Text.populate_screen_line_starting_pos(State, line_index)
  local num_screen_linesA = #line_cache.screen_line_starting_pos - Text.screen_line_index(line_cache.screen_line_starting_pos, line_cache.startpos) + 1
  local num_screen_linesB = 0
  if line.dataB and #line.dataB > 0 then

  local num_screen_lines = 0
  if line_cache.startpos then
    Text.populate_screen_line_starting_pos(State, line_index)
    num_screen_lines = num_screen_lines + #line_cache.screen_line_starting_pos - Text.screen_line_index(line_cache.screen_line_starting_pos, line_cache.startpos) + 1
  end
  if line.dataB and #line.dataB > 0 and line.expanded then

    num_screen_linesB = #line_cache.screen_line_starting_posB - Text.screen_line_index(line_cache.screen_line_starting_posB, line_cache.startposB)  -- no +1; first screen line of B side overlaps with A side

    num_screen_lines = num_screen_lines + #line_cache.screen_line_starting_posB - Text.screen_line_indexB(line_cache.screen_line_starting_posB, line_cache.startposB)  -- no +1; first screen line of B side overlaps with A side

  return y < line_cache.starty + State.line_height*(num_screen_linesA+num_screen_linesB)

  return y < line_cache.starty + State.line_height*num_screen_lines

function Text.screen_line_indexB(screen_line_starting_posB, posB)
  if posB == nil then
    return 0
  end
  assert(screen_line_starting_posB)
  for i = #screen_line_starting_posB,1,-1 do
    if screen_line_starting_posB[i] <= posB then
      return i
    end
  end
end