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For keyboard and controller navigation to work correctly, any node must be focused by
using code when the scene starts. Without doing this, pressing buttons or keys won't
do anything.
You can use the Control.grab_focus() method
to focus a control. Here is a basic example of setting initial focus with code:
Now when the scene starts, the "Start Button" node will be focused, and the keyboard
or a controller can be used to navigate between it and other UI elements.