[plt-scheme] A medley of questions

1.My teacher was redefine the function on-paint :
 (define/override on-paint
      (lambda () (send (get-dc) draw-bitmap $bitmap 0 0)))

This function brings up to date the window so that the graphs do not
disappear when one passes a window over the canvas. But when i draw a
element, i must send #f for masq the windows and send #t for can see the
element. Would it be possible to avoid making hide to return the window(i.e
not "flash" the windows) ?

2.I see in the documentation, the object message%, there would be an object
which would be able to post a string in a panel, taking parameters of the
type "blah ~a..." par1 ?