godot-demo-projects/plugins/addons/main_screen
..
README.md
handled_by_main_screen.gd
main_panel.tscn
main_screen_plugin.gd
plugin.cfg
print_hello.gd

README.md

Main Screen Plugin Demo

This plugin demo shows how to make a main screen plugin. The main screen appears as a button next to the "2D", "3D", "Script", and "AssetLib" buttons. It also shows up when a Node it handles is selected.

For more information, see this documentation article: https://docs.godotengine.org/en/latest/tutorials/plugins/editor/making_main_screen_plugins.html

If you would like to see a more complete example of what main screen plugins are capable of, check out the 2.5D demo project.