Prevent that a fixed element resizes when zooming on touchscreen

Question

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?

Answer 1

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.