Menus on windows with canvases

If I have a window with a canvas on it and a menu, when the menu is clicked on it appears but remains on the screen until the mouse is clicked again. You can see this in the code below. This is taken straight out of the help - the only difference is a single line that creates a canvas on the window. If You click on the file menu it appears, but if you move the mouse along to the other menus they appear as expected but the initial file menu is still shown on the screen as well. Removing the canvas makes the menu work normally.

So the question is what should I be doing to ensure my menu refreshes correctly when I have a canvas on the window?

Const MENU_NEW=101
Const MENU_OPEN=102
Const MENU_SAVE=103
Const MENU_CLOSE=104
Const MENU_EXIT=105

Const MENU_CUT=106
Const MENU_COPY=107
Const MENU_PASTE=108

Const MENU_ABOUT=109

window=CreateWindow("My Window",40,40,320,240)
Local canvas:TGadget=CreateCanvas(0,0,320,240,window)


filemenu=CreateMenu("&File",0,WindowMenu(window))
CreateMenu"&New",MENU_NEW,filemenu,KEY_N,MODIFIER_COMMAND
CreateMenu"&Open",MENU_OPEN,filemenu,KEY_O,MODIFIER_COMMAND
CreateMenu"&Close",MENU_CLOSE,filemenu,KEY_W,MODIFIER_COMMAND
CreateMenu"",0,filemenu
CreateMenu"&Save",MENU_SAVE,filemenu,KEY_S,MODIFIER_COMMAND
CreateMenu"",0,filemenu
CreateMenu"E&xit",MENU_EXIT,filemenu,KEY_F4,MODIFIER_COMMAND

editmenu=CreateMenu("&Edit",0,WindowMenu(window))
CreateMenu "Cu&t",MENU_CUT,editmenu,KEY_X,MODIFIER_COMMAND
CreateMenu "&Copy",MENU_COPY,editmenu,KEY_C,MODIFIER_COMMAND
CreateMenu "&Paste",MENU_PASTE,editmenu,KEY_V,MODIFIER_COMMAND

helpmenu=CreateMenu("&Help",0,WindowMenu(window))
CreateMenu "&About",MENU_ABOUT,helpmenu

UpdateWindowMenu window

While True
	WaitEvent 
	Select EventID()
		Case EVENT_WINDOWCLOSE
			End
		Case EVENT_MENUACTION
			Select EventData()
				Case MENU_EXIT
					End
				Case MENU_ABOUT
					Notify "Incrediabler~n(C)2005 Incredible Software"
			End Select
	End Select
Wend