Allow any widget or element to be draggable/resizable (add Draggable and Resizable properties to 4x API - View UI). This would allow popups to move or resize. Or a LayerList. Or Search widget. Or any widget or DOM element that would make sense to register with View UI as moveable. Also add an autofit property true/false and with padding parameter; to allow popups to autosize in height or widgth and with given extra padding or margin. Seems like the perfect place to add this vital functionality to the API.
UI | ArcGIS API for JavaScript 4.16