code: public int yToLine(int y) {
          FontMetrics fm = this.getFontMetrics(this.getFont());
          int height = fm.getHeight();
          Document doc = this.getDocument();
          int length = doc.getLength();
          Element map = doc.getDefaultRootElement();
          int startLine = map.getElementIndex(0);
          int endline = map.getElementIndex(length);
  
          return Math.max(0, Math.min(endline - 1, y / height + startLine)) - 1;
      }
docs: Converts a y co-ordinate to a line index.
  
  @param y The y co-ordinate