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.

Code:
        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
            End
            Procedure_Return iResult
        End_Procedure  // Notify
Frank Cheng