I learned something today.
Never put GitHub-managed folders in Dropbox folders because Git creates temporary and/or invisible files in folders, and Dropbox may do things that trigger unintended actions or side-effects in Git, causing random havoc which is not what you want.
If you get in trouble because you put a GitHub-managed folder in Dropbox, and you're using the Mac client, you can unhook from the repo by clicking on the word Repositories in the main window, right-click on the repo you are having trouble with, remove it. Then re-add it by going to the GitHub website, and click on the Clone in Desktop button to load it into the Mac desktop client.
I am putting this in a blog post so I can find it next time I get into this situation and forget how to get out of it. And in the meantime I'm moving my folders out of Dropbox hoping to never get here again!