nospam
undo only exists while you're using the app and it's within its undo history. at some point, it won't be undoable, generally when the file is saved but sometimes well before that, depending on the app. that's the whole problem with a traditional workflow.

with a non-destructive workflow, anything can be reversed at any time because the original data remains unaltered, including long after the app has been quit and even archived and later worked on with a different computer.

the difference is very important, and one which floyd and eric do not fully understand (or at all).

Eric Stevens
Of course I understand it!

You should stop and think a little about what you are actually doing. You are NOT editing the image until you export it in some way.