* {Zoom > 1, Zoom = 1, Zoom < 1} x tall node extends {above, below, both} viewport x {up arrow, down arrow, pageup, pagedown, mouse drag}
    - cursor remains within viewport
    - relative positions of nodes preserved
    - box and editor align

* Zoom = 1
    short node entirely within viewer
      arrow keys
      page-up positions cursor on top line
      page-down positioins cursor on bottom line
      mouse panning
    tall node extending below viewport
      repeated down arrow near bottom pans down
      repeated up arrow
      repeated up arrow near top pans up
      repeated down arrow
      repeated pagedown
      repeated pageup
      mouse panning
    tall node extending above viewport
      repeated up arrow near top pans up
      repeated down arrow
      repeated down arrow near bottom pans down
      repeated up arrow
      repeated pagedown
      repeated pageup
      mouse panning
    tall node extending both below and above viewport
      repeated up arrow near top pans up
      repeated down arrow
      repeated down arrow near bottom pans down
      repeated up arrow
      repeated pagedown
      repeated pageup
      mouse panning
* Zoom > 1
    short node entirely within viewer
      arrow keys
      page-up positions cursor on top line
      page-down positioins cursor on bottom line
      mouse panning
    tall node extending below viewport
      repeated down arrow near bottom pans down
      repeated up arrow
      repeated up arrow near top pans up
      repeated down arrow
      repeated pagedown
      repeated pageup
      mouse panning
    tall node extending above viewport
      repeated up arrow near top pans up
      repeated down arrow
      repeated down arrow near bottom pans down
      repeated up arrow
      repeated pagedown
      repeated pageup
      mouse panning
    tall node extending both below and above viewport
      repeated up arrow near top pans up
      repeated down arrow
      repeated down arrow near bottom pans down
      repeated up arrow
      repeated pagedown
      repeated pageup
      mouse panning
* Zoom < 1
    short node entirely within viewer
      arrow keys
      page-up positions cursor on top line
      page-down positioins cursor on bottom line
      mouse panning
    tall node extending below viewport
      repeated down arrow near bottom pans down
      repeated up arrow
      repeated up arrow near top pans up
      repeated down arrow
      repeated pagedown
      repeated pageup
      mouse panning
    tall node extending above viewport
      repeated up arrow near top pans up
      repeated down arrow
      repeated down arrow near bottom pans down
      repeated up arrow
      repeated pagedown
      repeated pageup
      mouse panning
    tall node extending both below and above viewport
      repeated up arrow near top pans up
      repeated down arrow
      repeated down arrow near bottom pans down
      repeated up arrow
      repeated pagedown
      repeated pageup
      mouse panning