function Text.tweak_screen_top_and_cursor()
--?   print('a', Selection1.line)
  if Screen_top1.pos == 1 then return end
  Text.populate_screen_line_starting_pos(Screen_top1.line)
  local line = Lines[Screen_top1.line]
  for i=2,#line.screen_line_starting_pos do
    local pos = line.screen_line_starting_pos[i]
    if pos == Screen_top1.pos then
      break
    end
    if pos > Screen_top1.pos then
      -- make sure screen top is at start of a screen line
      local prev = line.screen_line_starting_pos[i-1]
      if Screen_top1.pos - prev < pos - Screen_top1.pos then
        Screen_top1.pos = prev
      else
        Screen_top1.pos = pos
      end
      break
    end
  end
  -- make sure cursor is on screen
  if Text.lt1(Cursor1, Screen_top1) then
    Cursor1 = {line=Screen_top1.line, pos=Screen_top1.pos}
  elseif Cursor1.line >= Screen_bottom1.line then
--?     print('too low')
    App.draw()
    if Text.lt1(Screen_bottom1, Cursor1) then
--?       print('tweak')
      local line = Lines[Screen_bottom1.line]
      Cursor1 = {line=Screen_bottom1.line, pos=Text.to_pos_on_line(line, App.screen.width-5, App.screen.height-5)}
    end
  end
end

--
-- Make sure these coordinates are never aliased, so that changing one causes
-- action at a distance.

  Selection1 = {}  -- no support for shift drag while we're resizing
  Text.tweak_screen_top_and_cursor()

--?   print('press')

--?   print('release')

* Like any high-level language, it's easy to accidentally alias two non-scalar
  variables. I wish there was a way to require copy when assigning.