if pane._height == nil then
          -- the first time around we need to hydrate just to make clipping decisions
          hydrate(pane)
        end

          -- after the first time around, we only need to hydrate panes that intersect the viewport
          hydrate(pane)

          dehydrate(pane)

        dehydrate(pane)

end
-- optimizations to limit memory footprint
-- It's better to constantly load and save a few panes from disk,
-- than it is to keep an unbounded set of panes in memory.
-- Just don't forget pane._height.
function dehydrate(pane)
  if pane.next_save then return end
--?   print('dehydrating '..pane.id)
  pane.lines = nil
  pane.line_cache = nil

function hydrate(pane)
  if pane.lines then return end
--?   print('  hydrating '..pane.id)
  load_from_disk(pane)
  Text.redraw_all(pane)
end