FB9165971 - Web extension popups don't respect Show scroll bars Always in macOS System Preferences

[lapcat]

This bug has been filed with Apple as FB9165971

Steps to reproduce:

1. Open System Preferences, General, and set "Show scroll bars" to "Always"
2. Build and run the attached sample Xcode project ScrollBug
3. Open Safari
4. Select "Allow Unsigned Extensions" from the Develop menu
5. Open Safari Preferences, Extensions pane, and enable the ScrollBug extension
6. Click the ScrollBug toolbar item to open the extension popup

Expected results: The popup has a scrollbar.

Actual results: The popup does not have a scrollbar.

I can reproduce this bug with Safari 14 on macOS 10.14 and macOS 11, as well as with Safari 15 on the macOS 12 beta.

If you change the "Show scroll bars" system preference from "Always" to "When scrolling", then you do see a scrollbar in the extension popup when scrolling.

ScrollBug.zip

[lapcat]

By the way, Show scroll bars Always works as expected with the same extension in Google Chrome.

I found a workaround:

:root { max-height: 600px; overflow-y: hidden; }
body { max-height: 600px; overflow-y: auto; }

[carlosjeurissen]

Related WECG issue: https://github.com/w3c/webextensions/issues/692