As a Cocoa developer, I have investigated the source code of TextEdit, and found that the undo is not saved in the file at all. It merely allows (for the lifetime of the window) for undo past the last undo, a new trick of Tiger's Cocoa API. I repeat: the undos are not saved in the file. The backup file option has nothing to do with this either. When Cocoa saves a file, it writes the data to another file (the backup file) and then copies it onto the destination, so that in case of a power outage, nothing should go wrong. You may keep the backup file, but I have found no real use for it.

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In the beginning, there was open source. And it was good. Then Microsoft came.

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Slightly off topic but somewhat related:

Does this backup file mean that information lost when textedit crashes is recoverable? Where is this backup file saved?

Thanks

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