Why Does Sticky Positioning with `bottom: 0` Exhibit Reversed Behavior Compared to MDN Documentation?

Understanding the Opposite Behavior of Sticky Positioning

In the MDN documentation, sticky positioning is described as a hybrid of relative and fixed positioning, where an element acts like a relatively positioned element until a threshold is exceeded, after which it behaves as a fixed position element. However, when specifying bottom: 0 for sticky positioning, the behavior seems to be reversed.

Mechanism of Sticky Positioning

According to the MDN, sticky position elements operate with two states:

1. Relative Mode: The element is positioned relative to the flow of its container or static parent.
2. Fixed Mode: The element is positioned relative to the viewport, remaining at a fixed location.

The transition between these states occurs when the specified threshold is exceeded.

Scenario Explanation

Consider the following example:

<div class="container">
  <header></header>
  <main></main>
  <footer>
    <div class="footer"></div>
  </footer>
</div>

body {
  margin: 0;
}

.container {
  display: flex;
  height: 100vh;
}

.container>* {
  width: 100%;
}

footer {
  background: #faa;
  position: sticky;
  bottom: 0;
}

.footer {
  background: #aff;
  height: 100vh;
}

When bottom: 0 is specified for the footer element, it initially starts in the fixed state because the threshold (the bottom of the viewport) is exceeded from the start. As you scroll down, the threshold is still exceeded, so the footer remains in the fixed position.

However, when the footer element reaches the threshold (the bottom of the viewport), it transitions into the relative mode because the threshold is no longer exceeded. This is the opposite of what the MDN documentation suggests, which states that the transition should occur when the element is less than 0px from the bottom of the viewport.

Language Misinterpretation

The ambiguity arises from the language used in the MDN documentation. The statement "until the threshold is exceeded" implies that the relative state is the starting state for sticky positioning. However, this is not always the case. If the threshold is already exceeded when the element is declared sticky, it will start in the fixed state.

Conclusion

Understanding the two states of sticky positioning and when transitions occur between them is crucial for using it effectively. The behavior described in the MDN documentation can be reversed when the threshold is initially exceeded, resulting in a different behavior than expected.

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