Low level of MFC is the Win32 API. Open the context menu by right clicking on a call to CListCtrl::GetItemCount() and choose 'Go to definition'. You will see that it is an inline function that sends the LVM_GETITEMCOUNT message to the list control window. This message is not processed while your GUI thread is blocked.
A general rule is: Never block GUI threads. If you have to wait for events, do this in worker threads.