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What's the most accurate way of measuring scroll?

I'm making a slide scrolling page, and I'm trying to have it scroll like you're pulling a notecard up and with the next one right behind it.
To do this, I'm making them all fixed, and then moving their "top" position based off of scroll. But then I also need to make the body the size of the panel.
It's hard to describe what I'm doing, so here's the demo: https://codepen.io/NotDan/pen/vzraJE
Here's the particular piece of code that's causing my problem:
//what's going on here?
$(window).scroll(function(){
var panelNum = parseInt($(window).scrollTop()/$(window).height());//detemines panel number
var pixelMovement = ($(window).scrollTop())-(panelNum*$(".panel").height()); determines how many pixels to move the panel by
$('body').find(".panel:eq("+panelNum+")").css("top", -1*pixelMovement);
});
The problem is when the user scrolls quickly, the top position is not set accurately and there's some overhang. Again, hard to explain, but if you jump to the demo and scroll quickly you'll see what I mean.
Is there a more precise way of measuring scroll? Or is there a better way to do what I'm trying to? I've tried scrollmagic, and its "section wipe" feature is really close, but they bring the previous one up rather than move the current one up.

I tried making a condition to determine the panel number and everything started working.
var panelNum = 0;
var pixelMovement = 0;
$(window).scroll(function () {
pixelMovement = $(window).scrollTop() - panelNum * $(".panel").height(); // determines how many pixels to move the panel by
$("body")
.find(".panel:eq(" + panelNum + ")")
.css("top", -1 * pixelMovement);
if (Math.abs(pixelMovement) >= $(window).height()) {
panelNum++;
} else if (pixelMovement <= 0) {
panelNum--;
}
});
Here's the working demo: https://codepen.io/NotDan/pen/RYJeZq

Fade in div once user scrolls past element

Problem:
I'm trying to fadeIn and fadeOut div class="audioBox" once the user scrolls past the header. What I have seems to work fine, except for when the page is loaded and I'm already past the 835px height of the header/
Q: What I'm seeing is when I scroll the audioBox quickly fades in and then fades out, despite scroll >= header How do I prevent this from happening?
scripts.js
// If the reader scrolls past header, show audioBox
var scroll = $(window).scrollTop();
var header = $("header").height();
if (scroll >= header) {
$(".audioBox").fadeIn();
} else if (scroll <= header) {
$(".audioBox").fadeOut();
}

I tried implementing what you're describing in jsfiddle at http://jsfiddle.net/3wqfp2ch/1/.
I'd approach it a bit differently, based on the following ideas:
I personally prefer letting CSS take care of visual stuff via classes, and let jQuery take the simple responsibility of controlling when the classes should be added/removed. I think it makes for a better relationship between the two systems and allows each to do their thing better & more neatly.
I didn't see where you were listening for scroll events on the window, which is essential for figuring out whether a user's scroll position is before or after the header, so have included this in my code
I don't think we need multiple if conditions - there's just one question: "Is the scroll position greater than the header height?".
Here's the JS:
var headerHeight = $("header").height();
var audioBox = $('#audioBox');
$(window).on('scroll', function(){
var scrollPosition = $(window).scrollTop();
if (scrollPosition > headerHeight) {
audioBox.addClass('is-visible');
} else {
audioBox.removeClass('is-visible');
}
});
Check out my fiddle at http://jsfiddle.net/3wqfp2ch/1/ for the HTML & CSS that this relates to, and the working demo putting it all together.
I can't test whether this suffers from the same issue regarding you loading at a point already past the header height from jsfiddle unfortunately, but I wouldn't be expecting the behaviour you described using the code above.
Let me know how you get on!

Calling .fadeIn() or .fadeOut() all the time and having an overlap in the conditions might be the problem.
Try this:
// If the reader scrolls past header, show audioBox
var scroll = $(window).scrollTop();
var header = $("header").offset().top + $("header").height(); // should include the header's offset as well
if (scroll >= header) {
$(".audioBox:hidden").fadeIn();
} else if (scroll < header) {
$(".audioBox:visible").fadeOut();
}

Sticky sidebar is jolty if its shorter than the window height?

I have a sticky sidebar that when you scroll becomes fixed when the bottom of the sidebar is in view.
If the sidebar exceeds the length of the page as it does in this demo all works fine when you scroll and is exactly what you would expect.
However if the sidebar is shorter than the window height as in this demo, it seems to be jumping when you scroll and I can't work out how to get it to stop jumping and to be smooth. In other words it should only be fixed when the base of the sidebar hits the base of the window.
I'm not great with jQuery so any help would be greatly appreciated.
$(function () {
if ($('.leftsidebar').offset()!=null) {
var top = $('.leftsidebar').offset().top - parseFloat($('.leftsidebar').css('margin-top').replace(/auto/, 0));
var height = $('.leftsidebar').height();
var winHeight = $(window).height();
var footerTop = $('#footer').offset().top - parseFloat($('#footer').css('margin-top').replace(/auto/, 0));
var gap = 7;
$(window).scroll(function (event) {
// what the y position of the scroll is
var y = $(this).scrollTop();
// whether that's below the form
if (y+winHeight >= top+ height+gap && y+winHeight<=footerTop) {
// if so, ad the fixed class
$('.leftsidebar').addClass('leftsidebarfixed').css('top', winHeight-height-gap +'px');
}
else if (y+winHeight>footerTop) {
// if so, ad the fixed class
$('.leftsidebar').addClass('leftsidebarfixed').css('top', footerTop-height-y-gap + 'px');
}
else {
// otherwise remove it
$('.leftsidebar').removeClass('leftsidebarfixed').css('top', '0px');
}
});
}
});
Is it possible to combine the two instances? So if its shorter stay relative till the sidebar reaches the bottom, then act as it is now if the sidebar is longer?

The code works just as intended. This is actually a conceptual problem.
Picture how it would work first. The way you described it working seems to be exactly how it's working in your demo. When the sidebar is longer than the page, the scrolling page reaches the bottom of the sidebar before the leftsidebarfixed is added. That would be impossible with a shorter sidebar.
You may want to consider fixing the sidebar to the top, instead of the bottom (as most websites with sticky sidebars do) or having a taller header, so that the sidebar starts at the bottom.

Make a div appear when scrolling past a certain point of a page

My goal is to make a fixed div appear at the top of a page once someone scrolls a certain amount of pixels down the page. Basically once the header section is out of view, this div will appear.
I've looked at code similar to what I want; however, haven't seen anything that would allow me to easily modify the pixel count from the top of the page (if possible).
Here is a piece of code I saw dealing with making divs appear by scrolling.
// Get the headers position from the top of the page, plus its own height
var startY = $('header').position().top + $('header').outerHeight();
$(window).scroll(function(){
checkY();
});
function checkY(){
if( $(window).scrollTop() > startY ){
$('.fixedDiv').slideDown();
}else{
$('.fixedDiv').slideUp();
}
}
// Do this on load just in case the user starts half way down the page
checkY();
I just want to know how to make it appear. If someone knows of a piece of code already in tact with a slide up and slide down animation, that would be greatly appreciated as well but not required.

window.addEventListener("scroll",function() {
if(window.scrollY > 500) {
$('.fixedDiv').slideDown();
}
else {
$('.fixedDiv').slideUp();
}
},false);

Brandon Tilley answered my question in a comment...
You would change the first line, with the startY, to be the specific Y
position you need, rather than calculating based on the header's
position and height. Here's an updated fiddle:
jsfiddle.net/BinaryMuse/Ehney/1

window.addEventListener("scroll",function() {
$('.fixedDiv')[(window.scrollY > 500)?"slideDown":"slideUp"]();
},false);
DEMO: http://jsfiddle.net/DerekL/8eG2A/

Floating elements on scroll

I was wondering how sites like Facebook, with their timeline feature, float a certain element (usually a menu bar, or sometimes a social plugin, etc) when the user has scrolled past a point such that the top of the element is off the screen, etc.
This could be seen as a more general JavaScript (jQuery?) event firing when the user has scrolled to a certain element, or scrolled down a certain number of pixels.
Obviously it would require toggling the CSS property from:
#foo { position: relative; }
to
#foo { position: fixed; }
Or with jQuery, something like:
$('#foo').css('position', 'fixed');
Another way I have seen this implemented is with blogs, where a popup will be called when you reach the bottom, or near the bottom of a page. My question is, what is firing that code, and could you link or provide some syntax/ semantics/ examples?
Edit: I'm seeing some great JS variants coming up, but as I am using jQuery, I think the plugin mentioned will do just nicely.

Take a look at this jsfiddle: http://jsfiddle.net/remibreton/RWJhM/2/
In this example, I'm using document.onscroll = function(){ //Scroll event } to detect a scroll event on the document.
I'm then calculating the percentage of the page scrolled based on it's height. (document.body.scrollTop * 100 / (document.body.clientHeight - document.documentElement.clientHeight)).
document.body.scrollTop being the number of pixels scrolled from the top, document.body.clientHeight being the height of the entire document and document.documentElement.clientHeight being the visible portion of the document, a.k.a. the viewport.
Then you can compare this value to a target percentage, an execute JavaScript. if(currentPercentage > targetPercentage)...
Here's the whole thing:
document.onscroll = function(){
var targetPercentage = 80;
var currentPercentage = (document.body.scrollTop * 100 / (document.body.clientHeight - document.documentElement.clientHeight));
console.log(currentPercentage);
if(currentPercentage > targetPercentage){
document.getElementById('pop').style.display = 'block';
// Scrolled more than 80%
} else {
document.getElementById('pop').style.display = 'none';
// Scrolled less than 80%
}
}
​If you prefer jQuery, here is the same example translated into everybody's favorite library: http://jsfiddle.net/remibreton/8NVS6/1/
$(document).on('scroll', function(){
var targetPercentage = 80;
var currentPercentage = $(document).scrollTop() * 100 / ($(document).height() - $(window).height());
if(currentPercentage > targetPercentage){
$('#pop').css({display:'block'});
//Scrolled more than 80%
} else {
$('#pop').css({display:'none'});
//Scrolled less than 80%
}
});​

An idea would be to handle the window.scroll event and determine if the user has scrolled to the bottom of the page. Here is an example:
http://chrissilich.com/blog/load-more-content-as-the-user-reaches-the-bottom-of-your-page-with-jquery/
Hope it helps!

There is a jquery plugin that might help you in the right direction.
http://imakewebthings.com/jquery-waypoints/

I just answered basically the same question here. In that case it was a table and its header, and the basic idea is like this:
function placeHeader(){
var $table = $('#table');
var $header = $('#header');
if ($table.offset().top <= $(window).scrollTop()) {
$header.offset({top: $(window).scrollTop()});
} else {
$header.offset({top: $table.offset().top});
}
}
$(window).scroll(placeHeader);
Here's a demo.
Quoting myself:
In other words, if the top of the table is above the scrollTop, then
position the header at scrollTop, otherwise put it back at the top of
the table. Depending on the contents of the rest of the site, you
might also need to check if you have scrolled all the way past the
table, since then you don't want the header to stay visible.
To answer your question directly, it is triggered by checking the scrollTop against either the position of an element, or the height of the document minus the height of the viewport (for the scrolled to bottom use case). This check is done every time the scroll event is fired (bound using $(window).scroll(...)).