Определить вкладку под курсором

Private Sub TabStrip1_MouseUp(Button As Integer, Shift As Integer, x As Single, y As Single)
    
    If x <= TabStrip1.Tabs(1).Width Then
            MsgBox TabStrip1.Tabs(1).Caption
    ElseIf x <= TabStrip1.Tabs(1).Width + TabStrip1.Tabs(2).Width Then
            MsgBox TabStrip1.Tabs(2).Caption
    ElseIf x <= TabStrip1.Tabs(1).Width + TabStrip1.Tabs(2).Width + TabStrip1.Tabs(3).Width Then
            MsgBox TabStrip1.Tabs(3).Caption
    Else
            MsgBox TabStrip1.Tabs(4).Caption
    End If
End Sub

Option Explicit
Private Type POINTAPI
  x As Long
  y As Long
End Type
Private Type TCHITTESTINFO
  pt As POINTAPI
  flags As Long
End Type
Private Const TCM_FIRST As Long = &H1300
Private Const TCM_HITTEST As Long = (TCM_FIRST + 13)
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" ( _
  ByVal hwnd As Long, _
  ByVal wMsg As Long, _
  ByVal wParam As Long, _
  ByRef lParam As Any) As Long
' Есть ещё и такой вариант:
Private Declare Function SendMessageLong Lib "user32" Alias "SendMessageA" ( _
  ByVal hwnd As Long, _
  ByVal wMsg As Long, _
  ByVal wParam As Long, _
  ByVal lParam As Long) As Long
Private Sub TabStrip1_MouseDown(Button As Integer, Shift As Integer, x As Single, y As Single)
  Dim num As Long
  Dim nn As TCHITTESTINFO
  
  If Button = vbRightButton Then
    nn.pt.x = x / Screen.TwipsPerPixelX
    nn.pt.y = y / Screen.TwipsPerPixelY
    num = SendMessage(TabStrip1.hwnd, TCM_HITTEST, 0, nn)
    Me.Caption = num
    '
    ' и т.д. .....
    '
  End If
End Sub