This message is not sent by WinCreateWindow when a window is created, and so any size-related processing must be done during the WM_CREATE message processing in this instance.
This message is sent after the window has been actually sized, but before any repainting has been done. Any resizing or repositioning of child windows that might be necessary a a result of the size change is usually done during the processing of this message.
Note: It is generally unwise to output to the window during the processing of this message, because the area drawn might be redrawn, after the WM_SIZE processing is complete, by the WinSetWindowPos function.
The processing of this message for a window which is displaying an advanced VIO presentation space must be carried out by the default advanced VIO window procedure.