Prevent body scrolling but allow overlay scrolling

Theory

Looking at current implementation of the pinterest site (it might change in the future), when you open the overlay a noscroll class is applied to the body element and overflow: hidden is set, thus body is no longer scrollable.

The overlay (created on-the-fly or already inside the page and made visible via display: block, it makes no difference) has position : fixed and overflow-y: scroll, with top, left, right and bottom properties set to 0: this style makes the overlay fill the whole viewport.

The div inside the overlay is instead just in position: static then the vertical scrollbar you see is related to that element. As a result the content is scrollable but overlay remains fixed.

When you close the zoom you hide the overlay (via display: none) and then you could also entirely remove it via javascript (or just the content inside, it's up to you how to inject it).

As a final step you have to also remove the noscroll class to the body (so the overflow property returns to its initial value)

Code

Codepen Example

(it works by changing the aria-hidden attribute of the overlay in order to show and hide it and to increase its accessibility).

Markup
(open button)

<button type="button" class="open-overlay">OPEN LAYER</button>

(overlay and close button)

<section class="overlay" aria-hidden="true">
  <div>
    <h2>Hello, I'm the overlayer</h2>
    ...   
    <button type="button" class="close-overlay">CLOSE LAYER</button>
  </div>
</section>

CSS

.noscroll { 
  overflow: hidden;
}

.overlay { 
   position: fixed; 
   overflow-y: scroll;
   top: 0; right: 0; bottom: 0; left: 0; }

[aria-hidden="true"]  { display: none; }
[aria-hidden="false"] { display: block; }

Javascript (vanilla-JS)

var body = document.body,
    overlay = document.querySelector('.overlay'),
    overlayBtts = document.querySelectorAll('button[class$="overlay"]');

[].forEach.call(overlayBtts, function(btt) {

  btt.addEventListener('click', function() { 

     /* Detect the button class name */
     var overlayOpen = this.className === 'open-overlay';

     /* Toggle the aria-hidden state on the overlay and the 
        no-scroll class on the body */
     overlay.setAttribute('aria-hidden', !overlayOpen);
     body.classList.toggle('noscroll', overlayOpen);

     /* On some mobile browser when the overlay was previously
        opened and scrolled, if you open it again it doesn't 
        reset its scrollTop property */
     overlay.scrollTop = 0;

  }, false);

});

Finally, here's another example in which the overlay opens with a fade-in effect by a CSS transition applied to the opacity property. Also a padding-right is applied to avoid a reflow on the underlying text when the scrollbar disappears.

Codepen Example (fade)

CSS

.noscroll { overflow: hidden; }

@media (min-device-width: 1025px) {
    /* not strictly necessary, just an experiment for 
       this specific example and couldn't be necessary 
       at all on some browser */
    .noscroll { 
        padding-right: 15px;
    }
}

.overlay { 
     position: fixed; 
     overflow-y: scroll;
     top: 0; left: 0; right: 0; bottom: 0;
}

[aria-hidden="true"] {    
    transition: opacity 1s, z-index 0s 1s;
    width: 100vw;
    z-index: -1; 
    opacity: 0;  
}

[aria-hidden="false"] {  
    transition: opacity 1s;
    width: 100%;
    z-index: 1;  
    opacity: 1; 
}

overscroll-behavior css property allows to override the browser's default overflow scroll behavior when reaching the top/bottom of content.

Just add the following styles to overlay:

.overlay {
   overscroll-behavior: contain;
   ...
}

Codepen demo

Currently works in Chrome, Firefox and IE(caniuse)

For more details check google developers article.

Most solutions have the problem that they do not retain the scroll position, so I took a look at how Facebook does it. In addition to setting the underlaying content to position: fixed they also set the top dynamically to retain the scroll position:

scrollPosition = window.pageYOffset;
mainEl.style.top = -scrollPosition + 'px';

Then, when you remove the overlay again, you need to reset the scroll position:

window.scrollTo(0, scrollPosition);

I created a little example to demonstrate this solution

let overlayShown = false;
let scrollPosition = 0;

document.querySelector('.toggle').addEventListener('click', function() {
  if (!overlayShown) {
        showOverlay();
  } else {
    removeOverlay();
  }
  overlayShown = !overlayShown;
});

function showOverlay() {
    scrollPosition = window.pageYOffset;
    const mainEl = document.querySelector('.main-content');
    mainEl.style.top = -scrollPosition + 'px';
    document.body.classList.add('show-overlay');
}

function removeOverlay() {
        document.body.classList.remove('show-overlay');
    window.scrollTo(0, scrollPosition);
    const mainEl = document.querySelector('.main-content');
    mainEl.style.top = 0;
}

.main-content {
  background-image: repeating-linear-gradient( lime, blue 103px);
  width: 100%;
  height: 200vh;
}

.show-overlay .main-content {
  position: fixed;
  left: 0;
  right: 0;
  overflow-y: scroll; /* render disabled scroll bar to keep the same width */
/* Suggestion to put: overflow-y: hidden; 
Disabled scrolling still makes a mess with its width. Hiding it does the trick. */
}

.overlay {
  display: none;
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: rgba(0, 0, 0, 0.3);
  overflow: auto;
}

.show-overlay .overlay {
  display: block;
}

.overlay-content {
  margin: 50px;
  background-image: repeating-linear-gradient( grey, grey 20px, black 20px, black 40px);
  height: 120vh;
}

.toggle {
  position: fixed;
  top: 5px;
  left: 15px;
  padding: 10px;
  background: red;
}

/* reset CSS */
body {
  margin: 0;
}

<main class="main-content"></main>

  <div class="overlay">
    <div class="overlay-content"></div>
  </div>
  
  <button class="toggle">Overlay</button>

If you want to prevent overscrolling on ios, you can add position fixed to your .noscroll class

body.noscroll{
    position:fixed;
    overflow:hidden;
}