Subclassing a listbox allows you to monitor horizontal scrolling. To subclass a listbox, you store the original WndProc in the UserData area of the listbox, allowing a single replacement of WndProc to work for all ListBox controls. WndProc notifies your form of a horizontal scroll message by sending a WM_MOUSEMOVE message with negative coordinates. The MouseMove event receives negative X and Y values, plus a Button value when horizontally scrolled, which is impossible under normal operation. Be sure to restore the original WndProc in the Form_Unload event:

 '--- Form codePrivate Sub Form_Load()	SetWindowLong List1.hwnd, GWL_USERDATA, _		SetWindowLong(List1.hwnd,GWL_WNDPROC, _		AddressOf WndProc)	SetWindowLong List2.hwnd, GWL_USERDATA, _		SetWindowLong(List2.hwnd, GWL_WNDPROC, _		AddressOf WndProc)End SubPrivate Sub Form_Unload(Cancel As Integer)	SetWindowLong List1.hwnd, GWL_WNDPROC, _		GetWindowLong(List1.hwnd, GWL_USERDATA)	SetWindowLong List2.hwnd, GWL_WNDPROC, _		GetWindowLong(List2.hwnd, GWL_USERDATA)End SubPrivate Sub List1_MouseMove(Button As Integer, Shift _	As Integer, X As Single, Y As Single)	If Button > 0 And X < 0 And Y < 0 Then Debug.Print _		"List1 Horizontal Scroll"End SubPrivate Sub List2_MouseMove(Button As Integer, Shift _	As Integer, X As Single, Y As Single)	If Button > 0 And X < 0 And Y < 0 Then Debug.Print _		"List2 Horizontal Scroll"End Sub'--- Module codePublic Declare Function CallWindowProc Lib "user32" Alias _	"CallWindowProcA" (ByVal lpPrevWndFunc As Long, ByVal _	hwnd As Long, ByVal Msg As Long, ByVal wParam As Long, _	ByVal lParam As Long) As LongPublic Declare Function GetWindowLong Lib "user32" Alias _	"GetWindowLongA" (ByVal hwnd As Long, ByVal nIndex _	As Long) As LongPublic Declare Function SendMessage Lib "user32" Alias _	"SendMessageA" (ByVal hwnd As Long, ByVal wMsg As _	Long, ByVal wParam As Long, lParam As Any) As LongPublic Declare Function SetWindowLong Lib "user32" Alias _	"SetWindowLongA" (ByVal hwnd As Long, ByVal nIndex As _	Long, ByVal dwNewLong As Long) As LongPublic Const GWL_WNDPROC = (-4)Public Const GWL_USERDATA = (-21)Public Const WM_HSCROLL = &H114Public Const WM_MOUSEMOVE = &H200Public Function WndProc(ByVal hwnd As Long, ByVal Msg As _	Long, ByVal wParam As Long, ByVal lParam As Long) _	As Long	If Msg = WM_HSCROLL Then SendMessage hwnd, _		WM_MOUSEMOVE, 1, ByVal &HFFFF		WndProc = CallWindowProc(GetWindowLong(hwnd, _			GWL_USERDATA), hwnd, Msg, wParam, lParam)End Function

For brevity, this example omits the code that would add horizontal scrollbars to the List1 and List2 controls, but is readily available in the Knowledge Base (Q192184).