Windows Insider Preview Build 15031 introduces a new feature called Compact Overlay. A new window mode that functions like picture-in-picture on most TVs, it will allow a new level flexibility for users wanting to consume content while they multitask. The feature is part of UWA and will need to be programmed in.
http://www.notebookcheck.net/Windows-insider-preview-introduces-picture-in-picture-apps.196452.0.html