-- Since the editor deals with potentially changing data, it tries to maintain
-- just a screen's worth of screen-line data.
-- Since the larger note-taking app mostly deals with read-only data, it
-- maintains screen-line data for the entirety of each file.
-- To map between the two we have the following timeline of events:
--  - a keystroke that scrolls the editor modifies pane.screen_top1
--  - we update Display_settings.y to scroll all neighboring panes as well
--  - App.draw uses Display_settings.y to set pane.top and renders everything
--    from pane.top to bottom of screen