Here's a curious thing: a Mac-friendly file drawer GUI for MacVim. It was made by Eloy Durán, and is available at alloy / macvim on GitHub. To try it out, follow the build instructions on the fork's wiki.
The sidebar itself behaves as you'd expect: it's "global", so it always looks the same even when tabs are in use. It also works correctly in full-screen mode. File system commands like
:cd are reflected in the drawer, and switching buffers causes the highlighted file to change.
The General settings panel has options that pertain to the file drawer, so you can change the side which it appears on and make it always open by default.
It also adds a menu option for toggling the file browser.
There was a discussion about this in the mac_vim group where Bradford Smith asked about merging this fork:
It would be so lovely for MacVim to have a working file drawer. I see a fork exists which tries to add this behavior: https://github.com/alloy/macvim. Why not try to merge this into the main MacVim repository?
Björn replied with a detailed response:
Please don't take this as me saying I am opposed to having GUI drawers. I simply think that if it should be done at all it should be done right.
I have for a long time wanted to split MacVim into a view framework and an actual app. This way I could maintain MacVim as a "pure" gvim for Mac and anybody wishing to trick it out with drawers etc could simply reuse the view framework. I even started working on this last summer but got bitten by the "version 2.0 bug" (tried rewriting from scratch and ran out of time/energy).
There are those of us who have never been completely happy with file/project drawers in the first place. The best argument against this that I've found is the Oil and vinegar post by Drew Neil. But for those of you who have switched to Vim from GUI editors like TextMate or Sublime Text, then you might find Eloy's fork interesting.