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入力中文字表示をIMEで行うならDefWindowProc が要るんじゃなかろうか...
もう見ただろうけどSDKの説明(下記)を見ておもた。

　If the application has created an IME window, it should pass this message to that window.
　The DefWindowProc function processes this message by passing it to the default IME window.
　The IME window processes this message by updating its appearance based on the change flag given.