code: public void computeScrollSurfaceSize(Point out) {
          out.set((int) (maxViewport.width() * contentRectMinusAllMargins.width() / currentViewport.width()),
                  (int) (maxViewport.height() * contentRectMinusAllMargins.height() / currentViewport.height()));
      }
docs: Computes the current scrollable surface size, in pixels. For example, if the entire chart area is visible, this
  is simply the current size of {@link #contentRectMinusAllMargins}. If the chart is zoomed in 200% in both
  directions, the
  returned size will be twice as large horizontally and vertically.