-- scenarios:
--   line without B side
--   line with B side collapsed
--   line with B side expanded
--   line starting rendering in A side (startpos ~= nil)
--   line starting rendering in B side (startposB ~= nil)
--   my on final screen line of A side
--     mx to right of A side with no B side
--     mx to right of A side but left of B side
--     mx to right of B side
-- preconditions:
--  startpos xor startposB
--  expanded -> dataB

  local start_screen_line_index = Text.screen_line_index(line_cache.screen_line_starting_pos, line_cache.startpos)
  for screen_line_index = start_screen_line_index,#line_cache.screen_line_starting_pos do
    local screen_line_starting_pos = line_cache.screen_line_starting_pos[screen_line_index]
    local screen_line_starting_byte_offset = Text.offset(line.data, screen_line_starting_pos)
--?     print('iter', y, screen_line_index, screen_line_starting_pos, string.sub(line.data, screen_line_starting_byte_offset))

  if line_cache.startpos then
    local start_screen_line_index = Text.screen_line_index(line_cache.screen_line_starting_pos, line_cache.startpos)
    for screen_line_index = start_screen_line_index,#line_cache.screen_line_starting_pos do
      local screen_line_starting_pos = line_cache.screen_line_starting_pos[screen_line_index]
      local screen_line_starting_byte_offset = Text.offset(line.data, screen_line_starting_pos)
--?       print('iter', y, screen_line_index, screen_line_starting_pos, string.sub(line.data, screen_line_starting_byte_offset))
      local nexty = y + State.line_height
      if my < nexty then
        -- On all wrapped screen lines but the final one, clicks past end of
        -- line position cursor on final character of screen line.
        -- (The final screen line positions past end of screen line as always.)
        if screen_line_index < #line_cache.screen_line_starting_pos and mx > State.left + Text.screen_line_width(State, line_index, screen_line_index) then
--?           print('past end of non-final line; return')
          return line_cache.screen_line_starting_pos[screen_line_index+1]-1
        end
        local s = string.sub(line.data, screen_line_starting_byte_offset)
--?         print('return', mx, Text.nearest_cursor_pos(s, mx, State.left), '=>', screen_line_starting_pos + Text.nearest_cursor_pos(s, mx, State.left) - 1)
        if not line.expanded then
          -- no B side
          return screen_line_starting_pos + Text.nearest_cursor_pos(s, mx, State.left) - 1
        else
          local screen_line_posA = Text.nearest_cursor_pos(s, mx, State.left)
          if not line.expanded then
            -- B side is not expanded
            return screen_line_starting_pos + screen_line_posA - 1
          end
          local lenA = utf8.len(s)
          if screen_line_posA < lenA then
            -- mx is within A side
            return screen_line_starting_pos + screen_line_posA - 1
          end
          local max_xA = left+Text.x(s, len+1)
          if mx < max_xA + AB_padding then
            -- mx is in the space between A and B side
            return screen_line_starting_pos + screen_line_posA - 1
          end
          local mx = mx - max_xA - AB_padding
          local screen_line_posB = Text.nearest_cursor_pos(line.dataB, mx, --[[no left margin]] 0)
          return nil, screen_line_posB
        end
      end
      y = nexty
    end
  end
  -- look in screen lines composed entirely of the B side
  assert(line.expanded)
  assert(line_cache.startposB)
  local start_screen_line_indexB = Text.screen_line_indexB(line_cache.screen_line_starting_posB, line_cache.startposB)
  for screen_line_indexB = start_screen_line_indexB,#line_cache.screen_line_starting_posB do
    local screen_line_starting_posB = line_cache.screen_line_starting_posB[screen_line_indexB]
    local screen_line_starting_byte_offsetB = Text.offset(line.dataB, screen_line_starting_posB)
--?     print('iter', y, screen_line_indexB, screen_line_starting_pos, string.sub(line.dataB, screen_line_starting_byte_offsetB))

      if screen_line_index < #line_cache.screen_line_starting_pos and mx > State.left + Text.screen_line_width(State, line_index, screen_line_index) then

      if screen_line_indexB < #line_cache.screen_line_starting_posB and mx > State.left + Text.screen_line_widthB(State, line_index, screen_line_indexB) then

        return line_cache.screen_line_starting_pos[screen_line_index+1]-1

        return line_cache.screen_line_starting_posB[screen_line_indexB+1]-1

      local s = string.sub(line.data, screen_line_starting_byte_offset)
--?       print('return', mx, Text.nearest_cursor_pos(s, mx, State.left), '=>', screen_line_starting_pos + Text.nearest_cursor_pos(s, mx, State.left) - 1)
      return screen_line_starting_pos + Text.nearest_cursor_pos(s, mx, State.left) - 1

      local s = string.sub(line.dataB, screen_line_starting_byte_offsetB)
--?       print('return', mx, Text.nearest_cursor_pos(s, mx, State.left), '=>', screen_line_starting_posB + Text.nearest_cursor_pos(s, mx, State.left) - 1)
      return screen_line_starting_posB + Text.nearest_cursor_pos(s, mx, State.left) - 1