Comment by akkartik

Hmm, now I'm wondering if paste should save the clipboard contents before so undo can restore them.

But no. Your text editor is a cottage surrounded by an outdoors it doesn't control. It makes no more sense to try to take charge of the clipboard than of a flock of geese flying overhead. What if someone or some other app pasted something in between? Paste is an arrow shot, a message in a bottle.