If you don't want text to scroll off the bottom of the screen during evaluation, use the "Scroll Often" option, from the "Options" menu.
Pixie Scheme will keep the text most recently added in view, but will run a whole lot slower.
"Evaluate" is undoable, in the restricted sense that the "Undo" command removes from the Pixie Scheme window whatever text was printed during the last evaluation. Pixie Scheme cannot undo any of the other side effects of evaluation. Other side effects include binding or assignment of variables, and file input and output.