"Line" here refer to "visual line": A section of the DOM that should be treated as a line for cursor movement. Line breaks may come from the back-end, or be inserted by the line break algorithm.
The lineStarts array maps from a line number to the DOM location of the start of the corresponding line.
The lineEnds array maps to the end of each line.
Always points to a span node with the line attribute set.
Normally lineEnds[i] == lineStarts[i+1]; however, sometimes
lineStarts[i] is the start of a <div> or other block element.