Correct me if I am wrong, but isn't WS_CLIPCHILDREN supposed to stop parent window from drawing over its children window? I have a parent window with two child windows, yet whenever I move the parent window, strangely enough one of the child's window content is always invalidated. Is this because I am using openGL to draw the content within the window? A post here seemed to suggest the lack of respect in one toward the other.
EDIT1 I have found my flaw. Tried changing window background to other colours only to note that some background section of the window failed to comply with the change. It turned out that the value I use to clear my OpenGL color buffer bit had been wrong, hence making my window showing black all the time.
So the real issue now is, why isn't my textured quad render? I have already bound it. Depth test and alpha test are also disabled. The coordinate for texture and vertex are corrected. The viewport is also set to be that of the window, and the projection matrix and modelview matrix are both reset. Other windows that use the same logic seemed to work fine enough.