Affected version:

Nightly 80.0a1 (2020-07-20)

Tested on:

Windows 10/Windows 7

Steps to reproduce:

1. Launch Firefox Nightly with a new profile and go to Youtube.
2. Play a random video and select PiP.
3. Using TAB key go to the mute/unmute button (make sure that the mouse cursor is not in the PiP window).

Expected results:

After few seconds the control buttons should disappear from the PiP window.

Actual results:

The control buttons are still displayed when a control button is focused.

Note 1: In the youtube normal video window the control bar is hidden after few seconds even if a control button is focused.
Note 2: When the mute/unmute button is pressed by using the space/enter keys (in the PiP window), while the video is playing, the control buttons are not displayed in order to indicate the mute/unmute state. In the youtube main video window if the mute/unmute button is actuated by using space/enter keys the control buttons are displayed. (See the following video for a better understanding: https://streamable.com/vjg53u).

This is intentional behaviour. When the user is keyboard focusing, we keep the controls visible instead of hiding them after a timeout so that users don't lose the indication of their current focus state. Pressing Escape in this state causes the controls to fade away.