All menus are "scenes", including the main screen, which is called base scene in the SCENE_MANAGER.
This scene and it's "mouse mode" scene are available as HUD_SCENE and HUD_UI_SCENE.
https://wiki.esoui.com/SimpleNotebookTutorial/part6
https://wiki.esoui.com/Scene_Manager:_On_scene_change
Also look at:
SCENE_MANAGER:RegisterTopLevel(topLevel, locksUIMode)