The panel is the parent object of a visual treeAn object graph, made of lightweight nodes, that holds all the elements in a window or panel. It defines every UI you build with the UI Toolkit.
See in Glossary. It owns the rootVisualElement but itself isn’t a visual elementA node of a visual tree that instantiates or derives from the C# VisualElement class. You can style the look, define the behaviour, and display it on screen as part of the UI. More info
See in Glossary. 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 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.