Firefox Nightly 68.0a1 20190428094048 on Windows 8.1
The setup is a notebook with a 1920x1080 screen. Using that alone, triggering a picture-in-picture display of a video works like this:
- Open a youtube video.
- Right-click it.
- Dismiss the menu and right-click again.
- Choose 'Picture in picture'.
Actual result: small video shown at bottom left.
If a second screen (4K) is attached and placed above the working area of the main screen, the instructions above will open a window for the video which will be shown in the task bar and the task bar preview will show the content playing, but it's shown nowhere on the screens. It doesn't matter if the window for which the PiP is triggered is on the smaller or bigger screen.