Investigate why the deferred popup test needs sleeping
Background
The solution introduced by !369 (merged) is only a workaround. More info: https://gitlab.com/eyeo/adblockplus/abc/webext-sdk/-/merge_requests/369#note_999122596
Use case
We don't really understand why the !369 (merged) fix works. Some investigation is needed to understand and fix the underlying issue.
What to change
TBD