Neat. A couple thoughts: 1) it took me some time to figure out that I could scroll a given column when the mouse is over it... Also when the mouse is over a non-column area (background on far right/left) it would be useful if scrolling did a global scroll of all columns at the same time.

2) It could be nice if the sections M+1/M-1 (above/below) the selected section in column N were given a subtle distinct color, and then the appropriate sections in column N+1 that are nested in M+1/M-1 were given the same color. Does that make any sense? It would give a visual indication of which sections in column N+1 lie within sections M+1 and M-1, and also help to emphasize the tree nature of the layout. A different color could be chosen for M+/-2, +/-3, etc.

Good job! :)