ComboBox Scroll with Mouse wheel

[DarkGlobus1OO]

i tried to work with peter Peter Thornton (Excel MVP 2008-13) code which works amazing for userform Comboxes and listboxes but im a vba noobie and i cant seem to understand how to make this code work for a regular ComboBox on a worksheet

Module Code:

'Enables mouse wheel scrolling in controls
Option Explicit


#If Win64 Then
    Private Type POINTAPI
       XY As LongLong
    End Type
#Else
    Private Type POINTAPI
           X As Long
           Y As Long
    End Type
#End If


Private Type MOUSEHOOKSTRUCT
    Pt As POINTAPI
    hWnd As Long
    wHitTestCode As Long
    dwExtraInfo As Long
End Type


#If  VBA7 Then
    Private Declare PtrSafe Function FindWindow Lib "user32" _
                                            Alias "FindWindowA" ( _
                                                            ByVal lpClassName As String, _
                                                            ByVal lpWindowName As String) As Long ' not sure if this should be LongPtr
    #If  Win64 Then
        Private Declare PtrSafe Function GetWindowLongPtr Lib "user32" _
                                            Alias "GetWindowLongPtrA" ( _
                                                            ByVal hWnd As LongPtr, _
                                                            ByVal nIndex As Long) As LongPtr
    #Else
        Private Declare PtrSafe Function GetWindowLong Lib "user32" _
                                            Alias "GetWindowLongA" ( _
                                                            ByVal hWnd As LongPtr, _
                                                            ByVal nIndex As Long) As LongPtr
    #End  If
    Private Declare PtrSafe Function SetWindowsHookEx Lib "user32" _
                                            Alias "SetWindowsHookExA" ( _
                                                            ByVal idHook As Long, _
                                                            ByVal lpfn As LongPtr, _
                                                            ByVal hmod As LongPtr, _
                                                            ByVal dwThreadId As Long) As LongPtr
    Private Declare PtrSafe Function CallNextHookEx Lib "user32" ( _
                                                            ByVal hHook As LongPtr, _
                                                            ByVal nCode As Long, _
                                                            ByVal wParam As LongPtr, _
                                                           lParam As Any) As LongPtr
    Private Declare PtrSafe Function UnhookWindowsHookEx Lib "user32" ( _
                                                            ByVal hHook As LongPtr) As LongPtr ' MAYBE Long
    'Private Declare PtrSafe Function PostMessage Lib "user32.dll" _
    '                                         Alias "PostMessageA" ( _
    '                                                         ByVal hwnd As LongPtr, _
    '                                                         ByVal wMsg As Long, _
    '                                                         ByVal wParam As LongPtr, _
    '                                                         ByVal lParam As LongPtr) As LongPtr   ' MAYBE Long
    #If  Win64 Then
        Private Declare PtrSafe Function WindowFromPoint Lib "user32" ( _
                                                            ByVal Point As LongLong) As LongPtr    '
    #Else
        Private Declare PtrSafe Function WindowFromPoint Lib "user32" ( _
                                                            ByVal xPoint As Long, _
                                                            ByVal yPoint As Long) As LongPtr    '
    #End  If
    Private Declare PtrSafe Function GetCursorPos Lib "user32" ( _
                                                            ByRef lpPoint As POINTAPI) As LongPtr   'MAYBE Long
#Else
    Private Declare Function FindWindow Lib "user32" _
                                            Alias "FindWindowA" ( _
                                                            ByVal lpClassName As String, _
                                                            ByVal lpWindowName As String) As Long
    Private Declare Function GetWindowLong Lib "user32.dll" _
                                            Alias "GetWindowLongA" ( _
                                                            ByVal hWnd As Long, _
                                                            ByVal nIndex As Long) As Long
    Private Declare Function SetWindowsHookEx Lib "user32" _
                                            Alias "SetWindowsHookExA" ( _
                                                            ByVal idHook As Long, _
                                                            ByVal lpfn As Long, _
                                                            ByVal hmod As Long, _
                                                            ByVal dwThreadId As Long) As Long
    Private Declare Function CallNextHookEx Lib "user32" ( _
                                                            ByVal hHook As Long, _
                                                            ByVal nCode As Long, _
                                                            ByVal wParam As Long, _
                                                           lParam As Any) As Long
    Private Declare Function UnhookWindowsHookEx Lib "user32" ( _
                                                            ByVal hHook As Long) As Long
    'Private Declare Function PostMessage Lib "user32.dll" _
    '                                         Alias "PostMessageA" ( _
    '                                                         ByVal hwnd As Long, _
    '                                                         ByVal wMsg As Long, _
    '                                                         ByVal wParam As Long, _
    '                                                         ByVal lParam As Long) As Long
    Private Declare Function WindowFromPoint Lib "user32" ( _
                                                            ByVal xPoint As Long, _
                                                            ByVal yPoint As Long) As Long
    Private Declare Function GetCursorPos Lib "user32.dll" ( _
                                                            ByRef lpPoint As POINTAPI) As Long
#End  If


Private Const WH_MOUSE_LL As Long = 14
Private Const WM_MOUSEWHEEL As Long = &H20A
Private Const HC_ACTION As Long = 0
Private Const GWL_HINSTANCE As Long = (-6)
'Private Const WM_KEYDOWN As Long = &H100
'Private Const WM_KEYUP As Long = &H101
'Private Const VK_UP As Long = &H26
'Private Const VK_DOWN As Long = &H28
'Private Const WM_LBUTTONDOWN As Long = &H201
Dim n As Long
Private mCtl As Object
Private mbHook As Boolean
#If  VBA7 Then
    Private mLngMouseHook As LongPtr
    Private mListBoxHwnd As LongPtr
#Else
    Private mLngMouseHook As Long
    Private mListBoxHwnd As Long
#End  If
     
Sub HookListBoxScroll(frm As Object, ctl As Object)
    Dim tPT As POINTAPI
    #If VBA7 Then
        Dim lngAppInst As LongPtr
        Dim hwndUnderCursor As LongPtr
    #Else
        Dim lngAppInst As Long
        Dim hwndUnderCursor As Long
    #End  If
    GetCursorPos tPT
    #If  Win64 Then
        hwndUnderCursor = WindowFromPoint(tPT.XY)
    #Else
        hwndUnderCursor = WindowFromPoint(tPT.X, tPT.Y)
    #End  If
    If TypeOf ctl Is UserForm Then
        If Not frm Is ctl Then
               ctl.SetFocus
        End If
    Else
        If Not frm.ActiveControl Is ctl Then
             ctl.SetFocus
        End If
    End If
    If mListBoxHwnd <> hwndUnderCursor Then
        UnhookListBoxScroll
        Set mCtl = ctl
        mListBoxHwnd = hwndUnderCursor
        #If  Win64 Then
            lngAppInst = GetWindowLongPtr(mListBoxHwnd, GWL_HINSTANCE)
        #Else
            lngAppInst = GetWindowLong(mListBoxHwnd, GWL_HINSTANCE)
        #End  If
        ' PostMessage mListBoxHwnd, WM_LBUTTONDOWN, 0&, 0&
        If Not mbHook Then
            mLngMouseHook = SetWindowsHookEx( _
                                            WH_MOUSE_LL, AddressOf MouseProc, lngAppInst, 0)
            mbHook = mLngMouseHook <> 0
        End If
    End If
End Sub


Sub UnhookListBoxScroll()
    If mbHook Then
        Set mCtl = Nothing
        UnhookWindowsHookEx mLngMouseHook
        mLngMouseHook = 0
        mListBoxHwnd = 0
        mbHook = False
    End If
End Sub
#If  VBA7 Then
    Private Function MouseProc( _
                            ByVal nCode As Long, ByVal wParam As Long, _
                            ByRef lParam As MOUSEHOOKSTRUCT) As LongPtr
        Dim idx As Long
        On Error GoTo errH
        If (nCode = HC_ACTION) Then
            #If  Win64 Then
                If WindowFromPoint(lParam.Pt.XY) = mListBoxHwnd Then
                    If wParam = WM_MOUSEWHEEL Then
                        MouseProc = True
'                        If lParam.hWnd > 0 Then
'                            postMessage mListBoxHwnd, WM_KEYDOWN, VK_UP, 0
'                        Else
'                            postMessage mListBoxHwnd, WM_KEYDOWN, VK_DOWN, 0
'                        End If
'                        postMessage mListBoxHwnd, WM_KEYUP, VK_UP, 0
                        If TypeOf mCtl Is Frame Then
                            If lParam.hWnd > 0 Then idx = -10 Else idx = 10
                            idx = idx + mCtl.ScrollTop
                            If idx >= 0 And idx < ((mCtl.ScrollHeight - mCtl.Height) + 17.25) Then
                                mCtl.ScrollTop = idx
                            End If
                        ElseIf TypeOf mCtl Is UserForm Then
                            If lParam.hWnd > 0 Then idx = -10 Else idx = 10
                            idx = idx + mCtl.ScrollTop
                            If idx >= 0 And idx < ((mCtl.ScrollHeight - mCtl.Height) + 17.25) Then
                                mCtl.ScrollTop = idx
                            End If
                        Else
                             If lParam.hWnd > 0 Then idx = -1 Else idx = 1
                             idx = idx + mCtl.ListIndex
                             If idx >= 0 And idx <= mCtl.ListCount - 1 Then mCtl.ListIndex = idx
                        End If
                    Exit Function
                    End If
                Else
                    UnhookListBoxScroll
                End If
            #Else
                If WindowFromPoint(lParam.Pt.X, lParam.Pt.Y) = mListBoxHwnd Then
                    If wParam = WM_MOUSEWHEEL Then
                        MouseProc = True
'                        If lParam.hWnd > 0 Then
'                            postMessage mListBoxHwnd, WM_KEYDOWN, VK_UP, 0
'                        Else
'                            postMessage mListBoxHwnd, WM_KEYDOWN, VK_DOWN, 0
'                        End If
'                        postMessage mListBoxHwnd, WM_KEYUP, VK_UP, 0
                        If TypeOf mCtl Is Frame Then
                            If lParam.hWnd > 0 Then idx = -10 Else idx = 10
                            idx = idx + mCtl.ScrollTop
                            If idx >= 0 And idx < ((mCtl.ScrollHeight - mCtl.Height) + 17.25) Then
                                mCtl.ScrollTop = idx
                            End If
                        ElseIf TypeOf mCtl Is UserForm Then
                            If lParam.hWnd > 0 Then idx = -10 Else idx = 10
                            idx = idx + mCtl.ScrollTop
                            If idx >= 0 And idx < ((mCtl.ScrollHeight - mCtl.Height) + 17.25) Then
                                mCtl.ScrollTop = idx
                            End If
                        Else
                             If lParam.hWnd > 0 Then idx = -1 Else idx = 1
                             idx = idx + mCtl.ListIndex
                             If idx >= 0 And idx <= mCtl.ListCount - 1 Then mCtl.ListIndex = idx
                        End If
                        Exit Function
                    End If
                Else
                    UnhookListBoxScroll
                End If
            #End If
        End If
        MouseProc = CallNextHookEx( _
                                mLngMouseHook, nCode, wParam, ByVal lParam)
        Exit Function
errH:
        UnhookListBoxScroll
    End Function
#Else
    Private Function MouseProc( _
                            ByVal nCode As Long, ByVal wParam As Long, _
                            ByRef lParam As MOUSEHOOKSTRUCT) As Long
        Dim idx As Long
        On Error GoTo errH
        If (nCode = HC_ACTION) Then
            If WindowFromPoint(lParam.Pt.X, lParam.Pt.Y) = mListBoxHwnd Then
                If wParam = WM_MOUSEWHEEL Then
                    MouseProc = True
'                    If lParam.hWnd > 0 Then
'                    postMessage mListBoxHwnd, WM_KEYDOWN, VK_UP, 0
'                    Else
'                    postMessage mListBoxHwnd, WM_KEYDOWN, VK_DOWN, 0
'                    End If
'                    postMessage mListBoxHwnd, WM_KEYUP, VK_UP, 0
                    
                    If TypeOf mCtl Is Frame Then
                        If lParam.hWnd > 0 Then idx = -10 Else idx = 10
                        idx = idx + mCtl.ScrollTop
                        If idx >= 0 And idx < ((mCtl.ScrollHeight - mCtl.Height) + 17.25) Then
                            mCtl.ScrollTop = idx
                        End If
                    ElseIf TypeOf mCtl Is UserForm Then
                        If lParam.hWnd > 0 Then idx = -10 Else idx = 10
                        idx = idx + mCtl.ScrollTop
                        If idx >= 0 And idx < ((mCtl.ScrollHeight - mCtl.Height) + 17.25) Then
                            mCtl.ScrollTop = idx
                        End If
                    Else
                         If lParam.hWnd > 0 Then idx = -1 Else idx = 1
                         idx = idx + mCtl.ListIndex
                         If idx >= 0 And idx <= mCtl.ListCount - 1 Then mCtl.ListIndex = idx
                    End If
                    Exit Function
                End If
            Else
                UnhookListBoxScroll
            End If
        End If
        MouseProc = CallNextHookEx( _
        mLngMouseHook, nCode, wParam, ByVal lParam)
        Exit Function
errH:
        UnhookListBoxScroll
    End Function
#End  If

hope someone here can help me i have been scratching my head around this problem for days!

 

[ISY]

Hi

This approach uses the

PeekMessage API which is less invasive, more stable and not prone to potential crashings

Unfortunately it doesn't work well

The scroll only works when going up while if I try to go down, the combobox closes and the sheet slides down
Window 10 Excel 2016 64 bit

 

[Jaafar Tribak]

@ISY

First of all, thanks for testing my code and offering constructive feedback.

Interstingly, I too have excel 2016 64 bit just like you, and I can't reproduce the problem you described when scrolling down... everything works for me as advertised.

BTW, did you try using the code that uses the WH_MOUSE_LL hook in post#19 ? does it work well for you ?

Regards.

 

[ISY]

Hi

BTW, did you try using the code that uses the WH_MOUSE_LL hook in post#19 ? does it work well for you ?

I tested the code in post # 19 it goes in continuously In the crask, it works in both directions for a few moments at low speed for (1-2 seconds)...

In one case out of 10 co slow movements I was able to select a value.

 

[Jaafar Tribak]

Hi

I tested the code in post # 19 it goes in continuously In the crask, it works in both directions for a few moments at low speed for (1-2 seconds)...

In one case out of 10 co slow movements I was able to select a value.

Sorry what do you mean by "In the crask" ? does it crash in the end ?

 

[ISY]

Hi

One Type Debug as

Private Sub ComboBox1_MouseMove(ByVal Button As Integer, ByVal Shift As Integer, ByVal x As Single, ByVal y As Single)
    Call SetComboBoxHook(ComboBox1)
End Sub

Terminate with Crash

More... Crash Immediately

 

[Jaafar Tribak]

OK

I think I have found the reason why these mousewheel scroll codes I posted in this thread so far fail on some computers :

The reason is the vbeditor needs to be broken and then reset before running the code(s)

This is an issue I faced before when trying to subclass the excel application window.

I want to pull this one through either by running the code from within a dll or by resetting the vbeditor via code (this one is an ugly hack but should work fine)

Regards.

 

[Jaafar Tribak]

@ISY

Can you do me a favour ?

Before running the code, go to the VBEditor and click on the break button and then on the reset button.

Now, go back to the worksheet and try mouse-scrolling the combobox and see what happens .... It should now hopefully work fine.

Can you confirm that for me?

Regards.

 

[ISY]

Jaafar Tribak

Ok, Ok

The sequence:
Alt + F11 "the Open Vbe"
Ctrl + Break
I press Restores Vbe

After this step the code runs regularly without errors

 

[Jaafar Tribak]

Jaafar Tribak

Ok, Ok

The sequence:
Alt + F11 "the Open Vbe"
Ctrl + Break
I press Restores Vbe

After this step the code runs regularly without errors

Thanks ISY for testing and confirming.

Now, that we located the 'source of the problem', I will try to find a workaround and post back .

Regards.

 

[Jaafar Tribak]

@ISY

This new version should work accross different excel\windows platforms... I just followed the hook call by a message pump which seemed to work without the need to reset the vbeditor or to run the code from a dll.

Workbook Demo.

1- API code in a Standard Module:

Option Explicit

Type POINTAPI
    x As Long
    y As Long
End Type

Type RECT
    Left As Long
    Top As Long
    Right As Long
    Bottom As Long
End Type

Type MSLLHOOKSTRUCT
    pt As POINTAPI
    mouseData As Long
    flags As Long
    time As Long
    dwExtraInfo As Long
End Type

#If  VBA7 Then
    #If  Win64 Then
        Declare PtrSafe Function WindowFromPoint Lib "user32" (ByVal Point As LongPtr) As LongPtr
    #Else
        Declare PtrSafe Function WindowFromPoint Lib "user32" (ByVal xPoint As Long, ByVal yPoint As Long) As LongPtr
    #End If
    Declare PtrSafe Function GetClassName Lib "user32" Alias "GetClassNameA" (ByVal hwnd As LongPtr, ByVal lpClassName As String, ByVal nMaxCount As Long) As Long
    Declare PtrSafe Function GetParent Lib "user32" (ByVal hwnd As LongPtr) As LongPtr
    Declare PtrSafe Function GetActiveWindow Lib "user32" () As LongPtr
    Declare PtrSafe Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (Destination As Any, Source As Any, ByVal Length As LongPtr)
    Declare PtrSafe Function GetCursorPos Lib "user32" (ByRef lpPoint As POINTAPI) As LongPtr
    Declare PtrSafe Function SetFocus Lib "user32" (ByVal hwnd As LongPtr) As LongPtr
    Declare PtrSafe Function IsWindow Lib "user32" (ByVal hwnd As LongPtr) As Long
    Declare PtrSafe Function SetWindowsHookEx Lib "user32" Alias "SetWindowsHookExA" (ByVal idHook As Long, ByVal lpfn As LongPtr, ByVal hmod As LongPtr, ByVal dwThreadId As Long) As LongPtr
    Declare PtrSafe Function CallNextHookEx Lib "user32" (ByVal hHook As LongPtr, ByVal nCode As Long, ByVal wParam As LongPtr, lParam As Any) As LongPtr
    Declare PtrSafe Function UnhookWindowsHookEx Lib "user32" (ByVal hHook As LongPtr) As LongPtr
    Declare PtrSafe Function PostMessage Lib "user32" Alias "PostMessageA" (ByVal hwnd As LongPtr, ByVal wMsg As Long, ByVal wParam As LongPtr, ByVal lParam As LongPtr) As Long
    Declare PtrSafe Function GetClientRect Lib "user32" (ByVal hwnd As LongPtr, lpRect As RECT) As Long
    Declare PtrSafe Function GetSystemMetrics Lib "user32" (ByVal nIndex As Long) As Long
    Dim hwnd As LongPtr, lMouseHook As LongPtr
#Else
    Declare Function WindowFromPoint Lib "user32" (ByVal xPoint As Long, ByVal yPoint As Long) As Long
    Declare Function GetClassName Lib "user32" Alias "GetClassNameA" (ByVal hwnd As Long, ByVal lpClassName As String, ByVal nMaxCount As Long) As Long
    Declare Function GetParent Lib "user32" (ByVal hwnd As Long) As Long
    Declare Function GetActiveWindow Lib "user32" () As Long
    Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (Destination As Any, Source As Any, ByVal Length As Long)
    Declare Function GetCursorPos Lib "user32" (ByRef lpPoint As POINTAPI) As Long
    Declare Function SetFocus Lib "user32" (ByVal hwnd As Long) As Long
    Declare Function IsWindow Lib "user32" (ByVal hwnd As Long) As Long
    Declare Function SetWindowsHookEx Lib "user32" Alias "SetWindowsHookExA" (ByVal idHook As Long, ByVal lpfn As Long, ByVal hmod As Long, ByVal dwThreadId As Long) As Long
    Declare Function CallNextHookEx Lib "user32" (ByVal hHook As Long, ByVal nCode As Long, ByVal wParam As Long, lParam As Any) As Long
    Declare Function UnhookWindowsHookEx Lib "user32" (ByVal hHook As Long) As Long
    Declare Function PostMessage Lib "user32" Alias "PostMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long
    Declare Function GetClientRect Lib "user32" (ByVal hwnd As Long, lpRect As RECT) As Long
    Declare Function GetSystemMetrics Lib "user32" (ByVal nIndex As Long) As Long
    Dim hwnd As Long, lMouseHook As Long
#End  If

Const WH_MOUSE_LL = 14
Const WM_MOUSEWHEEL = &H20A
Const HC_ACTION = 0
Const WM_LBUTTONDOWN = &H201
Const WM_LBUTTONUP = &H202
Const MK_LBUTTON = &H1
Const SM_CXVSCROLL = 2


Public Property Let EnableMouseScroll(ByVal ComboBox As Object, ByVal Enable As Boolean)

    Dim tpt As POINTAPI, sBuffer As String * 256, lRet As Long
       
    Application.EnableCancelKey = xlDisabled
    On Error GoTo XitProp
   
    If Enable Then
        If lMouseHook = 0 Then
            GetCursorPos tpt
            #If  VBA7 And Win64 Then
                Dim lPt As LongPtr
                CopyMemory lPt, tpt, LenB(tpt)
                hwnd = WindowFromPoint(lPt)
            #Else
                hwnd = WindowFromPoint(tpt.x, tpt.y)
            #End  If
            lRet = GetClassName(GetParent(hwnd), sBuffer, 256)
            If InStr(Left(sBuffer, lRet), "MdcPopup") Then
                Debug.Print "Mousecroll Enabled for : " & ComboBox.Name
                SetFocus hwnd
                #If  VBA7 Then
                    lMouseHook = SetWindowsHookEx(WH_MOUSE_LL, AddressOf MouseProc, Application.HinstancePtr, 0)
                #Else
                    lMouseHook = SetWindowsHookEx(WH_MOUSE_LL, AddressOf MouseProc, Application.Hinstance, 0)
                #End  If
                Do While IsWindow(hwnd)
                    DoEvents
                Loop
                GoTo XitProp
            End If
        End If
    Else
        If lMouseHook Then
            GoTo XitProp
        End If
    End If
    Exit Property
   
XitProp:
   
    UnhookWindowsHookEx lMouseHook: lMouseHook = 0
    Debug.Print "Mousecroll Disabled for : " & ComboBox.Name

End Property


#If  VBA7 Then
    Function MouseProc(ByVal nCode As Long, ByVal wParam As LongPtr, lParam As MSLLHOOKSTRUCT) As LongPtr
#Else
    Function MouseProc(ByVal nCode As Long, ByVal wParam As Long, lParam As MSLLHOOKSTRUCT) As Long
#End  If
       
    Dim tRect As RECT
    Dim Low As Integer, High As Integer
   
    On Error GoTo errHandler
   
    If (nCode = HC_ACTION) Then
        If wParam = WM_MOUSEWHEEL Then
            #If  VBA7 And Win64 Then
                Dim lPt As LongPtr
                Dim lParm As LongPtr
                CopyMemory lPt, lParam.pt, LenB(lPt)
                If WindowFromPoint(lPt) = hwnd Then
            #Else
                Dim lParm As Long
                If WindowFromPoint(lParam.pt.x, lParam.pt.y) = hwnd Then
            #End  If
                    GetClientRect hwnd, tRect
                    If lParam.mouseData > 0 Then
                        Low = tRect.Right - (GetSystemMetrics(SM_CXVSCROLL) / 2)
                        High = tRect.Top + ((GetSystemMetrics(SM_CXVSCROLL) / 2) + 1)
                    Else
                        Low = tRect.Right - (GetSystemMetrics(SM_CXVSCROLL) / 2)
                        High = tRect.Bottom - ((GetSystemMetrics(SM_CXVSCROLL) / 2) + 1)
                    End If
                    lParm = MakeDWord(Low, High)
                    PostMessage hwnd, WM_LBUTTONDOWN, MK_LBUTTON, lParm
                    PostMessage hwnd, WM_LBUTTONUP, MK_LBUTTON, lParm
                End If
        End If
    End If

    MouseProc = CallNextHookEx(lMouseHook, nCode, wParam, ByVal lParam)
    Exit Function
   
errHandler:
    
     UnhookWindowsHookEx lMouseHook: lMouseHook = 0
   
End Function


Private Function MakeDWord(ByVal LoWord As Integer, ByVal HiWord As Integer) As Long
    MakeDWord = (HiWord * &H10000) Or (LoWord And &HFFFF&)
End Function

Code Usage:

2- Code in the worksheet module where the embedeed ActiveX comboboxes are located.

Option Explicit

Private Sub ComboBox1_MouseMove(ByVal Button As Integer, ByVal Shift As Integer, ByVal x As Single, ByVal y As Single)
    EnableMouseScroll(ComboBox1) = True
End Sub

Private Sub ComboBox1_LostFocus()
    EnableMouseScroll(ComboBox1) = False
End Sub


Private Sub ComboBox2_MouseMove(ByVal Button As Integer, ByVal Shift As Integer, ByVal x As Single, ByVal y As Single)
    EnableMouseScroll(ComboBox2) = True
End Sub

Private Sub ComboBox2_LostFocus()
    EnableMouseScroll(ComboBox2) = False
End Sub

EDIT:
Anyone using this code should bear in mind that while scrolling the combobox(es), any unhandled error could potentially cause the application to crash ... Once the combobox is not being used (ie: unselected) there is no longer an issue as the mouse hook is automatically removed when working on the worksheet.... So, please use propper error handling inside the combobox(es) event procedures.