Prevent body scrolling but allow overlay scrolling

I've been searching for a "lightbox" type solution that allows this but haven't found one yet (please, suggest if you know of any).

The behavior I'm trying to recreate is just like what you'd see at Pinterest when clicking on an image. The overlay is scrollable (as in the whole overlay moves up like a page on top of a page) but the body behind the overlay is fixed.

I attempted to create this with just CSS (i.e. a div overlay on top of the whole page and body with overflow: hidden), but it doesn't prevent div from being scrollable.

How to keep the body/page from scrolling but keep scrolling inside the fullscreen container?

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 (setting overflow: hidden) making the body no longer scrollable.

The overlay created on-the-fly or already injected in 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 (but now we are in 2022, so you may use inset: 0 instead).

The div inside the overlay is in position: static so the vertical scrollbar is related to that element. This is resulting in a scrollable but fixed overlay.

When you close the overlay, you have to hide it (using display: none) and you could even remove the node via javascript (or just the content inside, it's up to you but also depends on the nature of the content).

The final step is to also remove the noscroll class applied to the body (so the overflow property gets back to the value it had previously)

---

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" tabindex="-1">
  <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;
   inset: 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"]'),
    openingBtt;
    
[].forEach.call(overlayBtts, function(btt) {

  btt.addEventListener('click', function() { 
     
     /* Detect the button class name */
     var overlayOpen = this.className === 'open-overlay';
     
     /* storing a reference to the opening button */
     if (overlayOpen) {
        openingBtt = this;
     }
     
     /* 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;
     
      /* forcing focus for Assistive technologies but note:
    - if your modal has just a phrase and a button move the
      focus on the button
    - if your modal has a long text inside (e.g. a privacy
      policy) move the focus on the first heading inside 
      the modal
    - otherwise just focus the modal.

    When you close the overlay restore the focus on the 
    button that opened the modal.
    */
    if (overlayOpen) {
       overlay.focus();
    }
    else {
       openingBtt.focus();
       openingBtt = null;
    }

  }, false);

});

/* detect Escape key when the overlay is open */
document.body.addEventListener('keyup', (ev) => {
   if (ev.key === "Escape" && overlay.getAttribute('aria-hidden') === 'false') {
      overlay.setAttribute('aria-hidden', 'true');
      body.classList.toggle('noscroll', false);
      openingBtt.focus();
      openingBtt = null;
   }
})

---

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;
     inset: 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; 
}