Version: Unity 6.1 Alpha (6000.1)
Language : English
Introduction to visual elements and the visual tree
Draw order

Panels

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.
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. 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
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. 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.

Additional resources

Introduction to visual elements and the visual tree
Draw order