copy-pixels overrides background colour depending on window size
This works, draws random black circles on a white background:

     (defsketch bug
              ((width 800)
               (height 600)
               (copy-pixels t)
               (pen (make-pen :fill +black+)))
            (with-pen pen
              (circle (random width) (random height) (random 10))))
      (defmethod setup ((instance bug) &key &allow-other-keys)
                  (background +white+))

But if you change both the width & height to 400, for example, then the background goes black. It also seems like an initial window comes up, then gets closed and a new one opens up with the desired dimensions. I'm on Linux and have Intel graphics hardware, if that makes a difference.

edit: My guess is that sketch restarts the window in order to initialise it with the proper dimensions, but then doesn't call `setup` afterwards. I'm not sure why this behaviour is dependent on the width and height, though.