When the previous undo is not finished, if you operate the next undo with Ctrl + z, the software freezes in an hourglass state and the software is forcibly terminated.
Repeating heavy undo processing increases the probability of forced termination in the same way.
Please modify the program so that it ignores the input even if the next undo reservation input comes while the undo is not completed.

In order to make it even safer, I felt that it would be better to ignore all reservation inputs and wait for a certain amount of time before accepting further inputs.