Shoutbox

There's a difference between invalidating the control and allowing the next WM_PAINT to redraw it when it's convenient, and forcing a repaint unless SETREDRAW has been explicitly turned off.

WM_PAINT is the lowest priority message and gets special handling anyway to stop multiple paint messages being sent in quick succession; this should have been exploited rather than MS taking the lazy approach and repainting it every time something was changed, even if that has no visible effect, as is often the case.