(Only in windows) a canvas with a long text inside a LayOut Vertical flashes during scroll
I have a container with one big control canvas, something like a long page of text which has to be scrolled. In linux gtk it works well, but in windows when I scroll down the text with the scrollbar of the layout container I can see a flashing as if the control inside the the window was redrawn or the container relayouted. Is there something I can do to avoid it? In linux there is no problem, only in Windows.
This is the part of my code that writes the text using a loop on a sequences of array from a sqlite query (to make it work the seq must be replaced with a static one). 
```
# Definition of a custom widget
type MyWidget* = ref object of ControlImpl
  counter: int
  altezza_riga:int
  fontsize:float

type Stile = ref object of RootObj
  font_size: float
  font_tipo: string ="Arial"
  font_color:Color=rgb(0,0,0)
  xpos:int
  margin_left: int
  margin_top: int

let stile1 = Stile(font_size:24.0,font_color:rgb(0,0,255),margin_left:20,margin_top:0)
let stile2 = Stile(font_size:18.0,margin_left:30,margin_top:5)


proc addp(canvas:Canvas,testo:string,stile:Stile,xpos:int,ypos:int)=
  canvas.fontSize = stile.font_size
  let x:int = stile.margin_left+xpos
  let y:int = stile.margin_top+ypos
  canvas.textColor = stile.font_color
  canvas.drawText(testo,x,y)

proc loop_dei_p(canvas:Canvas)=
  let db = openDatabase("./pianificazioni.sqlite")
  var matr = sqlite_query(db,"SELECT * FROM '01_aree'")
  var y =1
  for riga in matr:
    var txt=riga[1].replace("\n", " ")
    truncate txt,100
    y=y+30
    canvas.addp(txt,stile1,1,y)
    var obiet=sqlite_query(db,"SELECT titolo_obiettivo FROM '02_obiettivi' WHERE id_area=" & riga[0])
    for riga1 in obiet:
      var txt1=riga1[0].replace("\n", " ")
      truncate txt1,100
      y = y+20
      canvas.addp(txt1,stile2,1,y)

# Custom widgets need to draw themselves
method handleDrawEvent(control: MyWidget, event: DrawEvent) =
  let canvas = event.control.canvas
  canvas.areaColor = rgb(255, 255, 255)
  loop_dei_p(canvas)

# Constructor (optional)
proc newMyWidget*(): MyWidget =
  result = new MyWidget
  result.init()
  result.height = 1700.scaleToDpi
  result.width = 1900.scaleToDpi

app.init()
var window = newWindow()

var vbox = newLayoutContainer(Layout_Vertical)
vbox.widthMode =WidthMode_Expand
vbox.heightMode =HeightMode_Expand
var myWidget = newMyWidget()



vbox.add(myWidget)
window.add(vbox)


window.show()

app.defaultBackgroundColor=rgb(255,255,255)
app.run()``
```