(maxgui) refresh canvas from another window

Hi !
i've the following problem. when i open the second window and move it on the first, the first canvas is not drawn correctly (see the picture).

How to resolve this ?

Thanks !

Strict

' -------------------------------------------------------------------------------------------

Extern "win32"
	Function GetActiveWindow()
	Function SetWindowPos(hWnd:Int,after:Int,x:Int,y:Int,w:Int,h:Int,flags:Int)
End Extern

Const SWP_NOSIZE:Int = $1
Const SWP_NOMOVE:Int = $2
Const HWND_TOPMOST:Int = -1
Const HWND_NOTOPMOST:Int = - 2

Function alwaysOnTop(hwnd:Int)
	SetWindowPos( hwnd, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOSIZE + SWP_NOMOVE )
End Function

Function notAlwaysOnTop(hwnd:Int)
	SetWindowPos( hwnd, HWND_NOTOPMOST, 0, 0, 0, 0, SWP_NOSIZE + SWP_NOMOVE )
End Function
		

' -------------------------------------------------------------------------------------------


Global win1 : Tgadget = CreateWindow ("Win1",100,100,400,300,Null,WINDOW_TITLEBAR)
Global Canvas1 : Tgadget = CreateCanvas (10,40,350,200,Win1)
Global Button1 : Tgadget = CreateButton ("Open Win2",10,10,100,20,Win1)
HideGadget Win1

Global win2 : Tgadget = CreateWindow ("Win2",200,200,400,300,Null,WINDOW_TITLEBAR)
Global Canvas2 : Tgadget = CreateCanvas (10,40,350,200,Win2)
HideGadget Win2

Function Draw_First_window ()

	ShowGadget win1
	ActivateWindow(Win1)
	
	Local hwnd:Int = GetActiveWindow()
	alwaysOnTop(hwnd)

	CreateTimer 60

	While WaitEvent()
		Print CurrentEvent.ToString()
		Select EventID()
		
			Case EVENT_TIMERTICK
			
				SetGraphics CanvasGraphics(Canvas1)
				Cls
				
				For Local i=0 To GadgetWidth (Canvas1) Step 32
					DrawLine i,0,i,GadgetHeight(Canvas1)
				Next
				
				SetColor 0,40,0
				For Local i=0 To GadgetHeight (Canvas1) Step 32
					DrawLine 0,i,GadgetWidth(Canvas1),i
				Next				

				Flip
				
			Case EVENT_GADGETACTION
				
				If EventSource() = Button1 Then
					Draw_second_window()
				End If
		
			Case EVENT_WINDOWCLOSE
				Exit
				
		End Select
	Wend
	
	HideGadget Win1

End Function

Function Draw_second_window ()

	ShowGadget win2
	ActivateWindow(Win2)
	
	Local hwnd:Int = GetActiveWindow()
	alwaysOnTop(hwnd)

	CreateTimer 60

	While WaitEvent()
	
		Select EventID()
		
			Case EVENT_TIMERTICK
			
				SetGraphics CanvasGraphics(Canvas2)
				Cls
				
				For Local i=0 To GadgetWidth (Canvas2) Step 32
					DrawLine i,0,i,GadgetHeight(Canvas2)
				Next
				
				SetColor 0,40,0
				For Local i=0 To GadgetHeight (Canvas2) Step 32
					DrawLine 0,i,GadgetWidth(Canvas2),i
				Next
				Flip
				
			Case EVENT_WINDOWCLOSE
				Exit
		End Select
	Wend
	
	HideGadget Win1

End Function

Draw_First_Window()