[layer8]

What happens when, for example, you atomically replace “mouse” by “elephant”, then select “epha”, and then region-undo?

The reason most editors don’t implement this is probably that it’s hard to conceive of how it should behave in the general case. (That’s not to say that the way Emacs is implementing it isn’t good and useful.)

[soamv]

Great question. I tried it and I got `elephantmouse`. I was surprised by this, but FWIW I've never encountered this edge case in actual usage.