A page looks correct on desktop but overflows or overlaps on smaller screens. How would you debug it?

I would verify the fix across narrow and wide viewports, 200% zoom, long translated labels, validation messages, loading states, and real data extremes. Container queries can make reusable components respond to their actual container rather than the entire viewport. I also check keyboard focus and sticky or fixed elements because clipping can make controls unreachable. Visual regression tests can protect known breakpoints, while DOM measurements or browser assertions catch overflow that a screenshot reviewer might miss.