Why Do CSS Transitions on Appended Elements Not Work Immediately?

Immediate CSS Transition on Appended Elements

Recently, it was noted that immediate CSS transitions on appended elements seem to be ignored, with the transition's end state displayed immediately. To delve into this behavior and its solutions, we will examine:

Causes

The root cause of this issue is a browser optimization technique called reflow batching. When an element is added or modified in one JavaScript cycle, browsers may opt to perform the necessary reflows (recalculations of layout and position) in a single batch after all changes have been made. This optimization prevents unnecessary repainting of the page multiple times.

Workarounds

Several methods effectively trigger the transition:

• setTimeout: Delaying the class addition to a subsequent JavaScript cycle forces the browser to calculate two distinct style values, resulting in a smooth transition.
• Directly accessing offsetWidth: This property forces a reflow, ensuring that the browser processes the initial and final style values.
• Focus on the element: Similarly to accessing offsetWidth, focusing on the element triggers a reflow, initiating the transition.

Preferred Solution

The preferred solution depends on the specific scenario. However, for reliability, accessing offsetWidth (or getComputedStyle()) is generally recommended. It ensures that the browser calculates the style values before the transition, reducing the likelihood of unexpected behavior due to it being skipped.

Additional Options

Alternative solutions include:

• Using requestAnimationFrame: This API allows you to schedule an animation callback to be executed on the next animation frame, ensuring that the transition is triggered after the browser has completed its reflow batching process.
• Forcing a layout recalculation: Calling document.documentElement.style.position = null; followed by document.body.style.position = null; forces the browser to recalculate the layout, triggering the transition.

By understanding the underlying causes and exploring the available workarounds, you can effectively trigger CSS transitions on dynamically appended elements, ensuring smooth and seamless animations in your web applications.

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