Tryst::UI::PaneValidator
@api private
A pane's own contract: it has to be declared directly inside a ui.split. WidgetDSL#pane already refuses to run anywhere else, so this is only reachable by building the tree by hand (direct Document/Node manipulation) - the same defense in depth TabValidator provides for a tab. Composed into WidgetValidators via :pane's own WidgetType#validator (see widget_types/pane.cr).