Steps to reproduce:

1. Set widget.disable-native-theme-for-content to true
2. Install https://addons.mozilla.org/firefox/addon/privacy-redirect/reviews/
3. Click icon in toolbar opening popup

What happens:

The form controls below "Nitter Instance" etc. are cut off on the right side.
The "More Options" link at the bottom of the popup is barely visible.

Expected result:

Nothing is cut off or not visible.

2020-03-28T20:38:15: INFO : Narrowed integration regression window from [73e691ab, 3e2d9328] (4 builds) to [02f22447, 3e2d9328] (2 builds) (~1 steps left)
2020-03-28T20:38:15: DEBUG : Starting merge handling...
2020-03-28T20:38:15: DEBUG : Using url: https://hg.mozilla.org/integration/autoland/json-pushes?changeset=3e2d9328d59b6af9ad780030437a629cb8ccdb12&full=1
2020-03-28T20:38:16: DEBUG : Found commit message:
Bug 1381938: Add native theme for Windows that avoids native system calls. r=mstange,dbaron

Differential Revision: https://phabricator.services.mozilla.com/D33310

2020-03-28T20:38:16: DEBUG : Did not find a branch, checking all integration branches
2020-03-28T20:38:16: INFO : The bisection is done.
2020-03-28T20:38:16: INFO : Stopped

This happens on Linux too, without the pref enabled. I think this is a bug in the extension, there's probably a box-sizing: border-box link missing here: https://github.com/SimonBrazell/privacy-redirect/blob/b01ac7c9662f83506713094fc54bfa346a3ab9fe/pages/styles.css#L88

(And this just happens because the theme has a bit more built-in padding on Windows, nothing else)

https://github.com/SimonBrazell/privacy-redirect/pull/29

Thanks Emilio!