When you're editing in the code editor, the history stack persists across different code cells.

So if I have the editor open on a python cell and type something, save it, then open another cell and hit "undo", it will revert the change I made in another code cell.

This is really odd. History stack should be on a per-cell basis. If I open a new cell where I haven't edited anything yet, hitting CMD+Z shouldn't do anything.