[plt-scheme] question about unexpected behavior using with-steps in Slideshow

I tried to run the program you posted, but it seems to be
mis-parenthesized and I couldn't figure out what you were trying to do
from the code, exactly, but maybe this slideshow is an example of what
you want to do?

(module tsp-show mzscheme
  (require (lib "slideshow.ss" "slideshow")
           (lib "step.ss" "slideshow"))
 
  (with-steps 
   (one two)
   (slide
    (frame
     (ct-superimpose
      ((vonly one)
       (vl-append (t "line one")
                  (t "line two")))
      ((vonly two) 
       (vl-append (t "just the one line"))))))))

The trick here is that both paragraphs are in both slides. Only one is
visible, but both contribute to the overall shape.

For more details on this kind of thing and elaborations of it, let me
encourage you to check out this paper:

http://people.cs.uchicago.edu/~robby/pubs/papers/jfp2005-ff-color.pdf

section 3 in particular.

hth,
Robby