Way to control which UIDocument gets input?

It's kind of unclear to me how input works for the current version of the UIToolkit preview package, but right now it seems like the UIDocument displaying over everything else is the one that gets input focus. This is fine for me with the current menu I'm building, but I think I should be able to control which panel the EventSystem dispatches events to. (I don't see any way to control this from the InputWrapper class.)