--?     print('', '', frag, 'comparing', x, frag_width, State.right)

    y=0,
    x=0,

    x=0, y=0,  -- <==== Top-left corner of the viewport into the surface

  update_pane_bounds()

  Surface.dirty = true

  update_pane_bounds()

function update_pane_bounds(options)
  local sx = Padding_horizontal + Margin_left
  for column_index, column in ipairs(Surface) do
    if overlap(sx, sx+Display_settings.column_width, Display_settings.x, Display_settings.x + App.screen.width) then
      local sy = Margin_top
      for pane_index, pane in ipairs(column) do
        if overlap(sy, sy + Margin_above + Cache[pane.id].height + Margin_below, Display_settings.y, Display_settings.y + App.screen.height) then
          if column_index == Cursor_pane.col and pane_index == Cursor_pane.row then
            -- update the cursor pane either if it's not editable, or
            -- if it was explicitly requested
            if not pane.show_cursor or options == nil or not options.ignore_editable_cursor_pane then
              if sy < Display_settings.y then
                pane.screen_top1 = schema1_of_y(pane, Display_settings.y - Margin_top)
              else
                pane.screen_top1 = {line=1, pos=1}
              end
            end
          end
--?           print(('%d,%d: adjusting pane top %d; screen_top is at line %d (max %d)'):format(column_index, pane_index, pane.top, pane.screen_top1.line, #pane.lines))
          pane.top = sy - Display_settings.y + Margin_above
--?           print(('pane top is %d'):format(pane.top))
          pane.left = sx - Display_settings.x
          pane.right = pane.left + Display_settings.column_width
          pane.width = pane.right - pane.left
        end
        sy = sy + Margin_above + Cache[pane.id].height + Margin_below + Padding_vertical
      end
    end
    sx = sx + Margin_right + Display_settings.column_width + Padding_horizontal + Margin_left
  end
end

  -- top > Margin_top or Screen_top.line > 1

--?   if Surface.dirty then
--?     print(('draw from surface y of %d'):format(Display_settings.y))
--?   end

--?           print('draw pane')
          if Surface.dirty then
--?             print(('%d,%d: adjusting pane top %d; screen_top is at line %d (max %d)'):format(column_index, pane_index, pane.top, pane.screen_top1.line, #pane.lines))
            pane.top = sy - Display_settings.y + Margin_above
--?             print(('pane top is %d'):format(pane.top))
            pane.left = sx - Display_settings.x
            pane.right = pane.left + Display_settings.column_width
            pane.width = pane.right - pane.left
          end
--?           print(column_index, pane_index, pane.cursor1.line, pane.cursor1.pos, pane.selection1.line, pane.selection1.pos)

--?           print('draw pane', column_index, pane_index, pane.cursor1.line, pane.cursor1.pos, pane.selection1.line, pane.selection1.pos)

  Surface.dirty = false

    Surface.dirty = true

    update_pane_bounds()

  Surface.dirty = true

  -- editable cursor pane will have already updated its screen_top, so don't clobber it here
  update_pane_bounds{ignore_editable_cursor_pane=true}

  local y_offset = y
  local line,pos_index = 1,1
  Text.populate_screen_line_starting_pos(pane, line)
  while y_offset >= Line_height do
    pos_index = pos_index + 1
    if pos_index > #pane.line_cache[line].screen_line_starting_pos then
      line = line+1
      pos_index = 1
      Text.populate_screen_line_starting_pos(pane, line)
    end
    y_offset = y_offset - Line_height
  end
  return {line=line, pos=pane.line_cache[line].screen_line_starting_pos[pos_index]}, y_offset

  assert(Text.le1(State.screen_top1, State.cursor1))

  if State.show_cursor then
    if not Text.le1(State.screen_top1, State.cursor1) then
      print('assertion failure: cursor is above screen top')
      print('screen top', State.screen_top1.line, State.screen_top1.pos)
      print('cursor', State.cursor1.line, State.cursor1.pos)
      assert(false)
    end
  end