GraphicsWidth(), Height() only returns

This works for me.

SuperStrict
SetGraphicsDriver GLMax2DDriver()

Global Window:TGadget = CreateWindow("Graphics",100,100,640,480)
Global Canvas:TGadget = CreateCanvas(0,0,ClientWidth(Window),ClientHeight(Window),Window)
SetGadgetLayout(Canvas,EDGE_ALIGNED,EDGE_ALIGNED,EDGE_ALIGNED,EDGE_ALIGNED)
Global timer:TTimer = CreateTimer(60)
AddHook EmitEventHook, MyHook

Repeat
	WaitEvent()
Forever

Function MyHook:Object(id:Int,Data:Object,Context:Object)
	Local Event:TEvent = TEvent(Data)
	
	Select Event.id
		Case EVENT_WINDOWCLOSE
			End
		Case EVENT_GADGETPAINT
			SetGraphics CanvasGraphics(Canvas)
			Cls
			DrawText GraphicsWidth()+" "+GraphicsHeight(),10,10
			Flip
			Return Null
		Case EVENT_WINDOWSIZE
			SetGraphics CanvasGraphics(Canvas)
			SetViewport 0,0,ClientWidth(Window),ClientHeight(Window)
			RedrawGadget Canvas
			Return Null
		Case EVENT_TIMERTICK
			RedrawGadget Canvas
			Return Null
	End Select
	Return data
End Function

Although I'd think you'd want to use GadgetWidth() and GadgetHeight() on a canvas.

Edit: Didn't notice you were having problems with only OpenGL, so I changed the above code to use OpenGL instead of DirectX. Still works though :)