STR:

1. Open a private browsing window (this is to increase the likelihood of viewing an advertisement on YouTube)
2. In that private window, visit https://www.youtube.com/watch?v=sb3FJdRk-tI
3. Hopefully you get an advertisement. If not, go back to 1.
4. While the advertisement is playing, put it into Picture-in-Picture mode. If the toggle doesn't appear for you (for example, if the ad is less than 45 seconds long), double-right click on the ad and use the context menu to put it into Picture-in-Picture
5. Once the ad is open in Picture-in-Picture mode, note the aspect ratio of the player window.
6. Click on "Skip Ad" or wait for the advertisement(s) to finish, and the music video to begin.
7. Note that the player window resizes itself to a new aspect ratio.
8. Attempt to manually resize the window.

ER:

The player window should maintain the new aspect ratio.

AR:

The player window snaps back to the original aspect ratio.

The easiest way to do this would probably be to check if we're locking the aspect ratio here: https://searchfox.org/mozilla-central/rev/1843375acbbca68127713e402be222350ac99301/widget/windows/nsWindow.cpp#2036

by checking if mAspectRatio != 0.0, and if so, to call LockAspectRatio(false), and then after setting the bounds, re-locking with LockAspectRatio(true).

https://hg.mozilla.org/mozilla-central/rev/b3574370c361