Opacity vs. Z-Index: Why Doesn't Z-Index Always Work with Semi-Transparent Elements?

Opacity vs. Z-Index: The Invisible Overlay

When layering elements in web browsers, two key properties come into play: opacity and z-index. Opacity controls the transparency of an element, while z-index determines its position in the stacking order. However, a surprising interaction between these properties has been discovered.

The Problem: Squarely Conflicting

Consider the scenario where a "popup window" is added on top of a navy square. Intuitively, one would expect the square to be completely hidden beneath the popup. Yet, when the opacity of the square is set below 1 (e.g., 0.3), part of the square remains visible underneath the popup window.

Unraveling the Mystery

This behavior is not a bug but rather a deliberate design choice by web browsers. As per the CSS color module, elements with opacity less than 1 create a new stacking context. Consequently, elements outside this context cannot be layered within it, regardless of their z-index values.

The Silent Stacking Context

By default, non-positioned elements with opacity less than 1 are treated as if they have 'z-index: 0' and 'opacity: 1'. This means that they will always be rendered at the same stacking order as if they were at the bottom of their parent stacking context, regardless of their actual z-index values.

Resolving the Conflict

To rectify this behavior, one can simply avoid using opacity values below 1 for elements that should be fully hidden. Alternatively, by positioning the element with opacity less than 1 using CSS properties like 'position: absolute' or 'position: fixed', the desired stacking order can be achieved, as the 'z-index' property will then control the stacking order within the new stacking context created by the opacity.

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