Unless this is set, child windows
are not scrolled. If this is set, and prclScroll is NULL, all the
child windows are scrolled by lDx and lDy units. If prclScroll
is not NULL, only those child windows that intersect prclScroll are
scrolled.
SW_INVALIDATERGN
The
invalid region created as a result of the scroll is added to the update
regions of those windows affected. This may result in sending WM_PAINT
messages to CS_SYNCPAINT windows before the call returns.