Is there a Finder plugin (maybe SIMBL plugin) which allows one to see the directory structure in the Finder's sidebar (instead of Favorites, Shared, Devices) or in an attached window?
Something like this:
There is Cover Flow, but it's not what I need.
Also I don't need any Finder replacement; I just want to fix this missing feature in Finder.
I doubt there's a SIMBL plugin to do that. What you're asking for would be a fairly extensive rewrite even if you had the source code for the Finder.
Your best bets are what @jtbandes and @Richard's comments on your question state: get a Finder replacement that does that or use list view in the Finder.
There is now a program called Owlfiles for MacOS that is the closest I've found to what you're wanting. I find the lack of a navigation pane in Finder irritating... Honestly Apple, just copy Microsoft's File Explorer already as it is far more superior for advanced file management tasks.
