Why Don\'t CSS Transitions Work on Appended Elements?

Triggering CSS Transitions on Appended Elements: Unveiling the Enigma

Introduction

Despite the pervasive use of CSS animations, certain anomalies arise when attempting to apply them to elements appended dynamically. As observed, immediate transitions are often ignored, resulting in an abrupt rendering of the transition's end state. This article delves into the cause of this behavior and explores various solutions proposed to trigger the desired transition effect.

Understanding the Problem

The peculiar behavior of transitions on newly-appended elements stems from browser optimizations. Specifically, browsers tend to batch reflows and CSS style recalculations to enhance efficiency. When elements are added and CSS classes are assigned in a single JavaScript cycle, this optimization can lead to scenarios where the first reflow is optimized out, resulting in only a single style value being presented to the rendering engine. Consequently, no transition is triggered.

Proposed Solutions

To address this issue, multiple approaches have been devised, each employing a unique technique:

1. Time-Based Delay using setTimeout

This method introduces a slight delay, separating the DOM manipulation and class addition into different JavaScript cycles. This ensures that when the class is added, there are two distinct style values, enabling the transition to occur.

2. Triggering Reflow with offsetWidth

Access to certain properties, such as offsetWidth, forces the browser to perform a reflow. By leveraging this property before adding the transition class, the reflow is triggered, creating the necessary style values for the transition to take effect.

3. Selecting and Adding Class Programatically

This approach involves selecting the appended element programmatically using JavaScript and then applying the transition class directly. This eliminates any potential batching optimizations and ensures that the transition occurs as intended.

Preferred Solution

The best choice among the presented solutions depends on the specific requirements and context. However, accessing offsetWidth generally stands out as the most reliable and efficient option, as it guarantees a reflow before the transition is applied. This strategy is particularly beneficial when dealing with complex or heavy DOM operations, as it ensures that the browser has the necessary style information to execute the transition smoothly.

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