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Do you think when I have done a small HTML viewer name "Gulliver" that I do a bitmap and then scroll it, I can't imagine do it, no but I not use VDI to draw I have an offscreen redraw then do a vro_cpyfm() but this bitmap is only the part I need redraw no more. This could be usefull to help display some pictures in awindows but no more, it can't be universal.RAM is cheap. Unlike the majority of people on this list, I'm actually using a system that has off-screen rendering. XP does not. OS X does, but OS X is also all hardware accelerated. Xorg using Xcomposite extensions is about the same as what you are talking about - you draw off-screen and then blit thechanges to the window. Scrolling is horrible!
Perhaps the implementation is not good! I do it for Gulliver and it's pleasant on an enough speed computer (It vos enough on MagicMac on my old PPC 8600 at 300Mhz!). It's more pleasant than software drawing directly on screen, this is quite easy to understand because you not have to erase before draw, that do blink. I don't know about Xcomposite never see, but if you draw your offscreen part before start scroll on screen there is no problem this is not horrible, of course if you not do this this could be very unpleasant. Ok it's slower than what you think, but what you wan't is impossible to do in a general case. I look if I put it on web and it look not be avaible, I will send you a version to see scrolling (this is not a good HTML viewer, there is a lot of things not working)
... Olivier