Created attachment 716 [details]
before fix

As reported in the URL.

The issue here is that the rounding problem (from double -> int for GDI+ pixels) will depend on the number of elements in the LegendScrollBar, meaning that the 'fix' of a pixel (an int of value '1') will not necessarily render correctly in all cases. 

For this code:
    private void InitializeChart()
    {
      Line series = new Line(tChart1.Chart);
      series.FillSampleValues(100);

      LegendScrollBar tool = new LegendScrollBar(tChart1.Chart);
      tool.ThumbBrush.Color = Color.Red;
      tool.Bevel.Inner = BevelStyles.None;
      tool.Bevel.Outer = BevelStyles.None;

      tChart1.CurrentTheme = ThemeType.Opera;

    }

the fix can be seen to affect rendering in the two attached images (at 3x magnification).