While the Stepper is running, it displays a read-only copy of the source in the source area. Therefore, you cannot edit the code in the source area, other than when the status is "Enter a form to step in the pane above.".
If you step a function for which the source has been edited since it was compiled, then the Stepper uses a copy of the compile-time source, not the edited source.
This copy is stepped in a new editor buffer created specially for it and this is displayed in the source area.