Visual aspect analyzer

visual-aspect-analyzer

When you have finished a UI change, run this as the final review step — once, at the end, not after every individual edit. It is the visual-regression gate: the change is not done until it reports score: 100 (Test a UI change).

Drive a real browser over a session and report, per auto-detected element, what actually changed on screen. You don't pick the elements — the instrument finds the page's relevant ones itself (Auto-detection). Impact is temporal — true if it held in any frame — so an element that only starts painting or moving mid-session is still caught.

Boundaries. It auto-scrolls but never clicks, types, or logs in, so defects behind an interaction are caught only via the embed interact hook. It sees the main document only — closed shadow roots and cross-origin iframes aren't pierced.

Run it

bun src/analyze-cli.ts <url>     # run from the skill directory

In the Tale sandbox this is pre-installed — the bundle, its pngjs dep, the playwright package (shared with the agent's browser tooling), and Chromium are all baked into the image, so there is no install step. (Working on the skill locally instead? Install the browser tooling once first: bun add -d playwright && bunx playwright install chromium.)

Paths below are relative to the skill directory. Needs Bun (bun.sh) — no npm/node fallback. The run is headless and self-driving: it loads the page hidden, auto-scrolls the whole session, then exits on its own (seconds for a light page) — nothing waits on you. Any reachable URL works, including a local dev server (http://localhost:3000) — start your server first; the tool drives a browser, it doesn't launch your app.

• No build step — Bun runs the TypeScript directly and bundles the in-page instrument in memory.
• Always on — pixel capture (dithering + the paint check) and auto-scroll (lazy content + CLS), so a bare run never silently misses a defect.
• stdout is pure JSON (the compact report); the health summary always prints to stderr. bun src/analyze-cli.ts <url> > report.json saves the report while the summary still shows in your terminal.
• One flag, --full (the faithful Report). It fixes the session knobs (keyframes 4, settleMs 500 → a ~2 s sample window); to tune sampling depth/window, dithering sensitivity (pixelThreshold), or to log in before sampling, use the embed path.
• Offline — re-analyze a saved recording with no browser: bun src/cli.ts <recording.json>.

Exit codes & failure modes

The agent must predict whether a run yields a report, an empty report, or a crash:

• Exit 0 on any successful analysis — even score 50 with defects. The exit code is not a gate. Non-zero (1) means it produced no report: missing url, no browser installed, or a navigation failure. To gate in CI, parse stdout and fail on score < 100 (or defects.length > 0) — never on $?.
• A dead or hanging URL throws after Chromium's ~30 s nav timeout → one-line stderr error, exit 1, no report.
• Auth-walled pages: the browser is fresh, with no cookies or credentials, so it analyzes the login wall. Use a pre-authenticated URL or the embed interact hook.
• An empty elements[] / audit.discovered: 0 is not "clean." A real failure throws (no report); a printed report with no elements usually means a served error page (a 404/500 still loads) or the wrong URL — confirm the page renders what you expect before trusting score: 100.

Output — compact, AI-first

stdout is a distilled report (--full keeps the faithful one). Shape:

• top — url, score (0–100), elements[], defects[]; plus, when present: hints (one fix per defect type present), audit (present only for a whole-page run: wholePage (always true), discovered, capped), transitions[] (non-smooth ones plus any with a known easing curve), a smoothTransitions count, and elementsOmitted/defectsOmitted (how many entries the worst-first caps dropped — surfaced so a cap is never a silent truncation).
• element — selector, label (role "name", e.g. nav "Main", else the selector), testid (or null), source (matched=auto-detected | affected=moved by one), impact (paints/layout), anchoredTo (screen/page/an ancestor selector/null), anchoredEdges (which edges stayed constant — or []), to (the settled box as [left, top, width, height] in page coords); plus from (the start box) when the element moved/resized, and affectedBy when present. (--full carries the raw start/end box in both screen and page coords under bounds.)
• defect — type (layout-shift/flicker/jank/dithering), selector, label, severity (0–1), window [start,end] ms, detail, metrics (the decision-relevant numbers per type: value for layout-shift; toggleCount+frequencyHz for flicker; droppedFrames+maxJumpPx for jank; noiseEnergy for dithering); plus count when >1.
• transition — selector, label, kind (move/resize/fade/color/composite), smoothness (smooth/janky/flicker/shift), quality (0–1); plus easing (linear/ease-in/ease-out/ease-in-out) when determinable.

Coalesced by type+selector, numbers rounded, fix hints hoisted to one entry per type — much smaller than the raw record. Worked example — examples/sample-report.json: score 92 (one layout-shift); footer returned source:"affected", affectedBy:["va-block"], fixed via the matching hints entry.

Test a UI change — the visual-regression gate

Run this as the final review step once your UI change is complete — once, at the end, not after every edit. There is no cross-run diffing built in, so baseline first to tell your defects from pre-existing ones:

1. Baseline the unchanged page: bun src/analyze-cli.ts <url> > before.json. Apply your change, run again to after.json. Defects in after but not before are yours. (A stable data-testid in your markup is reused as the element's testid, so your changed element is easy to find.)
2. Read after.json (compact JSON on stdout; a health line on stderr):
   • score — starts at 100; each defect type subtracts weight × min(Σ its severities, 1), capped per type: layout-shift −45, jank −30, flicker −25, dithering −12. Any reported defect forces score ≤ 99, so score === 100 is the only provably-clean result. There is no built-in fail threshold — pick your own, and prioritize by defect type+severity, not the score alone.
   • defects[] — what's broken, each with severity, window, metrics; the per-type fix is in top-level hints. Act on these first.
   • source:"affected" + affectedBy — causality. Every element your change moved comes back attributed to it; an absolutely/fixed-positioned element that did not actually move is correctly left out. This answers "what did my change shift?".
   • anchoredTo/anchoredEdges — is it held where you intended (screen/page/ancestor)?
   • to/from — where it ended up and (when it shifted) where it started, each [left, top, width, height] in page coords.
   • empty elements[] — not "clean"; see Exit codes & failure modes.
3. Blast radius of one element — find it in elements[] (by label/testid/selector), then scan for source:"affected" entries whose affectedBy lists it.

Done when score is 100 (≡ zero defects[]). Below 100 you are not done until every defect and every source:"affected" element traces to an intended change — one you can't attribute to your change is a regression; fix it, don't stop.

How it works

1. Auto-detect & tag the page's relevant elements, each with a stable data-testid (it pins one node through mutations) and a role "name" label — see Auto-detection.
2. Instrument — Mutation / Resize / Intersection / Performance(layout-shift) observers plus an rAF geometry/opacity/colour sampler; the driver captures pixels.
3. Segment on SPA nav (pushState/replaceState + a view-changing popstate; a #hash-only change never segments) and on reloads (recording persists via sessionStorage, so MPA accumulates).
4. Filter — scope (tracked and affected) → seen (entered viewport) → had visual impact.
5. Anchor and score each survivor.

Auto-detection

The instrument finds the page's developer-recognizable component roots itself — no selectors:

• Event boundaries (listeners.ts) — it wraps addEventListener before the page's own scripts run to record which elements get interaction handlers, a strong "this is a component" signal.
• A scored selection at settle (select.ts) — candidates from cheap signals (landmarks & roles, headings, interactive elements, the boundaries above, media, component-name hints like hero/card/nav, repeated list/grid items), then one batched layout pass scores each, collapses wrappers to the component root, and keeps the top set within a budget.
• Media + activity — replaced media (canvas/video/img) is always seeded, and the observers add anything that becomes active at runtime. Coverage is in the audit block (discovered, capped) — never a silent truncation. Each element is labeled by its ARIA role + accessible name (accname.ts), falling back to a CSS path. A decorative static element with no role, interactivity, media, or name hint carries no signal, so it may not be detected — give it a role/recognizable class/data-* if you need it.

Scope boundaries

What the instrument observes, and where it intentionally stops (these are scope, not bugs — it never crashes or fabricates a defect at any of them):

• Light DOM only — elements inside open or closed shadow roots aren't discovered (their CSS animations don't even surface — animationstart/transitionrun are composed:false). A page whose entire UI lives in a shadow root audits to discovered:0. Light-DOM effects of shadow content are still caught (a shadow subtree that grows and pushes a light-DOM sibling shows up as that sibling's layout-shift). The same holds for the UA shadow DOM of native controls — an <input type=range> thumb, <progress>/<meter> fill, or <select> dropdown lives in UA-internal pseudo-content, so its motion/fill is invisible; the control's own host box (a <textarea> auto-growing, <details> expanding) is fully tracked.
• Top document only — the instrument runs in the main frame, so an iframe's internal changes are opaque. The <iframe> element's own box (move/resize) and any CLS it causes in the parent are caught.
• Top layer is honored — <dialog>/popover/::backdrop occlusion is hit-tested via elementFromPoint, so a top-layer cover correctly occludes content even with a lower z-index. A modal ::backdrop makes the whole viewport read as occluded.
• Late reveal isn't back-dated — an off-screen content-visibility:auto (or :hidden) subtree is discovered only when first rendered, so its pre-reveal state isn't sampled (the general temporal/discovery boundary). The reveal's effect on neighbours (CLS) is still caught.
• Paint is style-gated, not 3D-projected — visibility/display/opacity/occlusion gate paint, not the computed transform matrix, so an element rotated past 90° with backface-visibility:hidden still counts as painting.
• Pseudo-elements aren't tracked — ::before/::after/::marker/::view-transition* are not DOM nodes, so a change confined to a pseudo-element isn't characterized (a ::before badge animating; a View Transitions cross-fade, which the browser renders in ::view-transition snapshots). The host element's own box/paint/visibility deltas still are — including the real before→after jump a view transition produces on the underlying element.

Invariants

• Impact is temporal — unioned over frames; the display:none counterfactual only confirms.- Two impact modes — paints (occlusion-gated) and affects layout, independent.
• Affected-by is causal — same-frame co-movement, confirmed by layout-shift sources and the counterfactual.- Motion ≠ scroll ≠ resize — a move translates the whole box in both coordinate spaces; a size change shifts one edge of a pair.- Each defect from the source that sees it — layout-shift from the API (CLS never recomputed), flicker from opacity, jank from frame timing, dithering from pixel noise.
• pixelThreshold governs every constant-vs-changed test, so results are deterministic.
• No any/as/unknown — boundary parsing uses guards in recording.ts.

Embed & test

import { buildInstrumentBundle } from './src/bundle';
import { analyzeSession } from './src/driver'; // any Playwright-shaped page works
const report = await analyzeSession(page, {
  url,
  instrumentBundle: await buildInstrumentBundle(),
  capturePixels: true, // dithering + the paint counterfactual — the CLI sets this; omit and you lose both
  // interact: async (p) => {/* log in / open a modal before sampling */},
  // keyframes: 4, settleMs: 500, pixelThreshold,  // session knobs the CLI fixes
});

bun test (unit) · bun run typecheck (strict) · bun run e2e (real Chromium, skips without a browser).

Companion files

• examples/run.md — CLI, embedding, and the e2e fixtures.

Implementation deep-dives live next to the code they document: src/impact-detection.md (impact + affected-by internals) and src/defects/transition-defects.md (defect algorithms and report schema).