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.
I'm pretty sure sharing the undo buffer was an issue for years. I remember teaching people that if they realised they had a whole bunch of things they needed to undo, they should close all other open workbooks before doing their undo-ing, or it might be their undoing!
I agree it would be good to separate these, but if working on linked files, there could be dependency issues (I remove or revert a formula which the other one is relying on). I assume this was always the logic for keeping them intertwangled.
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!