A free, open, from-scratch MV3 extension. An original reimplementation of what paid mobile-simulator extensions do, not a repack or patch of any existing product.

A local Brave/Chrome extension that previews the page you're on at any device resolution — in place, no separate tab. Click the toolbar icon → a popup with a searchable picker (168 devices) → the device frame overlays the current page → "Return to normal" removes it. Every preset is free, plus custom W×H. No paywall.

Paid mobile-simulator extensions are built around two things, both simple to do yourself:

1. A list of device width×height presets (devices.js — edit it freely).
2. Stripping the headers that stop a site from loading in an <iframe> (X-Frame-Options, CSP frame-ancestors). That's background.js, ~20 lines, scoped to the viewer tab only so your normal browsing is untouched.

It loads the target URL in a correctly-sized iframe and emulates the device so responsive sites reflow the way they do on a phone — both the CSS viewport (iframe width) AND the in-page JS environment.

A correctly-sized iframe gets CSS media-queries right, but any site that branches its layout on navigator.userAgent / touch / devicePixelRatio in JavaScript still thinks it's a desktop. emulate.js fixes that: it runs at document_start in the page's MAIN world and overrides navigator.userAgent, platform, vendor, maxTouchPoints (+ ontouchstart), devicePixelRatio, and navigator.userAgentData to match the selected device.

Scoping: viewer.js recreates the iframe each load and stamps the device profile onto the frame's window.name (set on the same-origin about:blank frame, which survives the navigation — the iframe name attribute does not propagate). emulate.js runs on every frame but returns immediately unless window.name carries the __rvsim__ marker, so your normal browsing is untouched.

1. Open brave://extensions (or chrome://extensions).
2. Toggle Developer mode on (top right).
3. Click Load unpacked → select this folder (the responsive-viewer folder).
4. Browse to any normal site, click the extension's toolbar icon → the popup opens.
5. Search/scroll the device list and click a device → the frame overlays the page in place. Use Rotate / Zoom in the popup. Click ✕ Return to normal (or Esc) to dismiss.

After editing any file, hit the ↻ reload on the extension card (it's a popup + content-script now, so just reopen the popup — no tab to refresh).

• popup.html/js/css — toolbar popup: searchable device list (brand-grouped), zoom, rotate, Return-to-normal. Owns the picker so there's no giant dropdown.
• overlay-inject.js — content script (top frame). On the popup's signal it drops a full-screen iframe (the viewer in ?embed=1 mode) over the current page and tears it down on Return. The real page is never navigated, so dismissing is instant and lossless.
• viewer.html/js/css — the device frame + sizing/emulation, now rendered inside that overlay iframe. The framed page is a fresh load of the current URL (a CSS container can't trigger a site's width media-queries — only a real iframe viewport can), with the original untouched underneath.

• Emulation patches the JS-visible environment (UA, touch, DPR) but does not move the visual viewport the way Chrome DevTools device mode does (that needs the DevTools Protocol, which can't apply to one iframe inside our toolbar tab). For the vast majority of responsive sites, iframe-width + the JS overrides match a real device; DevTools-grade pixel emulation is the only thing beyond reach.
• emulate.js is a MAIN-world content script on <all_urls>/all_frames — it runs (and instantly returns) on every page you browse. Cheap, but it is broad injection; that's the cost of document_start timing.
• http-only sites may be blocked as mixed content (the viewer is https-context).

• HD screenshot — chrome.tabs.captureVisibleTab then crop to the device's bounding box on a canvas. (Can't drawImage the cross-origin iframe directly — tab-capture + crop is the way.)
• GIF recording — repeated tab-capture frames encoded with a bundled gif encoder. Heavier; needs one vendored JS lib.
• Side-by-side multi-device — render N frames in a row, one configure rule per frame.

• manifest.json — MV3, declarativeNetRequestWithHostAccess + <all_urls>.
• background.js — opens the viewer, strips framing headers + sets request UA per tab.
• emulate.js — MAIN-world document_start script; overrides navigator/touch/DPR inside the framed page when tagged by the viewer.
• devices.js — the preset list: 168 devices across 15 brands (Apple, Samsung incl. Z Fold/Flip in both folded + unfolded states, Google, OnePlus, Nothing, Xiaomi/Redmi/Poco, Motorola incl. razr, Oppo, Vivo, Honor, Asus), each with viewport + dpr + cutout, grouped by brand. Accuracy tiers documented in the file header. This is the core of the tool.
• viewer.html / viewer.css / viewer.js — the toolbar + device frame UI.