With the Blackmagic Deck Control application, as soon as you remove focus from the playback window you will lose video playout. As our card has two different modes (capture and playback), this focus is important, as obviously by clicking on the capture window, you swap the card from playback mode into capture and visa versa.
Deck control is a very simple application for capture playback and is not designed to handle background video playout.
As far as I know i am afraid there are no settings to change this with Deck Control, however you may be able to achieve this background playback with a third party application.
Technical Support Consultant
Blackmagic Design EMEA