Comment 41 for bug 1350369

What I see is that undo always leaves a pam that points to the start of the undone area and a mark to the end of the undone area, even if that area is empty. (In the normal where there is a selection this can be seen by selecting something, deleting it, and undoing and the newly undeleted stuff is again selected)

The table overwrite/paste thing looks to see if a mark is set and goes off to "do something very complex" if its set. So if after each undo cycle, you physically click at the point where the cursor is flashing (which clears the mark) and then paste, undo, *click*, paste you get a wonderfully stable experience.

So it seems reasonable to me to "do the simple thing" if there is no mark, or if the mark and point are the same, i.e. there is nothing actually selected by the PaM.