Hey,
I'm using this on app.slack.com in whitelist mode. Up until a month or so ago this worked fine.

Now the extension seems to do nothing.

Diving into it a little bit, I tried using Extensions -> Debug and setting some breakpoints.. no breakpoints were hit, but the extension magically started working!
In addition, I (eventually, after a few minutes) saw this note in the console for the inspector for the background page:

Background event page was not terminated on idle because a DevTools toolbox is attached to the extension.

Is this maybe related?