I presume by 'when I hover my mouse over the box and scroll' you mean you are using the mousewheel. If so you need to use a couple of API's. The below assumes you are using access 2007 or later.
At the top of the form module put
Code:
'Scrolling Constants
Private Const WM_VSCROLL = &H115
Private Const SB_LINEUP = 0
Private Const SB_LINEDOWN = 1
Private Declare PtrSafe Function GetFocus Lib "USER32" () As LongPtr
Private Declare PtrSafe Function SendMessage Lib "USER32" Alias "SendMessageA" (ByVal hWnd As LongPtr, ByVal wMsg As Long, ByVal wParam As LongPtr, lParam As Any) As LongPtr
Function useMouseWheel(wCount As Long)
Dim i As Long
For i = 1 To Abs(wCount)
SendMessage GetFocus, WM_VSCROLL, IIf(wCount < 0, SB_LINEUP, SB_LINEDOWN), 0&
Next
End Function
and call it in the form on mouse wheel event - put the name of richtext control where indicated
Code:
Private Sub Form_MouseWheel(ByVal Page As Boolean, ByVal Count As Long)
'note to set the active control, it is necessary to click on the control to make it the active control
If ActiveControl.Name = "nameOfRichTextControl" Then useMouseWheel (Count)
End Sub