fix: a logic error in Step

[ndtoan96]

fix #231

[sxyazi]

Thank you for the PR.

But I can't reproduce the issue, and not quite clear on why "prev" would result in old < self.cursor, any tips here?

1.mp4

[ndtoan96]

Supposed you have two subfolders in the current folder

-->a
   b

You put the cursor at the second one (but don't enter it)

   a
-->b

Then cd to the first one.

Only debug build will get this issue, because --release build wraps around overflow by default.

[sxyazi]

I reproduced it.

Thanks for the report, it was a logic error of Step introduced by #213, not an overflow error.