Zero overflow in 10 minutes: responsive testing with Claude Code and Playwright

My markdown code viewer worked perfectly on desktop. On tablet, horizontal scroll. On mobile, buttons overflowing the viewport. The classic. Except this time, instead of spending 2 hours in DevTools manually resizing, I let Claude Code run the loop: screenshot → diagnose → fix → retest. 10 minutes, zero overflow across 10 resolutions. Here's the workflow. The problem — the overflow you don't see The scenario is always the same. You code on a 1920px screen. Everything lines up. The layout breathes. You ship. Then someone opens it on an iPad and a horizontal scrollbar appears. On an iPhone SE, a button bleeds past the right edge. The pre blocks expand beyond the viewport and drag the entire page with them. The real issue isn't the CSS. It's the feedback loop. On desktop you never see the overflow — because there's nothing to overflow. Detecting it requires resizing, which requires switching tools, which requires discipline you don't have at 11 PM when you're shipping a feature. There's a