test.cs

private void rulerBar1_MouseDoubleClick(object sender, MouseEventArgs e)
{
    // calculate zoom factor
    float fZoomFactor = (float)textControl1.ZoomFactor / 100;

    // get the click location in the ruler bar
    int iClickLocation = (int)((e.X * iDpiX) + (textControl1.ScrollLocation.X * fZoomFactor));

    // loop through all tab positions
    foreach (int tabPosition in textControl1.Paragraphs.GetItem(
        textControl1.Selection.Start).Format.TabPositions)
    {
        // retrieve the exact location including zoom factor
        int iTabLocation = (int)((tabPosition + 
            textControl1.Sections.GetItem().Format.PageMargins.Left) * fZoomFactor);

        // if click location matches tab position
        // open the tab dialog
        if (iClickLocation >= iTabLocation - 50 && iClickLocation <= iTabLocation + 50)
        {
            textControl1.TabDialog(); // open dialog
            break;
        }
    }
}