www.duckland.org/themes/diary/static/js/toc-collapse.js

var spy = function () {
  var elems = $(":header");
  if (elems.length == 0) {
    return;
  }
  var currentTop = $(window).scrollTop();
  var currentBottom = $(window).scrollTop() + $(window).height();
  var pageBottom = $('#EOF').offset().top;

  var meetUnread = false
  var currentIndex = elems.length - 1
  elems.each(function (idx) {
    var elemTop = $(this).offset().top;
    var id = $(this).attr('id');
    var navElem = $('#' + id + '-nav');
    if (currentTop + $(this).height() >= elemTop) {
      navElem.addClass('toc-active');
    } else {
      if (meetUnread == false) {
        meetUnread = true
        currentIndex = idx - 1
      }
      navElem.removeClass('toc-active');
    }
  })
  if (currentBottom >= pageBottom) {
    currentIndex = -1
  }
  //console.log(currentIndex);
  //Collapse them
  collapseOthers(currentIndex);
}

var onNavClick = function (name) {
  var elems = $(":header");
  elems.each(function (idx) {
    var id = $(this).attr('id');
    if (name == '#' + id + '-nav'){
      collapseOthers(idx);
      return;
    }
  });
}

var collapseOthers = function (currentIndex) {
  if (currentIndex == -1) {
    $(".collapse").each(function (idx) {
      $(this).collapse("hide");
    });
    return;
  }
  var elems = $(":header");
  //console.log(currentIndex);
  currentId = "#" + elems[currentIndex].id + "-nav";
  
  $(currentId).parents(".collapse").each(function (idx) {
    $(this).collapse("show");
  });
  $(currentId).parent().next().filter(".collapse").collapse("show");
  $(".collapse").not($(currentId).parents()).not($(currentId).parent().next()).each(function (idx) {
    $(this).collapse("hide");
  });
}
$().ready(function () {
  spy();
  $(window).bind('scroll', debounce(spy, 250, { 'maxWait': 1000 }));
});


//From https://github.com/lodash/lodash/blob/master/debounce.js
// and https://github.com/lodash/lodash/blob/master/isObject.js

function debounce(func, wait, options) {
  let lastArgs,
    lastThis,
    maxWait,
    result,
    timerId,
    lastCallTime

  let lastInvokeTime = 0
  let leading = false
  let maxing = false
  let trailing = true

  // Bypass `requestAnimationFrame` by explicitly setting `wait=0`.
  const useRAF = (!wait && wait !== 0 && typeof root.requestAnimationFrame === 'function')

  if (typeof func !== 'function') {
    throw new TypeError('Expected a function')
  }
  function isObject(value) {
    const type = typeof value
    return value != null && (type === 'object' || type === 'function')
  }

  wait = +wait || 0
  if (isObject(options)) {
    leading = !!options.leading
    maxing = 'maxWait' in options
    maxWait = maxing ? Math.max(+options.maxWait || 0, wait) : maxWait
    trailing = 'trailing' in options ? !!options.trailing : trailing
  }

  function invokeFunc(time) {
    const args = lastArgs
    const thisArg = lastThis

    lastArgs = lastThis = undefined
    lastInvokeTime = time
    result = func.apply(thisArg, args)
    return result
  }

  function startTimer(pendingFunc, wait) {
    if (useRAF) {
      root.cancelAnimationFrame(timerId)
      return root.requestAnimationFrame(pendingFunc)
    }
    return setTimeout(pendingFunc, wait)
  }

  function cancelTimer(id) {
    if (useRAF) {
      return root.cancelAnimationFrame(id)
    }
    clearTimeout(id)
  }

  function leadingEdge(time) {
    // Reset any `maxWait` timer.
    lastInvokeTime = time
    // Start the timer for the trailing edge.
    timerId = startTimer(timerExpired, wait)
    // Invoke the leading edge.
    return leading ? invokeFunc(time) : result
  }

  function remainingWait(time) {
    const timeSinceLastCall = time - lastCallTime
    const timeSinceLastInvoke = time - lastInvokeTime
    const timeWaiting = wait - timeSinceLastCall

    return maxing
      ? Math.min(timeWaiting, maxWait - timeSinceLastInvoke)
      : timeWaiting
  }

  function shouldInvoke(time) {
    const timeSinceLastCall = time - lastCallTime
    const timeSinceLastInvoke = time - lastInvokeTime

    // Either this is the first call, activity has stopped and we're at the
    // trailing edge, the system time has gone backwards and we're treating
    // it as the trailing edge, or we've hit the `maxWait` limit.
    return (lastCallTime === undefined || (timeSinceLastCall >= wait) ||
      (timeSinceLastCall < 0) || (maxing && timeSinceLastInvoke >= maxWait))
  }

  function timerExpired() {
    const time = Date.now()
    if (shouldInvoke(time)) {
      return trailingEdge(time)
    }
    // Restart the timer.
    timerId = startTimer(timerExpired, remainingWait(time))
  }

  function trailingEdge(time) {
    timerId = undefined

    // Only invoke if we have `lastArgs` which means `func` has been
    // debounced at least once.
    if (trailing && lastArgs) {
      return invokeFunc(time)
    }
    lastArgs = lastThis = undefined
    return result
  }

  function cancel() {
    if (timerId !== undefined) {
      cancelTimer(timerId)
    }
    lastInvokeTime = 0
    lastArgs = lastCallTime = lastThis = timerId = undefined
  }

  function flush() {
    return timerId === undefined ? result : trailingEdge(Date.now())
  }

  function pending() {
    return timerId !== undefined
  }

  function debounced(...args) {
    const time = Date.now()
    const isInvoking = shouldInvoke(time)

    lastArgs = args
    lastThis = this
    lastCallTime = time

    if (isInvoking) {
      if (timerId === undefined) {
        return leadingEdge(lastCallTime)
      }
      if (maxing) {
        // Handle invocations in a tight loop.
        timerId = startTimer(timerExpired, wait)
        return invokeFunc(lastCallTime)
      }
    }
    if (timerId === undefined) {
      timerId = startTimer(timerExpired, wait)
    }
    return result
  }
  debounced.cancel = cancel
  debounced.flush = flush
  debounced.pending = pending
  return debounced
}