Interpolation of element closest to point by sampling in expanding circles

closest.coffee

@closest = (x, y) ->
  el = document.elementFromPoint x, y
  unless el is null or el.nodeName is 'BODY' then el
  else
    slice = 2 * Math.PI / 8
    for radius in [5...200] by 8
      for step in [0...7]
        angle = slice * step
        [candX, candY] = [x + radius*Math.cos(angle), y + radius*Math.sin(angle)]
        cand = document.elementFromPoint candX, candY
        if cand != null and cand.nodeName != 'BODY' then return cand
    throw "Failed to find element close to (#{x}, #{y})"