You can modify the core to fix this. Jon Henning posted this as a solution in a different thread, I've implemented this on a few sites.

The easy fix is to add the following code to the SelectedIndexChanged event

        Protected Sub optMode_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles optMode.SelectedIndexChanged
            If Page.IsCallback = False Then
                SetPreview(True)
                Response.Redirect(Request.RawUrl, True)
            End If
        End Sub