Responsive QA, three viewports at once
Load any URL into mobile, tablet, and desktop panes side by side. Scroll, click, and navigate stay in sync. macOS desktop app, free public beta.
macOS Desktop App · Public Beta
Three viewports.
One browser.
Always in sync.
Vesper Mosaic loads one URL into three side-by-side panes at mobile (390), tablet (768), and desktop (1920) widths. Scroll one pane and the others follow. Click a link and all three navigate together. Built to replace bloated responsive-QA tools with one focused app.
v1.0.0-beta.1 · macOS · Apple Silicon and Intel · What's new
Requires macOS · Apple Silicon and IntelWhat it does
Three synced viewports
Mobile (390 x 844), Desktop (1920 x 1080), Tablet (768 x 1024) - rendered side by side in real Chromium webviews. One URL, three widths, one window.
Fit and 100% view modes
Fit mode scales every pane to share the screen so you can see all three at once. 100% mode shows each pane at its native pixel width with horizontal scrolling. Toggle with Cmd+1 and Cmd+2.
Anchor scroll sync
Scroll any pane and the other two follow, anchored to the same DOM element. Falls back to scroll percentage when no shared element is in view. No more scrubbing three windows independently.
Click and navigation sync
Click a link in any pane and all three navigate together. Submit a form, trigger an SPA route, follow a redirect - every pane stays in lockstep through a did-navigate broadcast.
Back, forward, all panes
The Back and Forward buttons in the toolbar step every pane through navigation history simultaneously. Each pane keeps its own history stack but the buttons drive them in sync.
Shared session
All three panes use one persistent partition (persist:mosaic-shared). Log in once and every pane is logged in. Useful for testing authenticated states, paywalls, or membership-only content.
Purge cache
One click clears the shared HTTP cache and reloads all three panes. Confirms whether you're seeing a CDN-cached asset or a fresh deploy without quitting and relaunching.
Viewport borders
A 1px lavender outline around each pane keeps the three viewports visually distinct without stealing focus from the content. Pane labels show exact dimensions at the top of each pane.
DevTools, every pane
Cmd+Alt+I opens DevTools for all three panes (detached). Cmd+Alt+J opens DevTools for the Mosaic chrome itself. Inspect, console-debug, or breakpoint each viewport independently.
Built for
Designers, developers,
and accessibility auditors.
Browser DevTools' responsive mode shows one viewport at a time. Stand-alone responsive-QA tools have grown bloated with device profiles, plugins, screenshot exporters, and per-pane toolbars that nobody asked for.
Vesper Mosaic does one thing well: three real Chromium viewports, kept in sync, in a single window. Spot a broken hamburger menu at 390. Confirm a tablet hero stacks correctly at 768. Verify desktop nav collapses cleanly. All three at once, with one URL bar, one set of navigation buttons, and one shared login.
Keyboard
Reach for the keyboard, not the mouse. The most common workflows are one-key away.
Cmd + 1
Switch to Fit mode (default). All three panes scale to share the available width.
Cmd + 2
Switch to 100% mode. Each pane renders at native pixel width with horizontal scroll.
Cmd + R
Reload all three panes simultaneously, preserving the current URL.
Cmd + Alt + I
Open DevTools for every pane (detached windows).
Cmd + Alt + J
Open DevTools for the Mosaic chrome itself.
Cmd + L
Focus the URL bar to type or paste a new address.