STR:

1. Visit https://developer.mozilla.org/en-US/docs/Web/HTML/Element/video#simple_video_example.
2. Click on a video embed to give it keyboard focus.
3. Press the PiP keyboard shortcut (Ctrl+Shift+]).

Nothing happens.

Works with fission.autostart = false or fission.webContentIsolationStrategy = 0.

Hey Micah, do you think you could find someone on the PiP team to take a look at this?

I narrowed the regression window to this:

Last good revision: 383986e2f5cb835a55c47bbd6e15d81035ca0784
First bad revision: d6e8528f0a936df88369c71f4e390e31a4d621a1
Pushlog:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=383986e2f5cb835a55c47bbd6e15d81035ca0784&tochange=d6e8528f0a936df88369c71f4e390e31a4d621a1

Although I realize this patch seems to be around the time we enabled fission, so we'd have to do some more digging into exactly what part of the fission caused the regression and when.

Quickly looking into this, I noticed that we no longer register the focused window (it's null) using the above the STR. I think this is where we can start.

We decided to address this in Fx 100

FWIW, I suspect we need to teach https://searchfox.org/mozilla-central/rev/8efeb8af3abb331df65c424ea79bab073140f173/toolkit/components/pictureinpicture/PictureInPicture.jsm#194-204 to send the async message to all of the subframes, rather than just the top actor. It would need some logic to avoid trying to open more than one video, and should perhaps start at the focused frame - the child would need to respond whether it found a video to toggle here.

https://hg.mozilla.org/mozilla-central/rev/2365c2fe841b

I managed to reproduce this issue on Firefox 98.0(20220304153049) on Win10 64-bits using the STR from the Description. Confirmed as fixed on Firefox 100.0b8(20220419190903) and Nightly 101.0a1(20220419214607) on Win10 64-bits and Linux x86_64(Ubuntu 20.04).