If you are having the same problem (it's still happening in VDF 17.0 Alpha 4), you can put the following code to your TreeView subclass to stop the annoying beep.

        Procedure Notify Integer wParam Integer lParam
            Integer iVoid iResult
            tWinNmHdr Header
            tWinNmTvKeyDown KeyInfo
            Forward Get Msg_Notify wParam lParam to iResult
            Move (MemCopy(AddressOf(Header), lParam, SizeOfType(tWinNmHdr))) to iVoid
            If (Header.Code = TVN_KEYDOWN) Begin
                Move (MemCopy(AddressOf(KeyInfo), lParam, SizeOfType(tWinNmTvKeyDown))) to iVoid
                If (KeyInfo.wVKey = VK_TAB) Procedure_Return TRUE
            Procedure_Return iResult
        End_Procedure  // Notify
Frank Cheng