Why Does My Full-Screen Background Image Jump on Mobile Chrome When the Address Bar Hides?

Background Image Offset on Address Bar Hide in iOS/Android/Mobile Chrome

Issue

When developing a responsive website using CSS, including a full-screen background image that rotates and fades between two divs, an unexpected behavior occurs: the background image jumps slightly when scrolling down on iOS Safari, Android Browser, or Android Chrome. This is caused by the address bar hiding and consequently adjusting the size of the background divs, which are fixed and set to 100% height.

Solution

To resolve this issue, two possible solutions exist:

1. Set #bg1 and #bg2 height to 100vh: This solution is theoretically elegant, using the viewport height to set the appropriate height. However, iOS has a vh bug that affects this solution.
2. Use JavaScript to set a static height: The viewport size, when determined by JavaScript, is unaffected by the address bar. Thus, JavaScript can set a static height for the background divs based on the viewport size. This is not an ideal solution due to its non-pure CSS nature and a slight image jump on page load, but it is viable considering iOS's vh bugs.

Using the following JavaScript code will adjust the background height dynamically:

var bg = $("#bg1, #bg2");

function resizeBackground() {
    bg.height($(window).height());
}

$(window).resize(resizeBackground);
resizeBackground();

Additional Considerations

While the above solution effectively prevents background resizing, it may cause a noticeable gap when users scroll down. This gap results from matching the background height to the screen minus the URL bar height. By adding 60px to the height, this issue can be resolved, but it means the bottom 60px of the background image won't be visible when the URL bar is present:

function resizeBackground() {
    bg.height( $(window).height() + 60);
}

It's worth noting that these resizing URL bars in iOS and Android have raised concerns due to their impact on website functionality. Although their purpose is understood, their behavior can cause unexpected challenges in CSS-based designs.

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