CODEWITHSUNDEEP
csstouch
bobsoap
bobsoap

Reputation: 5104

Prevent that a fixed element resizes when zooming on touchscreen

On an HTML website, you have a fixed element like this:

<div id="fixed">
  <p>Some content</p>
</div>

It has this CSS:

#fixed { height:150px; position:fixed; top:0; left:0; z-index:10000; }

When you view this page on a mobile device (or any touchscreen-enabled device), and you pinch the screen to zoom in, the fixed element zooms in along with all the other content (it gets bigger). When you zoom in far enough, it becomes so big that it almost fully overlaps all the content beneath it.

A practical use case would be a UI like a fixed navigation bar across the top, or a floating button in the corner of the screen.

How could you prevent a single element from resizing in the browser, and make it stay the same size at all times?

Upvotes: 35

Views: 72940

Answers (4)

DawnPaladin
DawnPaladin

Reputation: 1533

Guidance on MDN and StackOverflow suggests using either the resize event or various touch-specific APIs, but the one that's worked best for me has been the wheel event. It works cross-browser, with a bit of lag, although it's unreliable in Safari.

VisualViewport tells you the edges of the zoomed-in portion of the screen.

This will pin an element to the bottom of the screen and keep it there as you pinch-zoom and scroll around:

#pinned {
    position: fixed;
    bottom: 0px;
    left: 0px;
    right: 0px;
    width: 100%;
    height: 35px;
    transform-origin: 0% 100%;
    text-align: center;
}

const pinnedElement = document.getElementById('pinned');
const elementHeight = pinnedElement.getBoundingClientRect().height;
const viewport = window.visualViewport;
document.addEventListener('wheel', function(e) {
    const elementTop = (viewport.offsetTop + viewport.height - elementHeight);
    pinnedElement.style.top = elementTop + 'px';
    pinnedElement.style.bottom = elementTop + elementHeight + 'px';
    pinnedElement.style.left = viewport.offsetLeft + 'px';
    pinnedElement.style.transform = `scale(${1/viewport.scale})`;
});

Upvotes: 0

zrooda
zrooda

Reputation: 3673

Demo to this answer
Dialog widget library I wrote based on this answer.
Demo for the dialog widget Try it on mobile, zoom around and tap the link.

While the gesture event seems to hold some metadata about the scale factor of something, we might be better off in control and find it manually. Since we want to keep the effect up while the user moves around live, we need to recalculate during every scroll event.

window.addEventListener('scroll', function(e){ /* coming next */ })

We calculate the zoom factor and apply it to the rescaled element as a CSS3 transform:

el.style["transform"] = "scale(" + window.innerWidth/document.documentElement.clientWidth + ")";

This will rescale the element back to zoom 1 ratio relative to the current viewport zoom, but it's likely it is still incorrectly placed on the page. This means we have to get new values for it's position. What will actually happen on screen is dependant on the element's CSS position value, fixed will behave differently than absolute and on mobile Safari specifically, fixed position has an added smoothed out effect while the page is zoomed, which creates some inconvenient problems for this cause - I would suggest having a 100% content height element as a relative parent for your intended scaled element, then in your script, position your element absolutely inside of the parent. The following values work for this example demo:

overlay.style.left = window.pageXOffset + 'px';
overlay.style.bottom = document.documentElement.clientHeight - (window.pageYOffset + window.innerHeight) + 'px';

You might also pay attention to using transform-origin CSS property depending on from which anchor point you want the scaling to take effect - this has a direct effect on alignment.

Upvotes: 28

Max Knox
Max Knox

Reputation: 145

I'm looking to do the same thing that @bobsoap was trying to do BUT My solution is the following:

1. Disable zooming in your viewport tag:

<meta name="viewport" content="user-scalable=no, initial-scale=1, maximum-scale=1, minimum-scale=1" />

2. Use a plugin like: TouchSwipe.js:

https://github.com/mattbryson/TouchSwipe-Jquery-Plugin

And only zoom in the Div or divs that you want & suit to your needs.

I have 2 divs on a page (left & right) The left div has a fixed scrolling menu and the right has small text & images. I want to pinch/zoom and have only the right div zoom so that the user can read the text better if necessary. Rather than make the entire viewport zoomable and disable zoom on my left div, I'm doing exactly the opposite: Make only my right div zoomable via TouchSwipe plugin.

I'll share my code when I'm done if anyone is interested in how I implemented the TouchSwipe plugin.

Upvotes: 9

Tammy Shipps
Tammy Shipps

Reputation: 945

You're missing a width attribute and you can use things like max-width and max-height to help keep the box the size you wanted. However, zooming allows the user to get pretty granular with a page, so there's always going to be a chance they have that issue.

#fixed { height:150px; width: 200px; max-width: 200px; max-height: 150px; position:fixed; top:0; left:0; z-index:10000; }

Upvotes: -5

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