Run an Excel macro if cell "A500" gets visible / on screen - excel

Question

I'd like to get an Excel macro running as soon as cell "A500"
becomes visible on the screen when scrolling down/up the worksheet.
I remember reading somewhere about an active-x or standard control
that has an "on scrolling into view" event, so this could be done
by placing a control directly on the worksheet near the desired cell.
Finding this control currently eludes me.
A better way of course would be a cell formula, subclassing still is
a bad idea in the long run i guess :)
Sub temp_01() 'Excel Vba
'user scrolls down from cell "A1"
'when the user reaches cell "A500" show the following message:
MsgBox "Chapter 2"
End Sub

Answer 1

As mentioned above, with the help of the Onupdate event, (catches the mousewheel, not clicking on the scrollbars) (Change Sheetname(s) and Range(s) to yours)
In Class called ClsMonitorOnupdate:
Option Explicit
Private WithEvents objCommandBars As Office.CommandBars
Private rMonitor As Range
Private scrol As Boolean
Public Property Set Range(ByRef r As Range): Set rMonitor = r: End Property
Public Property Get Range() As Range: Set Range = rMonitor: End Property
Private Sub Class_Initialize()
Set objCommandBars = Application.CommandBars
End Sub
Private Sub Class_Terminate()
Set objCommandBars = Nothing
End Sub
Private Sub objCommandBars_OnUpdate()
Dim myrng As Range
If ActiveWorkbook.Name <> ThisWorkbook.Name Then Exit Sub
If ActiveSheet.Name <> rMonitor.Parent.Name Then Exit Sub
If TypeName(Selection) <> "Range" Then Exit Sub
If Intersect(Selection, rMonitor) Is Nothing Then Exit Sub
Set myrng = Application.Intersect(ActiveWindow.VisibleRange, ActiveSheet.Range("a500"))
If Not myrng Is Nothing And Not scrol Then scrol = True: MsgBox "chapter"
If myrng Is Nothing And scrol Then scrol = False
End Sub
In the ThisWorkbook section:
Option Explicit
Private sRanges As String
Private cMonitor As ClsMonitorOnupdate
Private Sub Workbook_BeforeClose(Cancel As Boolean)
Set cMonitor = Nothing
End Sub
Private Sub Workbook_Open()
Zetaan ActiveSheet
End Sub
Sub Zetuit()
Set cMonitor = Nothing
End Sub
Sub Zetaan(sht As Worksheet)
Select Case sht.Name
Case "Sheet1": sRanges = "A1:ZZ1000"
Case "Other Sheet": sRanges = "A1:ZZ1000"
Case Else: Exit Sub
End Select
Set cMonitor = New ClsMonitorOnupdate
Set cMonitor.Range = Sheets(sht.Name).Range(sRanges)
End Sub
Private Sub Workbook_SheetActivate(ByVal Sh As Object)
Zetaan Sh
End Sub
Private Sub Workbook_SheetDeactivate(ByVal Sh As Object)
Set cMonitor = Nothing
End Sub