Windows: Can't paint in WM_PAINT

For some reason, I need to paint something in WM_PAINT, but if I use BeginPaint in WM_PAINT in a QWindow/QWidget's nativeEvent function, the whole window will become blank. If I do it in a native event filter and install it, the window won't become blank but all the changes I made in WM_PAINT are lost.

If it's a bug, please tell me the work around and I'll submit a patch if you don't have time.

If it's not a bug, I wish you can give me some advice on how to achieve this. Hacks are acceptable as well.