The simplest way to scroll is to erase the current contents of the client area, and then paint the new information. This is the method an application is likely to use with SB_PAGEUP, SB_PAGEDOWN, SB_TOP, and SB_END requests where completely new contents are required.
For some requests, such as SB_LINEUP and SB_LINEDOWN, not all the contents need to be erased, since some will still be visible after the scroll. The ScrollWindow function preserves a portion of the client area's contents, moves the preserved portion the specified amount, and prepares the rest of the client area for painting new information. ScrollWindow uses the BitBlt function to move a specific part of the client area to a new location within the client area. Any part of the client area that is uncovered (not in the part to be preserved) is invalidated and will be erased and painted over at the next WM_PAINT message.
ScrollWindow also lets an application clip a part of the client area from the scroll. This is to keep items that have fixed positions in the client area, such as child windows, from moving. This action automatically invalidates the part of the client area that is to receive the new information so that the application does not have to compute its own clipping regions.