It would be good to know what the “undo” button will be undoing.
I’m assuming it must keep a stack of actions, so could clicking it show the last N items in its history? Or at least the last one on hover?
Main reason is that when you undo something, you don’t always see its effect. And if the last thing you did is not what you thought it was, you may have no idea what was just undone!
controversial edit - I’d further suggest that you either remove ctl-z as a keyboard shortcut for undo, as there’s also no way to know what you have just undone, or that preferably you trap it to pop up a history to choose from or a confirmation box to stop ctl-z spasms causing chaos. I appreciate this edit might not be popular 
