Affected platform:
ALL

Affected versions:
Nightly 99.0a1

Preconditions:
Have media.videocontrols.picture-in-picture.display-text-tracks.enabled set to "true".

Steps to reproduce:

1. Launch Firefox and reach coursera.org (or any site with WebVTT available)
2. Start one of the videos and enable the captions
3. Click on the PiP Toggle (or use the shortcuts to open it)
4. Adjust the PIP window and watch over the font size of the caption

Expected result:
The font size of the captions are adjusted accordingly to the size of the PIP window.

Actual Result:
The font size of the captions have a fixed size and do not change when the PIP window is adjusted.

Notes:
Setting a severity of S4.
Not a regression as the WebVTT has been available from Fx99.

The current patch in bug 1751231 (at least partially) addresses this.

I can confirm that this issue no longer occurs on Nightly 99.0a1 (2022-03-01) on Windows 10, Mac OS 11 and Ubuntu 20.

Ania could you please set this to fixed so I can verify it? Thank you.