Touch interfaces should work

[gracenotes]

This has worked sometimes and regressed.

There are a few possible interfaces:

1. A very large start target, and subsequent taps should automatically move towards that part of the grid. Maintaining a queue of these and interpreting user intent is difficult, however.
2. Dragging. This gets messy because your finger is covering the line, so it needs to be offset, and it is confusing all around.

[gracenotes]

Although this works now, two remaining problems should be solved before considering this fixed:

1. Tapping on the line should backtrack to that point on the line. What happens otherwise (it just runs into itself) is not that intuitive.
2. Rendering should be incremental, i.e. not replacing the complete path SVG every time.

A nice-to-have would be smarter navigation, so that the solve path does not go down segments where it will be blocked, such as where it crosses the existing path or where a disjoint segment is.