Why Do Fixed Elements Move When Non-Positioned Siblings Have Margin?

Understanding the Movement of Fixed Elements with Non-Positioned Siblings

In the realm of web design, the phenomenon of fixed elements interacting with their non-positioned siblings can be puzzling. Why does a fixed header move when a sibling element has a margin applied to it? This article delves into the underlying mechanism behind this behavior.

The Hypothesis

The hypothesis suggests that fixed elements, while removed from document flow, are calculated relative to the viewport. The viewport, in turn, is determined by the first element within the document flow. As the first in-flow element is the non-header div here, the viewport incorporates the applied margin-top. This shift in the viewport consequently moves the fixed header down.

The Explanation

The correct answer aligns with the hypothesis. When an element is positioned fixed, it is taken out of the normal document flow. The first element still within the flow (main) possesses a margin-top of 90px in our scenario.

The parent of main is body, which by default has a margin-top of 8px. Due to CSS margin collapsing, these margins combine, causing both body and main to shift down 90px. Consequently, the fixed header, which is positioned relative to the viewport, follows suit and moves down by the same amount.

Conclusion

In conclusion, the movement of fixed elements in relation to non-positioned siblings stems from the interplay between document flow, the viewport calculation, and margin collapsing. By understanding this mechanism, we can confidently control the positioning of elements, ensuring a predictable and responsive design.

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