If usChangedFirst equals usChangedLast, only one system value has changed.

If an application changes the settable system values, it is the responsibility of the application to post this message to all main windows.

This message is processed by WC_FRAME windows by doing any frame-specific processing (such as sending WM_SETBORDERSIZE messages to the size border if SV_CX/CYSIZEBORDER system values have changed) and then sending the message to the client window if one exists.

This message is only posted when settable system values change.