public override void Update(IGUIContext ctx)
{
// this sets RowManager.FirstParagraphOnScreen
// to a common value for the following OnPaintBackground and OnPaint
// so that all paintings are guaranteed synchronized and thus flicker-free
// An important point about such implementations:
// * The "manager" calculates everything in it's own absolute (0/0) metrics.
// * The "presenter" (this class) finally adds all kinds of "Offsets" (Paddings, Scrolling, ..)
// otherwise you'd have to deal with strange feedback effects
// which can make life very very hard.
RowManager.OnLayout(-ScrollOffsetY);
base.Update(ctx);
}