> Only marker originally pointing inside the deleted region were recorded
> here, and this is to make sure they regain their previous position after
> the reinsertion.
>

I removed this part of the code to see what changes.

1. I put a marker in a line
2. Deleted the line
3. Undo-ed the delete

The marker is still in the proper place. So markers in the deleted string
do seem to regain their position properly.