How does the browser decide when to reflow the layout on a page?

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How does the browser decide when to reflow the layout on a page?

Asked on Nov 28, 2025

Answer

Browsers trigger a reflow whenever the layout of a webpage needs to be recalculated, which can be due to changes in the size, position, or visibility of elements. Reflow is a critical process for rendering web pages correctly.

Example Concept: Reflow is a process where the browser recalculates the positions and sizes of elements on a webpage. This occurs when changes are made to the DOM, such as adding or removing elements, changing element styles, or resizing the browser window. Reflow can be triggered by JavaScript operations that modify the DOM or CSS properties that affect layout, like width, height, margin, and padding.

Additional Comment:

Reflow is computationally expensive, so minimizing unnecessary changes can improve performance.
Using CSS classes to apply style changes can reduce the number of reflows.
Batching DOM changes together can help in minimizing reflow operations.

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