if x == 0 and bpos == 0 then
          assert(false, ("Infinite loop while line-wrapping. Editor is %dpx wide; window is %dpx wide"):format(State.width, App.screen.width))
        end
  -- Perform some early sanity checking here, in hopes that we correctly call
  -- this whenever we change editor state.
  if State.right <= State.left then
    assert(false, ('Right margin %d must be to the right of the left margin %d'):format(State.right, State.left))
  end
  State.line_cache = {}
  for i=1,#State.lines do
    State.line_cache[i] = {}
  end
        pos = pos + bpos
        local boffset = Text.offset(frag, bpos+1)  -- byte _after_ bpos
        frag = string.sub(frag, boffset)
--?         if bpos > 0 then
--?           print('after chop:', frag)
--?         end
        frag_width = App.width(frag)
      end
--?       print('screen line:', pos)

        -- everything works if bpos == 0, but is a little inefficient

        if x == 0 and bpos == 0 then
          assert(false, ("Infinite loop while line-wrapping. Editor is %dpx wide; window is %dpx wide"):format(State.width, App.screen.width))
        end

  -- Perform some early sanity checking here, in hopes that we correctly call
  -- this whenever we change editor state.
  if State.right <= State.left then
    assert(false, ('Right margin %d must be to the right of the left margin %d'):format(State.right, State.left))
  end