I finally discovered why I can't copy and paste in RStudio: there's an interaction with Alfred's clipboard manager. I was noticing that every time I copy something from RStudio, it gets appended to the last thing I copied. Alfred has a feature where double-tapping C when you copy using Cmd-C appends the next selection to the most recent item in the clipboard. Disabling this feature resolves the problem. I guess RStudio must be sending a double-tapped C when copying.
Thanks for posting this — that does sound like a confusing interaction to track down and I think having a post here will definitely will help others find an answer .
In case it's helpful to anyone in the future, I can reproduce this problem on:
Running via Firefox 68.0.1 on the same Mac as above
I didn't see any issues in the Github tracker about this weird interaction. If you want to consider filing one, here's your starting point: RStudio IDE Guide: Writing Good Bug Reports