in static/js/scroll-apiref.js [150:166]
function checkNodePositions(nodes, flatNodeMap, scrollPosition) {
var activeNode;
for (var i = 0; i < nodes.length; i++) {
var item = nodes[i];
var node = flatNodeMap[item.section];
var nodeTop = node.offset().top - 50;
if (scrollPosition >= nodeTop) {
activeNode = {token: item.section, node: node};
if (item.subsections) {
activeNode.subsections = item.subsections;
}
break;
}
}
return activeNode;
}