[Notes]:

• I am not able to see the PiP button on the trailers carousel, so I’m not sure why they are opened via PiP.
• This issue is not reproducible with clicking the PiP button instead of using the shortcut.
• This issue is not reproducible when using the shortcut and the video plays instead of being paused.

[Affected versions]:

• Firefox Release Candidate 80.0 (Build ID: 20200818235255)

[Affected platforms]:

• Windows 10 x64
• Linux Mint 19.2

[Prerequisites]:

• Have the Firefox browser installed.
• Have a valid account for Amazon Prime.
• Have a VPN connection set to the US.
• You are enrolled in any of the experiment’s branches.

[Steps to reproduce]:

1. Open the Firefox browser and navigate to the https://www.primevideo.com/ page.
2. Sign in with credentials from prerequisites.
3. On the Homepage of Amazon Prime, hover 1 video from the listed ones (different from the carousel's trailers).
4. Click the "Play" button.
5. Pause the video.
6. Press "Ctrl+Shift+]" keys.
7. Observe the behavior.

[Expected result]:

• The PiP window is displayed with the video paused.
• The original screen is black and displays the PiP info message.

[Actual result]:

• The PiP window is displayed with one of the trailers from the Home page carousel paused.
• The original screen displays the video paused.

[Additional Notes]:

• This issue is reproducible on all the branches of the experiment.
• I wasn't able to verify this issue on mac OS due to issue 1599376.
• Attached a screen recording of the issue here.

Quoting Mike:

this bug is because of the naive algorithm that we use to select which video to PiP when the user uses the keyboard shortcut. I'm pretty sure we end up just choosing the first one in the DOM, which is not sufficient in this case.
https://searchfox.org/mozilla-central/rev/62f6cc5d9c829bc0c6f18e25f93203a98681ac97/toolkit/actors/PictureInPictureChild.jsm#1476-1481
We probably want to filter out paused videos, sort by video dimensions, and choose the largest one that's in the viewport.

I'd be interested in knowing if bug 1661061 fixed this problem?

• I have tested this scenario using latest Nightly 84.0a1 (Build ID 20201103213933) on Windows 10 x64 and macOS 10.15 and the issue is still reproducible.
• Worth to mention that the carousel from the Homepage doesn't contain trailers anymore. However, I have managed to reproduce the issue by using the "Prime Previews for you" section.
• In order to reproduce the issue, I made sure that the video from the "Prime Previews for you" section is playing, while I have followed the steps from the description (starting with step 3).
• Here is a video of the issue reproducible using the "Prime Previews for you" section.