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🤖 fix: avoid upscaling capped browser preview frames - #3396
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Render browser preview frames at their intrinsic captured bitmap size when agent-browser provides lower-resolution screencast frames, and keep pointer mapping aligned with the rendered frame bounds.
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_Generated with `mux` • Model: `openai:gpt-5.5` • Thinking: `xhigh` • Cost: `87963{MUX_COSTS_USD:-0.00}`_
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May 27, 2026
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yermakoffivan pushed a commit
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Aug 22, 2026
## Summary Fix the Browser Preview panel rendering pixelated/blurry on HiDPI (Retina) displays by capping the rendered frame at `naturalSize / devicePixelRatio` CSS pixels, so one stream bitmap pixel is never stretched past one physical display pixel. Fixescoder#3115 ## Background Browser Preview frames come from agent-browser's stream server, which captures via CDP `Page.startScreencast`. That capture is bounded by the remote session's CSS viewport size (in DIPs): `deviceScaleFactor` never raises the bitmap above CSS resolution, and older agent-browser versions additionally capped frames at 1280x720. Xum is a passive session viewer and cannot increase capture resolution. PR coder#3396 stopped upscaling frames beyond their intrinsic size in CSS pixels. On a HiDPI display that is still up to `devicePixelRatio`x physical upscaling: a 1280x720 bitmap rendered at 1280 CSS px covers 2560 physical px on a 2x Mac, which is exactly the "renders at 1x DPI and gets scaled up" blur reported in the issue. ## Implementation - New `useDevicePixelRatio` hook (`useSyncExternalStore` + a self-re-arming `matchMedia` resolution listener) so the cap tracks monitor moves and zoom changes. - `BrowserViewport` caps the frame `<img>` with inline `max-width`/`max-height: min(100%, naturalSize/dpr px)`. A smaller-but-crisp centered preview is preferred over fill-and-blur; at 1x displays behavior is unchanged. - Pointer-input mapping (`mapDomPointToViewport`/`getRenderedFrameRect`) mirrors the cap (`min(1, 1/dpr)` scale bound) so clicks, drags, and wheel events land correctly; the metadata-only fallback branch is untouched. ## Validation - `bun test src/browser/features/RightSidebar/BrowserTab/` (58 pass, including new DPR mapping + component cap tests) - `make static-check` - Remote dogfood UAT on this exact head (PASS): at emulated dpr=2 the img caps at `min(100%, 1080px)` for a 2160px frame and letterboxes crisply when the cap binds; at dpr=1 behavior is unchanged; click mapping accurate to ~2px, gutter clicks ignored, wheel + typing forwarded. ## Risks Low. Display sizing and input mapping change only when `devicePixelRatio > 1`; capture side and bridge protocol untouched. --- _Generated with `xum` • Model: `anthropic:claude-fable-5` • Thinking: `xhigh`_ <!-- xum-attribution: model=anthropic:claude-fable-5 thinking=xhigh -->
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Summary
Avoid making capped agent-browser screencast frames blurrier by rendering Browser Preview screenshots at their intrinsic captured bitmap size instead of always stretching them to fill the preview pane.
Background
Issue #3115 reports that Browser Preview becomes blurry at high viewport resolutions. Mux receives already-encoded frames from agent-browser, so it cannot restore detail lost by upstream capture caps. This change keeps Mux from further degrading capped streams by upscaling the received bitmap.
Implementation
Validation
bun test src/browser/features/RightSidebar/BrowserTab/BrowserViewport.test.tsxnix shell --extra-experimental-features 'nix-command flakes' nixpkgs#shfmt nixpkgs#shellcheck nixpkgs#hadolint -c make static-checkRisks
Low-to-moderate Browser Preview risk: rendering and input mapping changed for sessions where captured frame dimensions differ from the preview panel. Tests cover capped-frame gutters and existing object-contain coordinate behavior.
Generated with
mux• Model:openai:gpt-5.5• Thinking:xhigh• Cost:88851{MUX_COSTS_USD:-0.00}