#include <WindowsConstants.au3>
#include <StructureConstants.au3>
#include <GUIConstantsEx.au3>

Global Const $GUIChildStyle = BitOr($WS_CHILD,$WS_TABSTOP,$WS_Border), $GUIChildExStyle = $WS_EX_WINDOWEDGE

Local $GUI = GUICreate("Test", 1000, 1000, -1, -1, BitOR($WS_MINIMIZEBOX, $WS_MAXIMIZEBOX, $WS_CAPTION, $WS_POPUP, $WS_SYSMENU, $WS_SIZEBOX, $WS_CLIPCHILDREN, $WS_CLIPSIBLINGS), -1, 0), _
$ChildGUI = GUICreate("", 940, 940, 30, 30, $GUIChildStyle, $GUIChildExStyle, $GUI), _
$ChildGUI2 = GUICreate("", 880, 880, 30, 30, $GUIChildStyle, $GUIChildExStyle, $ChildGUI)

GUISwitch($GUI)
Local $Tab0 = GUICtrlCreateTab(0, 0, 1000, 1000, -1, -1), _
$Tab0Item0 = GUICtrlCreateTabItem("Level 1")
GUICtrlCreateTabItem("")
GUISwitch($ChildGUI)
Local $Tab1 = GUICtrlCreateTab(0, 0, 940, 940), _
$Tab1Item0 = GUICtrlCreateTabItem("Level 2")
GUICtrlCreateTabItem("")
GUISwitch($ChildGUI2)
GUICtrlCreatelabel("Stuff...", 10, 10, 100, 20)

GUISetBkColor(0x000000, $GUI)
GUISetBkColor(0xAAAAAA, $ChildGUI)
GUISetBkColor(0xFFCC00, $ChildGUI2)

GUISetState(@SW_SHOW, $GUI)
GUISetState(@SW_SHOW, $ChildGUI)
GUISetState(@SW_SHOW, $ChildGUI2)
Global Const $User32Dll = DllOpen("user32.dll")
GUIRegisterMsg($WM_SIZE, "WM_SIZE")
While 1
	Switch GUIGetMsg()
		Case -3
			GUIRegisterMsg($WM_SIZE, "")
			DllClose($User32Dll)
			Exit
	EndSwitch
WEnd

Func WM_SIZE($hWnd, $iMsg, $wParam, $lParam)
	#forceref $iMsg, $wParam
	If $hWnd = $GUI Then;This is the main, level 0 GUI
		Local $tRECT = DllStructCreate($tagRECT)
		DllCall($User32Dll, "bool", "GetClientRect", "hwnd", $hWnd, "struct*", $tRECT)
		Local $GUIWidth = $tRECT.Right - $tRECT.Left, $GUIHeight = $tRECT.Bottom - $tRECT.Top
		;Adjust the level 1 GUI
		WinMove($ChildGUI, "", $tRECT.Left + 30, $tRECT.Top + 30, $GUIWidth - 60, $GUIHeight - 60, 0)
		;Adjust the level 2 GUI
		;=========================> Uncomment the line below to see the error. <=========================
		;WinMove($ChildGUI2, "", $tRECT.Left + 30, $tRECT.Top + 30, $GUIWidth - 120, $GUIHeight - 120, 0)
		;=========================> Uncomment the line above to see the error. <=========================
	EndIf
	Return $GUI_RUNDEFMSG
EndFunc