code: TwoDimensional.Position currentLine() {
          int parIdx = getCurrentParagraph();
          Cell<Paragraph<PS, SEG, S>, ParagraphBox<PS, SEG, S>> cell = virtualFlow.getCell(parIdx);
          int lineIdx = cell.getNode().getCurrentLineIndex(caretSelectionBind.getUnderlyingCaret());
          return paragraphLineNavigator.position(parIdx, lineIdx);
      }
docs: Returns the current line as a two-level index.
  The major number is the paragraph index, the minor
  number is the line number within the paragraph.
  
  <p>This method has a side-effect of bringing the current
  paragraph to the viewport if it is not already visible.