mh-nexus.de

It's very inconvenient now that I can't just CTRL+Z after file saving to reverse changes

Not possible due to technical constraints as mentioned is a similar feature request.

The diff history will not be valid anymore after the underlying file is changed (i.e., saved). Also after saving the file is unlocked and any other program can change it, which would make any diff information invalid as well.

These constraints are necessary to allow for big file support (i.e., not keeping files in memory entirely).

It is a little inconvenient though
Can I suggest a workaround?
If "Create backups (BAK files)" is checked and the program saves the file, it can also save the diff history separately.
When you press Ctrl+Z after the file has been saved, the program can ask you if you really want it to change back. If you reply yes, it can then combine the current with the previous diff histories and undo the previous change. I understand that another program can change and save the file, but half of the times, it's HxD, and in that case, the user decides. This way, it won't keep anything more in memory and it'll have the feature. It may be a little slow with huge files, but only when you combine the two diff histories, only for a second.