Text.populate_link_offsets(State, line_index)

      -- render fragment

      end

      -- render any link decorations
      for _,link_offsets in ipairs(line_cache.link_offsets) do
        local s,e,filename = unpack(link_offsets)
        local lo, hi = Text.clip_filename_with_screen_line(line, line_cache, i, s, e)
        if lo then
          button(State, 'link', {x=State.left+lo, y=y, w=hi-lo, h=State.line_height, color={1,1,1},
            icon = icon.hyperlink_decoration,
            onpress1 = function()
                         run.switch_to_file(filename)
                       end,
          })
        end

      -- If fragment is a filename relative to the current directory, register
      -- a button for clicks on it.
      local filename = rtrim(frag)  -- compute_fragments puts whitespace at the end
      if file_exists(filename) then
        local filename_text = App.newText(love.graphics.getFont(), filename)
        button(State, 'link', {x=x,y=y, w=App.width(filename_text), h=State.line_height, color={1,1,1},
          icon = icon.hyperlink_decoration,
          onpress1 = function()
                       run.switch_to_file(filename)
                     end,
        })

      -- render fragment

function Text.populate_link_offsets(State, line_index)
  local line = State.lines[line_index]
  local line_cache = State.line_cache[line_index]
  if line_cache.link_offsets then
    return
  end
  line_cache.link_offsets = {}
  local pos = 1
  -- try to wrap at word boundaries
  local s, e = 1, 0
  while s <= #line.data do
    s, e = line.data:find('%w+', s)
    if s == nil then break end
    local word = line.data:sub(s, e)
    if file_exists(word) then
--?       print('filename:', s, e, word)
      table.insert(line_cache.link_offsets, {s, e, word})
    end
    s = e + 1
  end
end
-- Intersect the filename between byte offsets s,e with the bounds of screen line i.
-- Return the left/right pixel coordinates of of the intersection,
-- or nil if it doesn't intersect with screen line i.
function Text.clip_filename_with_screen_line(line, line_cache, i, s, e)
  local spos = line_cache.screen_line_starting_pos[i]
  local soff = Text.offset(line.data, spos)
  if e < soff then
    return
  end
  local eoff
  if i < #line_cache.screen_line_starting_pos then
    local epos = line_cache.screen_line_starting_pos[i+1]
    eoff = Text.offset(line.data, epos)
    if s > eoff then
      return
    end
  end
  local loff = math.max(s, soff)
  local hoff
  if eoff then
    hoff = math.min(e, eoff)
  else
    hoff = e
  end
--?   print(s, e, soff, eoff, loff, hoff)
  return App.width(line.data:sub(1, loff-1)), App.width(line.data:sub(1, hoff))
end

  State.line_cache[line_index].link_offsets = nil