const
  Wheel_Scroll_Amount = 8;
  
newPos := ImageEnView1.ViewY + WheelDirection * Wheel_Scroll_Amount;
ImageEnView1.ViewY := nPos;

var
  FOldControl :TWinControl;

procedure TForm1.EnViewMouseEnter(Sender: TObject);
begin
 FOldControl := ActiveControl;
 if ImageEnView1.CanFocus then ImageEnView1.SetFocus;
end; 

procedure TForm1.ImageEnViewMouseLeave(Sender: TObject);
begin
 if Assigned(FOldControl) and FOldControl.CanFocus then
   FOldControl.SetFocus;
end;