I recently added a few action extensions to my Mac app. They work fine and I display their UI and progress in a dedicated window. For some reason though, Finder also open a window where it displays the process name and an undefined progress bar and says "estimating time remaining. 
I've looked everywhere, but can't find documentation on if this can prevented from popping up or how it could be controlled. Easy way to recreate is to start a project with one of Apple's template action extensions in Xcode or download samples here: https://developer.apple.com/documentation/appkit/app_extensions/add_functionality_to_finder_with_action_extensions