Hi Tom, I understand re not having an undo for deleting or closing molecules, but un-deleting parts of models would definitely be nice unless it is technically difficult/impossible to implement.
Basically, as universal and reversible an undo/redo function as possible, preferably bound to Cmd-Z/Shft-Cmd-Z to undo/redo the most recent action, would make using Chimera much easier, particularly for new users who are still learning their way around the interface.
Having granular control over which of several recent actions to undo is a nice-to-have, but not essential - usually if I want to undo something, it will be the last thing I did which I have just realized was a mistake - deleting the wrong selection, closing the wrong molecule, etc etc. Along the same lines, having an autosave or history function that saves the state of the session at regular intervals in the background would also be very handy.
Cheers,