Why Do My CSS Transitions Sometimes Fail to Animate?

Understanding CSS Reflow Issues in Browser Transitions

In creating a responsive image slider using CSS3 transitions, a common challenge arises when attempting to trigger an animation when CSS properties are modified. Despite initializing the transition property, browsers may ignore the changes and not execute the desired animation.

Solution: Forcing Reflow to Trigger Animation

The solution to this issue lies in forcing a browser reflow, which recalculates the layout and rendering of the page. This can be achieved by accessing an element's offsetHeight property. The following JavaScript function accomplishes this:

function reflow(elt) {
  console.log(elt.offsetHeight);
}

Implementation:

To implement the solution, call the reflow() function after modifying the CSS properties that trigger the animation. For example:

ul.style.transition = 'none 0s linear';
ul.style.left = '-600px';
ul.style.transition = 'all 0.2s ease-out';
reflow(ul); // Forces a reflow
ul.style.left = '0px';

Note:

A recent optimization involves replacing console.log(elt.offsetHeight) with void(elt.offsetHeight), as the optimizer is less likely to ignore this statement as a potential side effect.

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