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How do I prevent scroll back up with JavaScript or jQuery?

I have a webpage where there is a full height intro image. Underneath this image is the main body of the site with a regular site header at the top, I'm trying to create an effect where once the user scrolls down to the site header, they cannot scroll back up to view the intro image.
CSS Classes:
Main Intro Image: .cq-fullscreen-intro
Site Header: .nav-down
I had a poke around on StackOverflow but I can't find anything that addresses this circumstance, can anyone point me in the right direction to achieve this using jQuery?

you can use JQuery scrollTop function like this
$(function() {
$(window).scroll(function() {
var scroll = $(window).scrollTop();
// set the height in pixels
if (scroll >= 200) {
// after the scroll is greater than height then you can remove it or hide it
$(".intro-image").hide();
}
});
});

So instead of scrolling, I personally think it would be better to have it be actionable. Forcing the user to manually do the transition (and all in between states) is a bad idea. If the user scrolls half way, and see's something actionable (menu, button, input field) is it usable? If it is, what happens if they submit... very awkward. If it isn't usable, how do they know when it is? How do they know it's because they haven't scrolled all the way. It's very poor user experience.
In the following example, I've created a pseudo-screenport for you to see what's actually going on. The .body container in your real site would be the body element.
Code Pen Example
$(document).ready(function(){
$('.splash-screen').on('click', function(){
$('.splash-screen').addClass("is-hidden");
});
})
html, body{
background: #eee;
margin: 0;
padding: 0;
}
.flex-root {
display: flex;
align-items: center;
justify-content: center;
height: 100vh;
width: 100vw;
}
.web-container {
width: 640px;
height: 480px;
background: #fff;
}
.body {
font-size: 0; // this is only to prevent spacing between img placholders
position: relative;
}
.splash-screen{
position: absolute;
transition: transform 1s ease-in-out;
}
.splash-screen .fa {
position: absolute;
z-index: 1;
font-size: 24px;
color: #fff;
left: 50%;
bottom: 15px;
}
.splash-screen.is-hidden {
transform: translateY(-110%);
transition: transform 1s ease-in-out;
}
<link href="https://stackpath.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet"/>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="flex-root">
<div class="web-container">
<div class="body">
<div class="splash-screen">
<img src="https://via.placeholder.com/640x480?text=Splash+Screen"/>
<i class="fa fa-chevron-circle-up"></i>
</div>
<img src="https://via.placeholder.com/640x60/cbcbcb?text=Menu"/>
<img src="https://via.placeholder.com/640x420/dddddd?text=Site Body"/>
<div>
</div>
</div>

While its not direclty preventing you from scrolling up and its not jQuery, I would suggest to remove/hide the element once its out of view.
You could get the current scroll position, relative to the top of the page, and check if its greater than the elements height:
const target = document.getElementById('my-target')
const targetHeight = target.getBoundingClientRect().height
const scrollEventListener = () => {
if (
document.body.scrollTop > targetHeight ||
document.documentElement.scrollTop > targetHeight
) {
target.remove()
window.removeEventListener('scroll', scrollEventListener)
document.documentElement.scrollTop = 0
}
}
window.addEventListener('scroll', scrollEventListener)
Here is a codepen https://codepen.io/bluebrown/full/aboagov

Scrolling Nav Sticks to Top

My problem is along the lines of these previous issues on StackOverflow but with a slight difference.
Previous issues:
Stopping fixed position scrolling at a certain point?
Sticky subnav when scrolling past, breaks on resize
I have a sub nav that starts at a certain position in the page. When the page is scrolled the sub nav needs to stop 127px from the top. Most of the solutions I have found need you to specify the 'y' position of the sub nav first. The problem with this is that my sub nav will be starting from different positions on different pages.
This is the JS code i'm currently using. This works fine for one page but not all. Plus on mobile the values would be different again.
var num = 660; //number of pixels before modifying styles
$(window).bind('scroll', function () {
if ($(window).scrollTop() > num) {
$('.menu').addClass('fixed');
} else {
$('.menu').removeClass('fixed');
}
});
I'm looking for a solution that stops the sub nav 127px from the top no matter where on the page it started from.

You can use position: sticky and set the top of the sub-nav to 127px.
See example below:
body {
margin: 0;
}
.main-nav {
width: 100%;
height: 100px;
background-color: lime;
position: sticky;
top: 0;
}
.sub-nav {
position: sticky;
width: 100%;
height: 50px;
background-color: red;
top: 100px;
}
.contents {
width: 100%;
height: 100vh;
background-color: black;
color: white;
}
.contents p {
margin: 0;
}
<nav class="main-nav">Main-nav</nav>
<div class="contents">
<p>Contents</p>
</div>
<nav class="sub-nav">Sub-nav</nav>
<div class="contents">
<p>More contents</p>
</div>
Please see browser support for sticky here

You should change your code to the below, should work fine:
$(window).bind('scroll', function () {
if ($(window).scrollTop() > $(".menu").offset().top) {
$('.menu').addClass('fixed');
} else {
$('.menu').removeClass('fixed');
}
});

Maybe you can try this:
Find navigation div (.menu)
Find the top value of the .menu (vanilla JS would be menuVar.getBoundingClientRect().top, not sure how jQuery does this).
Get top value of browserscreen.
Calculate the difference - 127px.
When the user scrolls and reaches the top value of the menu -127px -> addClass('fixed').

jQuery fixed navbar that shrinks to a smaller size on scroll is being super buggy

When using a combination of jQuery and CSS to trigger my navbar to shrink on scroll, it get's buggy when you scroll back up to a certain position, I have linked a video as an example.
I have tried two different methods. The first is using $(window).scrollTop) with an if statement and a series of .addClass and .removeClass. The second thing I have tried is using $(window).scrollTop) with a series of .css dynamic style modifications. Both of these attempts render the same end result that is shown in this video https://youtu.be/YXKsrL1cghs .
My first jQuery attempt:
$(document).ready(function () {
$(window).on("scroll", function () {
if ($(window).scrollTop() >= 40) {
$(".navbar").removeClass("py-5");
$(".navbar").addClass("compressed");
} else {
$(".navbar").addClass("py-5");
$(".navbar").removeClass("compressed");
}
});
});
My second jQuery attempt:
$(document).ready(function () {
$(window).on("scroll", function () {
if ($(window).scrollTop() >= 40) {
$(".navbar").css({ "padding-top": "10px" });
$(".navbar").css({ "padding-bottom": "10px" });
} else {
$(".navbar").css({ "padding-top": "3rem" });
$(".navbar").css({ "padding-bottom": "3rem" });
}
});
});
My CSS:
.navbar.compressed {
padding-top: 10px;
padding-bottom: 10px;
}
My expected results would be a smooth scrolling fixed navbar that shrinks to a smaller size after scrolling beyond a certain point.
What actually occurs is that when you scroll down past a certain point, for 20px worth of height, it gets super buggy and starts bouncing up and down. Once you clear those 20 or so px it's perfectly fine, but when you scroll back up it acts the same within those 20px.

When watching the video, I noticed that your .navbar has transition: all .3s. It could be the reason that when you remove the class py-5 and add class compressed, it triggers the transition twice.
It would be helpful if you can provide the HTML markup and CSS as well.

The script is manipulating the DOM quite a lot. I am not sure if this is going to fix your problem but it might be a good idea to only change the classes if the have not yet been applied.
$(document).ready(function() {
$(window).on("scroll", function() {
let navbar = $(".navbar");
if ($(window).scrollTop() >= 40) {
if (navbar.hasClass("py-5")) {
navbar.removeClass("py-5");
navbar.addClass("compressed");
}
} else {
if (navbar.hasClass("compressed")) {
navbar.addClass("py-5");
navbar.removeClass("compressed");
}
}
});
});
body {
height: 10000px;
position: relative;
}
.navbar {
width: 100%;
position: fixed;
height: 50px;
top: 0;
transition: all .3s
}
.py-5 {
background-color: blue;
padding-top: 10px;
padding-bottom: 10px;
}
.compressed {
background-color: red;
padding-top: 0px;
padding-bottom: 0px;
}
<html>
<head></head>
<body>
<nav class="navbar py-5">Navigation</nav>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
</body>
</html>

How to make fixed navbar transparent based on page scroll?

I want mynavbar to be transparent when the page is scrolled to the top, however when the user scrolls I would like it to be made opaque. I tried this with javascript, but something still isn't working.
http://jsfiddle.net/6A6qy/
function myFunction() {
if ($(window).scrollTop() < 50) {
document.getElementById("masthead").style.opacity = "0.5";
}
}
#masthead {
position: fixed;
top: 0;
left: 0;
z-index: 9999;
width: 100%;
height: 50px;
background-color: #00a087;
opacity: 1;
}
#container {
background-color: blue;
height: 1000px;
display: block;
margin-top: -50px;
}
<body onload="myFunction()">
<nav id="masthead">
<!-- Fixed navigation bar content -->
</nav>
<div id="container"></div>
</body>

How about this:
JS:
// listen for scroll
$(window).scroll( function() {
// apply css classes based on the situation
if ($(".masthead").offset().top > 100) {
$(".masthead").addClass("navbar-scrolled");
} else {
$(".masthead").removeClass("navbar-scrolled");
}
}
CSS:
.navbar-scrolled {
/* some css for navbar when scrolled */
}
JSFiddle example:
http://jsfiddle.net/8ruwnaam/
And then of course you could add some optimization to not apply the classes all the time if they are already there. But it works quite fine without such things as well.
Additional things:
The first version of this answer and your question use IDs for styling, which is not really a good idea according to a lot of people. Styling IDs goes against the DRY principles, and causes all these funny little problems when you forget to think about CSS specificity. IDs are quite alright for a lot of things when it comes to the logic in the JS or something, but try to use classes for styling.

You should create an .opaque css class and attach it based on actively scrolling or if scrollTop is < 50:
.opaque {
opacity: 0.5;
}
Then attach that class on('scroll') or at scrollTop (this is using the debounce plugin):
function myFunction() {
var $masthead = $('#masthead')
, $window = $(window);
// currently scrolling
$window.scroll($.debounce( 250, true, function(){
$masthead.addClass('opaque');
}));
// done scrolling
$window.scroll($.debounce( 250, function(){
// if at the top, add or keep opaque class
if($(this).scrollTop() < 50) {
if(!$masthead.hasClass('opaque')) {
$masthead.addClass('opaque');
}
} else {
$masthead.removeClass('opaque');
}
}));
}

You need to set it to be transparent by default (as it will be on the top) like that
#masthead {
position: fixed;
top: 0;
left: 0;
z-index: 9999;
width: 100%;
height: 50px;
background-color: #00a087;
opacity: 0.5; /*edited the opacity to be 50% by default*/
}
then use this script to achieve your needs:
$(document).ready(function () {
$(window).scroll(function(){
var ScrollTop = parseInt($(window).scrollTop());
if (ScrollTop < 100) {
document.getElementById("masthead").style.opacity = "0.5";
} else {
document.getElementById("masthead").style.opacity = "1";
}
});
});

How to trigger the layout to change?

I've a sticked element which gets the top-alignment from current scroll-offset. Problem is, that the layout is not "retriggerd" if the space from it is free. So there stays a ghost-gap where the sticked element was...
http://fiddle.jshell.net/pPc4V/
The markup is pretty simple:
...
as well as the js:
var $win = $(this);
var sticked = document.querySelector('a.sticked');
$win.on('scroll', function () {
var scrollTop = $win.scrollTop();
sticked.style.top = scrollTop + 'px';
// $win.resize();
});
...and the css looks good so far:
a {
display: inline-block;
width: 90px;
height: 90px;
background: deepskyblue;
}
.sticked {
position: relative;
top: 0;
left: 0;
background: tomato;
}
I tried to trigger the resize-event on scroll (as you see above uncommented), but no success! Any ideas, how to retrigger the layout so that the free-gap is filled with the next floated element?
Update
To clarify what I mean I made a simple image-timelime:
Step 1
Step 2
Step 3

The issue is that you are setting position fixed on an element which is displayed inline. That will cause that space to occur. I have redid your jsFiddle with proper alignment.
To fix it, I added the class "stuck" only when the document's scrollTop position is greater than the scrollTop position of your target element.
jsFiddle: http://fiddle.jshell.net/pPc4V/44/
HMTL:
<div id="grid">
etc...
</div>
CSS:
#grid {
height:1000px;
overflow:hidden;
float:left
}
#grid > a {
display: inline-block;
width: 90px;
height: 90px;
background: deepskyblue;
}
.stuck {
position: fixed;
background: navy !important;
}
JS:
$(window).on('scroll', function () {
var $doc = $(document),
parentElement = $('#grid'),
childToGetStuck = parentElement.find('a:nth-child(5)');
if ($doc.scrollTop() > childToGetStuck.scrollTop()) {
childToGetStuck.addClass('stuck');
//console.log($('.stuck').scrollTop())
} else {
childToGetStuck.removeClass('stuck');
}
});