If you don't have a mouse on your PC, you can't scroll SYSMETS3 at all. Scroll bars created as part of your application window do not have an automatic keyboard interface. Because Windows can be installed without a mouse, it is highly recommended that you write programs that do not require the mouse.
In the next chapter you'll learn how to use the keyboard and how to add a keyboard interface to SYSMETS. You'll notice that SYSMETS3 seems to process WM_VSCROLL messages where wParam equals SB_TOP and SB_BOTTOM. I mentioned earlier that a window procedure doesn't receive these messages for scroll bars, so right now this is superfluous code. When we come back to this program in the next chapter, you'll see the reason for including this code.