Open System Preferences, General, and set "Show scroll bars" to "Always"
Build and run the attached sample Xcode project ScrollBug
Open Safari
Select "Allow Unsigned Extensions" from the Develop menu
Open Safari Preferences, Extensions pane, and enable the ScrollBug extension
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.
This bug has been filed with Apple as FB9165971
Steps to reproduce:
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