Why Does Reduced Opacity Affect Element Stacking Order in CSS?

Opacity and Z-Index: Unraveling Stacking Order in Browsers

When working with stacking contexts, designers often encounter perplexing behavior. A common scenario involves the unexpected visibility of elements when their opacity is less than 1, even when subsequent elements are positioned over them.

Consider a scenario where a "popup window" in JavaScript has been added over a navy square with an opacity of 0.3. Despite the popup's position atop the square, a portion of the square remains visible.

To understand this behavior, we need to delve into the concept of stacking contexts.

According to the CSS Visual Formatting Module, an element with an opacity less than 1 creates a new stacking context. This means that content outside this element cannot be layered within its content interms of z-order.

Crucially, if the element with reduced opacity is not positioned, it will be painted within the parent stacking context as if it had a z-index of 0 and an opacity of 1. This explains why the visible edges of the square appear even though the popup has a higher z-index.

However, if the element with reduced opacity is positioned, the z-index property applies as per CSS21, treating "auto" as "0" since a new stacking context is always created.

This subtle nuance in the stacking order highlighted the importance of understanding the intricacies of rendering when developing complex layouts. By appreciating the role of opacity in creating new stacking contexts, we can avoid such unexpected behavior and achieve seamless graphical effects.

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