FB9188460 - iOS 15 Safari extension: incorrect popup margin

[lapcat]

This bug has been filed with Apple as FB9188460

Steps to reproduce:

1. Download and open the Apple sample code project Sea Creator: https://developer.apple.com/documentation/safariservices/safari_web_extensions/adopting_new_safari_web_extension_apis
2. In the popup.css file, add "border: solid thin red;" to the body CSS
3. Run the iOS app on an iPad with iPadOS 15 installed

Expected results: The margins are all 8px

Actual results: The right margin is clipped.

If you add "margin: 0px;" to the body CSS, then all of the margins are correct.

[lapcat]

This bug does not occur with Safari web extensions on macOS.