Delphi scrollBox image display flickering issue

Question

I have a MDI application that has MDI child windows.

Those windows have a TEventScrollBox

TEventScrollBox   =   class(TScrollBox)
  private
          FVScrollEvent:   TNotifyEvent;
          FHScrollEvent:   TNotifyEvent;
          FCanvas      :   TCanvas;

          Procedure   WMVScroll(   Var   msg:   TWMVScroll   );   Message   WM_VSCROLL;
          Procedure   WMHScroll(   Var   msg:   TWMHScroll   );   Message   WM_HSCROLL;

          Procedure WMPaint( Var Message: TWMPaint ); Message WM_PAINT;
  public

          PaintProc  : TNotifyEvent;

          Procedure   DoVScroll;
          Procedure   DOHSCroll;
          property    OnVScroll:   TNotifyEvent read   FVScrollEvent   write   FVScrollEvent;
          property    OnHScroll:   TNotifyEvent read   FHScrollEvent   write   FHScrollEvent;

          property    Canvas   :   TCanvas read FCanvas;

          Constructor Create(Owner: TComponent); override;
          Destructor  Destroy; override;
  end; 

Where I paint an image inside it using its ScrollBox.Canvas.Handle and in the end SetDIBitsToDevice() in the WMpaint call

It works OK, but from time to time it flickers. Setting Doublebuffered to true does not help. I have tried with putting "invalidate" but this is worse.

Here is the implementation of the WMPaint

Procedure TEventScrollBox.WMPaint( Var Message: TWMPaint );
Begin
   Inherited;

   If Assigned(PaintProc) then
     PaintProc(Self);
End;

the constructor of TEventScrollBox

constructor TEventScrollBox.Create(Owner: TComponent);
begin
  inherited Create(Owner);
  PaintProc                       := Nil;
  FCanvas                         := TControlCanvas.Create;
  TControlCanvas(FCanvas).Control := Self;   
end; 

I tried many things without a clear success... Flickering is still visible. Any idea ?