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How do I recalculate the section's height on scroll for a fixed navigation that changes class past certain anchors?

Context
I am working on a one-page website where the fixed navigation's class changes as it scrolls through the different sections in order to match the section's background color. To achieve this effect, I used and modified the 2nd solution listed here.
Issue
While it works great most of the time, the navigation code breaks when I resize the browser (or leave the page and click back). More specifically, the navigation's background color changes too early or too late and no longer matches the section's background.
I'm guessing that this happens because the section's height are calculated on page load. Ideally, they would be recalculated on every scroll - but I am a novice and that's just a guess. Any help to solve this issue would be appreciated.
JavaScript
FYI: there are four sections in the websites (Hero, Work, About, Contact). Navigation's bg color should be transparent in Hero, white in Work and Contact, and teal in About.
var afterhero = $('#hero-section').offset().top + $('#hero-section').height();
var afterwork = afterhero + $('#work-section').height();
var afterabout = afterwebsites + $('#about-section').height();
$(window).on('scroll', function() {
stop = Math.round($(window).scrollTop());
if (stop > afterabout) {
$('header').removeClass('teal');
$('header').addClass('white');
} else if (stop > afterwork) {
$('header').addClass('teal');
} else if (stop > afterhero) {
$('header').removeClass('teal');
$('header').addClass('white');
} else {
$('header').removeClass('teal');
$('header').removeClass('white');
}
});

Just try adding all your size variables into your scroll event handler:
$(window).on('scroll', function() {
var afterhero = $('#hero-section').offset().top + $('#hero-section').height();
var afterwork = afterhero + $('#work-section').height();
var afterabout = afterwebsites + $('#about-section').height();
stop = Math.round($(window).scrollTop());
if (stop > afterabout) {
$('header').removeClass('teal');
$('header').addClass('white');
} else if (stop > afterwork) {
$('header').addClass('teal');
} else if (stop > afterhero) {
$('header').removeClass('teal');
$('header').addClass('white');
} else {
$('header').removeClass('teal');
$('header').removeClass('white');
}
});
Now afterhero, afterwork and afterabout should all be recalculated on a page scroll.

How do I add a class to the body using jQuery, scrollTop, and toggleClass

I have a question concerning jQuery's scrollTop functionality, and it's ability to toggle a class based on the amount of vertical scroll.
What I am trying to accomplish is on any page with a "section.banner" class, that after you scroll past the banner a class is applied to the body tag. This will allow me to change the fill colors of several SVGs that are in the site's header, as well as a fixed positioned side nav that is for pagination.
I am terrible at javascript, and have been stuck searching and trying to get this for hours. any help will be greatly appreciated. Here's the code that I'm working with now (CodeKit is telling me it is wrong, which I am not surprised). The value of 200 is just a placeholder and will be calculated by the height of a fluid image. Full disclosure, I have no idea if the brackets and parenthesis are correct.
// Header/Fixed Pagination Banner Scroll Recoloriing (toggle class)
// Check If '.banner' Exists
if( $('section.banner').length > 0) {
$('body').scrollTop(function)()
{
if $(window).scrollTop >= 200 {
$('body').toggleClass('downtown');
return false;
}
}
}

Try something like this :
if( $('section.banner').length > 0) {
$(window).scroll(function() {
{
if ($(window).scrollTop() >= $('section.banner').scrollTop()) {
$('body').toggleClass('downtown');
return false;
}
});
}
UPDATE
There was little mistake in my code : http://jsfiddle.net/t2yp15hq/
var top = $('section.banner').position().top;
if($('section.banner').length > 0) {
$(window).scroll(function() {
if ($(this).scrollTop() >= top) {
$('body').addClass('downtown');
}
else
{
$('body').removeClass('downtown');
}
});
}
It does not work with toogleClass, the background is flashing.
UPDATE
http://codepen.io/anon/pen/wBWzXy
The solution is to recalculate the top when the window is resized :
$(window).resize(function(){
top = $('section.story-intro').offset().top - 90;
});

How can I set my browser window's scrollbar or a div scrollbar to scroll in increments using animate and scrollTop?

The general idea to the site i am designing is to scroll through a set of menu items horizontally and incrementally underneath a static div that will magnify(increase dimensions and pt size) the contents of a menu items. I don't really need help with the magnify portion because i think it's as simple as adding a mag class to any of the menuItem divs that go underneath the static div. I have been messing with this for a few weeks and the code I have for incrementally scrolling, so far, is this:
$(document).ready(function () {
currentScrollPos = $('#scrollableDiv').scrollTop(120); //sets default scroll pos
/*The incrementScroll function is passed arguments currentScrollPos and UserScroll which are variables that i have initiated earlier in the program, and then initiates a for loop.
-The first statement sets up the variables: nextScrollPos as equal to the currentScrollPos(which by default is 120px) plus 240px(the distance to next menuItem), prevScrollPos as equal to the currentScrollPos(which by default is 120px) minus 240px(the distance to next menuItem).
-The second Statement checks to see if the user has scrolled using var userScroll
-The third statement sets: var CurrentScroll equal to the new scroll position and var userScroll to false*/
function incrementScroll(currentScrollPos, userScroll) {
for (var nextScrollPos = parseInt(currentScrollPos + 240, 10),
prevScrollPos = parseInt(currentScrollPos - 240, 10); //end first statement
userScroll == 'true'; console.log('dude'), //end second statement and begining of third
currentScrollPos = scrollTop(), userScroll = 'false') {
if (scrollTop() < currentScrollPos) {
$('#scrollableDiv').animate({
scrollTop: (parseInt(prevScrollPos, 10))
}, 200);
console.log('scrolln up')
} else if (scrollTop() > currentScrollPos) {
$('#scrollableDiv').animate({
scrollTop: (parseInt(nextScrollPos, 10))
}, 200);
console.log('scrolln down')//fire when
}
}
}
$('#scrollableDiv').scroll(function () {
userScroll = 'true';
_.debounce(incrementScroll, 200); //controls the amount of times the incrementScroll function is called
console.log('straight scrolln')
});
});
I have found a variety of solutions that are nigh close: such as a plugin that snaps to the next or previous div horizontally demo, another solution that also snaps and is based on setTimeout demo, but nothing that nails incrementally scrolling through divs. I also found a way to control the rate at which a user may scroll through the menuItems using debounce which is included in the above code.
The console.logs inside the loop do not fire when I demo the code in jsfiddle which leads me to believe the problem lies within the loop. I'm a noob though so it could be in syntax or anywhere else in the code for that matter. Also in the second demo, i have provided the css for the horizontal static div, but the moment I put it in my html it keeps the js from working.
I would like to write the code instead of using a plugin and any help would be appreciated! Also, thank you ahead of time!

Try this fiddle. Menu container height is 960px to show 4 menu items. "Zoom" div is positioned absolutely at top. When you scroll mouse over this div, menu items shifts to top/bottom. I had to add additional div to bottom to be able to scroll to last 3 menu items. JS code:
jQuery(document).ready(function($){
var current = 0;
var menu = $('.menu-container').scrollTop(0);
var items = menu.find('.menu-item');
var zoom = $('.zoom');
function isVerticalScroll(event){
var e = event.originalEvent;
if (e.axis && e.axis === e.HORIZONTAL_AXIS)
return false;
if (e.wheelDeltaX)
return false;
return true;
}
function handleMouseScroll(event){
if(isVerticalScroll(event)){
var delta = event.originalEvent.wheelDelta * -1 || event.originalEvent.detail;
current += (delta > 0 ? 1 : -1);
if(current < 0)
current = 0;
if(current >= items.length){
current = items.length - 1;
}
menu.stop().animate({
"scrollTop": current * 240
}, 300);
items.removeClass('current').eq(current).addClass('current');
event && event.preventDefault();
return false;
}
}
zoom.on({
"MozMousePixelScroll": handleMouseScroll,
"mousewheel": handleMouseScroll
});
});
Hope it will help.

Get divs within viewport inside a wrapper div

Is there a way to get elements which is:
Inside a div with overflow: scroll
Is in viewport
Just like the following picture, where active div (5,6,7,8,9) is orange, and the others is green (1-4 and >10) :
I just want the mousewheel event to add "active" class to div 5,6,7,8,9 (currently in viewport). View my JSFiddle
$('.wrapper').bind('mousewheel', function (e) {
//addClass 'active' here
});

You could do something like this. I would have re-factored it, but only to show the concept.
Firstly I would attach this to scroll event and not mousewheel. There are those among us that likes to use keyboard for scrolling, and you also have the case of dragging the scrollbar. ;) You also have the case of touch devices.
Note that with this I have set overflow:auto; on wrapper, thus no bottom scroll-bar.
With bottom scrollbar you would either have to live with it becoming tagged as in-view a tad to early, or tumble into the world of doing a cross-browser calculating of IE's clientHeight. But the code should hopefully be OK as a starter.
»»Fiddle««
function isView(wrp, elm)
{
var wrpH = $(wrp).height(),
elmH = $(elm).height(),
elmT = $(elm).offset().top;
return elmT >= 0 &&
elmT + elmH < wrpH;
}
$('.wrapper').bind('scroll', function (e) {
$('div.box').each(function(i, e) {
if (isView(".wrapper", this)) {
$(this).addClass('active');
} else {
$(this).removeClass('active');
}
});
});
Note that you should likely refactor in such a way that .wrapper height is only retrieved once per invocation, or if it is static, at page load etc.
Update; a modified version of isView(). Taking position of container into account. This time looking at dolphins in the pool.
»»Fiddle««
function isView(pool, dolphin) {
var poolT = pool.offset().top,
poolH = pool.height(),
dolpH = dolphin.height(),
dolpT = dolphin.offset().top - poolT;
return dolpT >= 0 && dolpT + dolpH <= poolH;
}

Change z-index after load

I have a function that gets called when a user scrolls to check for scrollTop() and after a certain scroll happens it changes the menu's z-index from -1 to 1. However this only occurs on a scroll so if the user refreshes the site the menu is virtually unusable until the next scroll occurs.
Is there a way for me to call this and check if the amount of screen scrolled (after the refresh not a user scroll) meets the criteria change the z-index?
My JS:
function getPosition(){
var y = $(window).scrollTop();
var status = (y > 880) ? true : false;
//console.log(status);
if(status)
$('#actual-menu').css('z-index', 1);
else
$('#actual-menu').css('z-index', -1);
}

z-index property has no effect on non-positioned elements, the element must be either relatively positioned ,absolutely positioned, or fixed.Replace your line with this:
Try adding this first ,
$("#actual-menu").css('position', 'relative');
$('#actual-menu').css('z-index', 1);

Just run getPosition inside of your ready event.
Functioning Example
var limit = 50;
$(function () {
function getPosition() {
var y = $(window).scrollTop();
var status = (y > limit) ? true : false;
if (status) {
$('#actual-menu').css('zIndex', 1);
} else {
$('#actual-menu').css('zIndex', -1);
}
}
$(document).on('scroll', getPosition);
getPosition();
});