Option Explicit
 
' Benötigte API-Deklarationen
Private Declare Function GetCursorPos Lib "user32" ( _
  lpPoint As POINTAPI) As Long
 
Private Declare Function PtInRect Lib "user32" ( _
  lpRect As RECT, _
  ByVal x As Long, _
  ByVal y As Long) As Long
 
Private Declare Function GetWindowRect Lib "user32" ( _
  ByVal hwnd As Long, lpRect As RECT) As Long
 
Private Type POINTAPI
  x As Long
  y As Long
End Type
 
Private Type RECT
  Left As Long
  Top As Long
  Right As Long
  Bottom As Long
End Type

Private Sub Form_Load()
  ' Timer initialisieren
  Timer1.Interval = 50
  Timer1.Enabled = True
End Sub

Private Sub Timer1_Timer()
  Dim P As POINTAPI
  Dim R As RECT
 
  ' akuelle Mauszeiger-Position ermitteln
  GetCursorPos P
 
  ' Position und Größe der Form
  GetWindowRect Me.hwnd, R
 
  ' Befindet sich der Mauszeiger innerhalb der Form?
  If PtInRect(R, P.x, P.y) = 0 Then
    MsgBox "Maus außerhalb der Form!"
  End If
End Sub

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