Why Is My CSS Transition Skipped or Callback Omitted in My Game?

CSS Transition Skipped or Callback Omitted

In this multifaceted inquiry, the developer expresses perplexity over intermittent inconsistencies in CSS transitions and callback invocations within a gaming project. Despite replacing jQuery with pure JavaScript, attempts to transition projectile positions using CSS failed to function as anticipated, necessitating a 1ms timeout to initiate the transition. Furthermore, the transition callback occasionally failed to execute, leaving the developer bewildered.

To delve into the intricacies of this issue, we must acknowledge that CSS transitions rely on changes in an element's computed style. When inline styles are set, as in this case, the browser may not immediately update the computed style, resulting in a mismatch between expected and actual element properties.

When the transition property is applied, the computed left and top values may already match the intended values, leaving the transition with no action to perform. Hence, it is skipped, and the transitionend event is not triggered.

To rectify this, the developer can employ techniques to force a style recalculation, such as accessing the element's offsetHeight or triggering a reflow using DOM methods. For instance:

<code class="javascript">let animdiv = document.getElementById('animid');
animdiv.addEventListener("transitionend", function(event) { ... }, false);
// force a reflow
animdiv.offsetTop;
Object.assign(animdiv.style, {left: "100px", top: "100px" });</code>

By coercing a style recalculation, the computed style will be up-to-date, ensuring that the transition can accurately track changes in the element's position and correctly fire the transitionend event.

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