Called when the VisualElement visual contents need to be (re)generated.
When this delegate is handled, you can generate custom geometry in the content region of the VisualElement. For an example, see the MeshGenerationContext documentation.
This delegate is called only when the VisualElement needs to regenerate its visual contents. It is not called every frame when the panel refreshes. The generated content is cached, and remains intact until any of the VisualElement's properties that affects visuals either changes, or VisualElement.MarkDirtyRepaint is called.
When you execute code in a handler to this delegate, do not make changes to any property of the VisualElement. A handler should treat the VisualElement as 'read-only'. Changing the VisualElement during this event might cause undesirable side effects. For example, the changes might lag, or be missed completely.