Affected versions:

Nightly 84.0a1

Tested on:

Ubuntu 18.04, Windows 10

Steps:

1. Launch Firefox Nightly with new profile and go to https://whereby.com
2. Join a meeting.
3. Click on the PiP icon.

Actual result:

The PiP description is displayed incorrectly (is displayed like in RTL).

Expected result:

The PiP description should be displayed accordingly like in any video.

Note: The PiP icon is not displayed in Beta 83 and Release 82 on whereby.com

I'm assuming they are doing something to mirror the video by applying transform: scaleX(-1)?

Can we set transform: initial !important on the PiP CSS?

Yeah, I think generally transform: scaleX(-1) is how this works. Unfortunately, setting transform: initial !important doesn't work on child elements for something set to scaleX(-1);:

https://jsbin.com/duyuzebewu/edit?html,css,output

We can, however, maybe used the privileged getTransformToViewport method here: https://searchfox.org/mozilla-central/rev/dbced93f1c57942501c23d4633d01ce59d9087a1/dom/webidl/Element.webidl#169. If we call that on the <video>, then if it (or one of its ancestors) is transformed such that the .a field on the DOMMatrix is -1, then we should set transform: scaleX(-1) on the UAWidget, perhaps when the mouse first moves over the video.

https://hg.mozilla.org/mozilla-central/rev/1ecbca0ad84f

This happens for me on Beta 83.0b10 build ID 20201108174701

This happens for me on Beta 83 build

And has existed longer according to the duplicate bugs like bug 1592909.

Verified - Fixed in latest Nightly 84.0a1 (2020-11-09) (build id: 20201109215349).