EVENT_GADGETPAINT prevents update?

Hi, I think that the event EVENT_GADGETPAINT used by Canvas prevents anything from being updated while it's in progress. This behavior is most probably only on Mac OS, because on WinXP I didn't had this 'bug' in my app.
Here's the code:

SuperStrict

Global MainWindow:TGadget = CreateWindow("Click and drag mouse to move slider", 100, 100, 400, 300)
Global MainCanvas:TGadget = CreateCanvas(0,0,300,200, MainWindow)
Global Slider:TGadget = CreateSlider(0,240,400,15, MainWindow, SLIDER_HORIZONTAL | SLIDER_SCROLLBAR)
SetSliderRange(slider, 50, 250)
Global global_slide_pos:Int = 0

AddHook EmitEventHook,MyHook

Repeat
	WaitEvent() 
	If EventID() = EVENT_APPTERMINATE Or EventID() = EVENT_WINDOWCLOSE Then End
Forever

Function MyHook:Object(iId:Int,tData:Object,tContext:Object)
	Local Event:TEvent=TEvent(tData)
	
	If Event.source=MainCanvas And Event.ID=EVENT_GADGETPAINT
		' Update here: it doesn't work!
		UpdateSlider() 
		Return Null
	EndIf

	If Event.ID=EVENT_MOUSEMOVE
		If(event.data = 1) Then
			global_slide_pos = event.x
			' Update here: it works!
			'UpdateSlider() 
		EndIf
	EndIf
	
	Return tData
End Function

Function UpdateSlider()
	SetSliderValue(slider, global_slide_pos)
	SetGadgetText(MainWindow, "Slider pos="+ global_slide_pos)
End Function

You'll see UpdateSlider at two differents place: in the example shown (inside GADGETPAINT), nothing happens if you click and drag the mouse around. However, if you resize the window, the slider and window title will get updated. Seems like the gadgets are 'dirty' but don't get drawn.

If you comment UpdateSlider and then uncommented the one in EVENT_MOUSEMOVE, then the program behaves correctly: the slider should move when you click and drag.
Can anyone confirm?