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1. go to https://www.theverge.com/ and click on a video play button right from their home page (not an article)
2. see the PiP affordance and click it

Expected: PiP video
Actual: nothing

I'm on the 2020-06-22 Nightly on Windows 10

Actually, I can make it happen on https://www.theverge.com/2020/6/24/21299924/ios-14-iphone-wwdc-2020-apple-home-screen-widgets-app-library-clips-pages-complexity, too.

Can't reproduce on beta or nightly on macOS.

Can you check a clean profile? Is this happening only with fission enabled, maybe? What OS are you seeing this on? Any errors in the browser console when PiP fails to open?

Sorry, I verified it works in a non-Fission window. Nothing in the console, FWIW.

Tracking for Fission M6b Nightly.

The severity is N/A here as Fission is still off by default (even though more people are turning it on). As we get closer to enabling Fission by default on Nightly, we should adjust the Severity field.

Can confirm the issue in OP with most recent nightly build and fission.autostart = true.
To add to the discussion, I've opened a similar issue on webcompat here https://github.com/webcompat/web-bugs/issues/54274 but it was closed and the bugherder never replied about re-testing with fission enabled.

(In reply to Mike Conley (:mconley) (:⚙️) from comment #5)

The severity is N/A here as Fission is still off by default (even though more people are turning it on). As we get closer to enabling Fission by default on Nightly, we should adjust the Severity field.

This should be a S3 severity instead of N/A since the work around for the bug is leaving the feature disabled. As more people are exposed to the feature, then up-rating the severity would be, as you said, the next right thing.

Mike, this is a blocker for Fission Nightly. Is it blocked on anything from DOM?

No, I think we have everything we need from DOM. I'll see if we can get this fixed next week.

https://hg.mozilla.org/mozilla-central/rev/c499848501df