Major Description
This patch fixes a painting bug that bothered me: when clicking on an element of the taskbar, the sizer became an ugly border. This was due to uxtheme handling WM_NCACTIVATE message and repainting the non-client area itself no matter if WM_NCPAINT was already handled by the window proc (as our explorer does).
It seems that uxtheme doesn't trigger a paint on WM_NCACTIVATE under certain conditions, for instance, if a window doesn't have WS_CAPTION style.