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Prevent Page Scrolling When a Modal is Open

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Please stop me if you've heard this one before. You open a modal, scroll through it, close it, and wind up somewhere else on the page than you were when you opened the modal.
That's because modals are elements on a page just like any other. It may stay in place (assuming that's what it's meant to do) but the rest of page continues to behave as normal.
Sometimes this is a non-issue, like screens that are the exact height of the viewport. Anything else, though, we're looking at Scroll City. The good news is that we can prevent that with a sprinkle of CSS (and JavaScript) trickery.

#Let's start with something simple

We can make a huge dent to open-modal-page-scrolling™ by setting the height of the entire body to the full height of the viewport and hiding vertical overflow when the modal is open:
body.modal-open {
  height: 100vh;
  overflow-y: hidden;
}
That's good and all, but if we've scrolled through the <body> element before opening the modal, we get a little horizontal reflow. The width of the viewport is expanded about 15 pixels more, which is exactly the with of the scroll bar.
Let's adjust the right padding of the body a bit to avoid that.
body {
  height: 100vh;
  overflow-y: hidden;
  padding-right: 15px; 
}
Note that the modal needs to be shorter than the height of the viewport to make this work. Otherwise, the scroll bar on the body will be necessary.

#Great, now what about mobile?

This solution works pretty great on desktop as well as Android Mobile. That said, Safari for iOS needs a little more love because the body still scrolls when a modal is open when tapping and moving about the touchscreen.
We can set the body to a fixed position as a workaround:
body {
  position: fixed;
}
Works now! The body will not respond when the screen is touched. However, there's still a "small" problem here. Let's say the modal trigger is lower down the page and we click to open it up. Great! But now we're automatically scrolled back up to the top of the screen, which is just as disorientating as the scrolling behavior we're trying to resolve.
Boo!

#That's why we've gotta turn to JavaScript

We can use JavaScript to avoid the touch event bubble. We all know there should be a backdrop layer when a modal is open. Unfortunately, stopPropagation is a little awkward with touch in iOS. But preventDefault works well. That means we have to add event listeners in every DOM node contained in the modal — not just on the backdrop or the modal box layer. The good news is, many JavaScript libraries can do this, including good ol' jQuery.
Oh, and one more thing: What if we need scrolling inside the modal? We still have to trigger a response for a touch event, but when reaching the top or bottom of the modal, we still need to prevent bubbling. Seems very complex, so we're not totally out of the woods here.

#Let's enhance the fixed body approach

This is what we were working with:
body {
  position: fixed;
}
If we know the top of the scroll location and add it to our CSS, then the body will not scroll back to the top of the screen, so problem solved. We can use JavaScript for this by calculating the scroll top, and add that value to the body styles:

document.body.style.position = 'fixed';
document.body.style.top = `-${window.scrollY}px`;


document.body.style.position = '';
document.body.style.top = '';
This works, but there's still a little leakage here after the modal is closed. Specifically, it appears that the page already loses its scroll position when the modal is open and the body set to be fixed. So we have to retrieve the location. Let's modify our JavaScript to account for that.

const scrollY = document.body.style.top;
document.body.style.position = '';
document.body.style.top = '';
window.scrollTo(0, parseInt(scrollY || '0') * -1);
That does it! The body no longer scrolls when a modal is open and the scroll location is maintained both when the modal is open and when it is closed. Huzzah!
CSS-Tricks
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