看起來只有在子窗體的 Activate 事件里做文章
但是也有問題，就是要打開的窗體一定會打開，

Dim sLastForm As String
Dim sThisForm As String '正要打開的子窗體的caption屬性

Private Sub Form_Activate()
Debug.Print "Form_Activate" & Me.Caption
sThisForm = Me.Caption
End Sub

Private Sub Form_Deactivate()
Debug.Print "Form_Deactivate" & Me.Caption
sLastForm = Me.Caption
End Sub