Separate Excel windows shall not share an UNDO buffer.
Sorry Microsoft, but you really blew it in the 2016 update. Separate Excel windows used to be separate Excel windows. Now they share the same buffers for Undo, copy/paste, and highlighting. This is really limiting when working comparatively on two spreadsheets.
Please upvote that one as well if you already haven't.
Yes, I find this really annoying. Undoing a batch of actions because I'm in the early stages of development, and suddenly I'm in the middle of undoing another sheet's recent actions merely because it is also open!