Reproduces on Windows 10 (other platforms not tested), on both 82 Release and 85 Nightly.
- Ensure Side View is installed. This was a 2018 test pilot experiment.
- Go to a video site, svtplay.se in my case.
- Click the "Open Side View" extension icon.
- Select the site from step 2.
- In the side view, play a video.
- Click the PiP button on the playing video. You might have to widen the side view so it shows.
Expected: Video stops playing in side view, starts playing in PiP window.
Actual: Video continues playing in side view, PiP window opens but is solid white.
Expected: Clicking the X to close the PiP window closes it.
Actual: Clicking the X in the PiP window does nothing.
At least one can close the PiP window by alt+tab and then clicking the X there. Other platform methods probably apply.