Embedding Tk into a native Win32 GUI

In the window below, that is a Win32 API based progam, with a HWND embedded into the client area of the toplevel window. In that HWND is a Tk window with a packed frame with 2 buttons and a canvas. The canvas draws little circles when you click on it. This is about 35 lines of Tcl/Tk, and about 45 lines of C.

Download the source here