The panel is the parent object of the visual tree. A visual tree must connect to a panel for the visual elements inside a tree to render. All panels belong to either an Editor Window or a runtime UIDocument. The panel also handles focus control and the event dispatching for the visual tree.
Every element in a visual tree holds a direct reference to the panel that holds the visual tree. To verify the connection of a VisualElement to a panel, you can test the panel property of this element. When the visual element isn’t connected, the test returns null.