and even many mod screens.
If the menu bar isn't visible in the settings screen, you probably have a mod installed that overrides that screen in a way that makes it uneditable, like OptiFine for example.

Editing stuff inside scroll areas (like in some settings screens, the singleplayer screen and multiplayer screen) is not possible and will never be.