Scalable canvas / Graphics mode 3

BlitzPlus Forums/BlitzPlus Programming/Scalable canvas / Graphics mode 3

I've forgotten how to do this. Or, rather, I've done what I thought should do it, but it's not happenning =] I'm trying to set up a 800x600 canvas on a window that covers the full screen:

Global dw=GadgetWidth(Desktop())
Global dh=GadgetHeight(Desktop())
Global pWindow=CreateWindow("",0,0,800,600,0,16+3)
Global menubuffer=CreateCanvas(0,0,800,600,pWindow)
SetGadgetLayout menubuffer,1,1,1,1
SetGadgetShape pWindow,0,0,dw,dh
SetBuffer CanvasBuffer(menubuffer)
ClsColor 0,0,0
Cls
FlipCanvas menubuffer
MouseWait
End

But as you can see, it remains an 800x600 canvas with the remaining space acting as a dead zone.