User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:148.0) Gecko/20100101 Firefox/148.0

Steps to reproduce:

1. Open testcase.html.
2. Click the Toggle PiP button.
3. Switch to another browser tab.

Actual results:

The animation in the Picture‑in‑Picture window becomes significantly slower.

Expected results:

The animation in the Picture‑in‑Picture window should run at normal speed.

When switching to another tab, the opener becomes inactive and the script that controls the PiP becomes throttled. If the script ran in the PiP document, I think it should not slow down.

I looked through the code and debugged a bit. PresShell::ComputeActiveness determines isActiveTab=false, we call nsRefreshDriver::SetActivity(false) and this throttles the opener.

This doesn't happen in Chrome and I verified that in general, they throttle inactive documents too (rAF slows down / stops). So I guess PresShell::ComputeActiveness needs adjustment. (doesn't help)