In Linux (Ubuntu/Unity 16.04 LTS, kernel 4.4.0-28), using "Playing Now" and then selecting "Ctrl + 6" or right clicking in the cover art area to do the same thing -- to "detach display", results in a partial small image of the currently playing cover art to appear in the lower right corner of MC21. The small (so called detached window of the cover art) can not be dragged onto the Linux desktop, which I presume, is the point of the feature. It remains locked within the main window of MC21 in the lower right corner. Furthermore, the entire main window of MC21 starts to pulse around the entire perimeter of the main window and has an additional adverse effect on using other running programs like Firefox (on a different workspace) until the small "detached" window and MC21 are both closed and MC21 restarted.
Attached is a screen shot which I hope helps depict the problem.
Addendum: After a little more trouble shooting I found that the "Screenlets" Now Playing app, which remains invisible on my desktop until Quod Libet is started, and which I have set in the lower right corner of my desktop was negatively interacting with MC21. Quod Libet wasn't running at the time of the problem. When I closed the "Screenlets" the pulsating MC21 main window stopped. However, detaching the MC21 now playing window still does not work. Now, nothing happens when I hit "CTRL +6". There is no MC21 now playing detached window visible anywhere on the desktop or within the MC21 main window. The cover art that was visible, simply disappears.