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Keine Antwort auf die Frage, aber vielleicht trotzdem interessant:
However, the ComboLBox (list box) window for combo boxes that have the CBS_DROPDOWN or CBS_DROPDOWNLIST style is not a child of the ComboBox window. Instead, each ComboLBox window is a child of the desktop window. This is required so that, when the drop-down list box is dropped, it can extend outside the application window or dialog box. Otherwise, the list box would be clipped at the window or dialog box border.
Because the ComboLBox window is not a child of the ComboBox window, there is no simple method to get the handle of one window, given the other. For example, given a handle to the ComboBox, the handle to any associated drop- down list box is not readily available. The ComboLBox is a private class registered by USER that is a list box with the class style CS_SAVEBITS.