Sidebar scrolled position lost when exiting video fullscreen

[SoftVision-PaulOiegas]

[Affected versions]:

• Firefox 50.1.0 and above
• Tab Center 1.29.0 ( build 01.16.2017)

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• The "xpinstall.signatures.required" is set to "false" and latest Tab Center add-on is installed on a new clean profile.
• Have multiple tabs opened in order to have sidebar scrollable.

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites.
2. On one of the top tabs, navigate to a https://youtube.com.
3. Click on one of the suggested videos.
4. Scroll the sidebar to reach bottom tabs without switching focus from video.
5. Full screen the video.
6. Exit video full screen and observe the sidebar position.

[Expected result]:

• The sidebar scrolled position is kept after exiting full screen.

[Actual result]:

• The video tab on top is refocused and scrolled position of the sidebar is lost.

[Notes]:

• This issue reproduces also with reverse tab order option checked.
• The issue is reproducible also with other websites players (eg: Vimeo).
• Here is a screen recording of the issue: https://goo.gl/b9vVQS.

[Keith94]

Affected by this bug too. TC seems to scroll down to the selected tab by itself when exiting fullscreen.