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If a child window does not process this message and instead calls DefWindowProc, DefWindowProc will send the message to its parent window. If a top level window does not process this message and instead calls DefWindowProc, DefWindowProc will call a shell hook with the hook code equal to HSHELL_APPCOMMAND.
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Unlike other windows messages, an application should return TRUE from this message if it processes it. Doing so will allow software that simulates this message on Microsoft Windows systems earlier than Windows 2000 to determine whether the window procedure processed the message or called DefWindowProc to process it.