I used this script: #include <GUIConstantsEx.au3> #include <WindowsConstants.au3> GUIRegisterMsg($WM_GETMINMAXINFO, "WM_GETMINMAXINFO") $Form1 = GUICreate("Form1", 615, 438, -1, -1, BitOR($GUI_SS_DEFAULT_GUI, $WS_MAXIMIZEBOX, $WS_SIZEBOX, $WS_THICKFRAME, $WS_TABSTOP)) GUISetState(@SW_SHOW) $Form2 = GUICreate("Form2", 200, 200, 500, 20, BitOR($WS_THICKFRAME, $WS_SYSMENU, $WS_POPUP), $WS_EX_MDICHILD, $Form1) GUISetState(@SW_SHOW) While 1 $nMsg = GUIGetMsg() Switch $nMsg Case $G