When in linear display mode, "foreground" nodes can sometimes appear behind faded "background" nodes, which makes mouse interactions wrong/wonky. I gather this is because SVG uses element definition order directly as z-level.

Should fix this by using d3 to order foreground nodes after background nodes.

Display and/or calcs can get pretty hairy when the app attempts to display a loop in the graph structure. Currently there's just a gap in later occurrences of where a given node should be whenever this happens.

I suppose it would be nice to show something like a virtual node that says "loop to", perhaps with a different colour/shape to true nodes, and that shows a link back to the upstream connection point.

Feedback from beta tester: rather than having the app trace through a route to the end straight off the bat, it would be more intuitive to walk a desired route by selecting each node along the way one-by-one, having it reveal potential 'next-step' child nodes each time.

When using the load form to open a new file (or reload the same one), some nodes seem to keep some/all attributes from the previous file (e.g. name).

Should flatten the whole programme state whenever a file is loaded.