How is reflow different from repaint, and why does frequent reflow hurt performance?
~4 min by voice
How to approach it
The interviewer checks whether the candidate understands the stages of rendering in the browser (layout → paint → composite), can tell expensive changes from cheap ones, and knows practical ways to avoid triggering unnecessary layout, first and foremost forced synchronous layout.
The interviewer will ask this question and listen. Think out loud and ask for a hint if you need one. When you're done, pause or tap “That's my answer”. In about a minute, your score and the points you covered show up here.