Panning (test these at multiple zoom levels):
* When no node has cursor, arrow keys and pageup/pagedown pan surface around, preserving relative positions of nodes
* When a node has cursor in viewport, arrow keys move the cursor within the node, keeping cursor visible, panning surface if scrolling is needed
  * Pageup/pagedown scroll a whole viewport height at a time if possible, but keep cursor visible if not possible.
* When a node has cursor, pageup/pagedown pan surface around
* Position a node with some of its area above (outside) viewport. Position cursor at topmost row on screen. Hit up arrow. Surface pans.
* Increase/decrease zoom, pan around using cursor inside node. The box for the editor surrounds the entire editor.
* Increase/decrease zoom, pan around using cursor outside node. The box for the editor surrounds the entire editor. (not perfect yet)

Panning:
* {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

			Viewport.y = Cursor_node.y + y_of_schema1(Cursor_node.editor, Cursor_node.editor.screen_top1)

			Viewport.y = Cursor_node.y + y_of_schema1(Cursor_node.editor, Cursor_node.editor.screen_top1)/Viewport.zoom