窗口未最大化时丢失的WM_NCLBUTTONUP消息的奇怪问题

I've got a window that I handle WM_NCLBUTTONUP messages, in order to handle clicks on custom buttons in the caption bar. This works great when the window is maximised, but when it's not, the WM_NCLBUTTONUP message never arrives! I do get a WM_NCLBUTTONDOWN message though. Strangely WM_NCLBUTTONUP does arrive if I click on the right of the menu bar, but anywhere along the caption bar / window frame, the message never arrives.I've got a window that I handle WM_NCLBUTTONUP