Comment 10 for bug 1622757

Step Up/Step Down mean increment/decrement the value shown in the box. This is the same meaning as the spinner arrows and cursor up/down when the cursor is in the box. It is the same behavior in bulk edit, single edit, and on the book list. In all cases the value you see *in the box* is the value changed.

One possible source of confusion is that "undefined" has an actual value: -1,000,000. That is why incrementing "undefined" came up with -999,999. My changes make incrementing undefined result in the box containing zero.

The spinner arrows work for me in bulk edit.

As for "sensibly": they do what I expect even though I didn't implement the behavior. You expect something different.

It seems that you want to use bulk edit increment the value in each book. It doesn't do that. However, you can do it in Search & Replace using "Template" as a source. Example:
Search field: template
Template: program: re(add(raw_field('#myint'), 1), "\.0$", "")
Search for: (.*)
Replace with: \1
Destination field: #myint