Are there any MKV video players that feature Picture in Picture?
IINA (https://iina.io/)

I found that IINA can use macOS's native Picture in Picture mode.


  For and only for modern macOS.
  
  IINA is born to be a modern macOS application, from its framework to
  the user interface. It adopts the post-Yosemite design language of
  macOS and keeps up the pace of new technologies like Force Touch,
  Touch Bar, and Picture-in-Picture.


Source: GitHub