Glossary

Glossary

Domain terms used across the extension UI, docs, and discussions. If you've ever wondered "what does that label mean?", check here.

React internals

Fiber A unit of work in React's reconciler. Every React element (component instance, DOM element, text node, etc.) has a corresponding fiber. The fiber tree mirrors the component tree but with extra metadata: pending props/state, effect lists, lanes, flags. Our extension walks this tree to detect renders.

Commit / commit phase The moment React applies pending changes to the actual DOM. onCommitFiberRoot fires once per commit. Multiple setState calls in one event handler are usually batched into a single commit.

Render / render phase The phase where React calls your component function to compute the new output. Distinct from "commit" — a component can render without committing (e.g., concurrent renders that get discarded).

fiber.alternate React maintains two fibers per element: the current one and the work-in-progress one. After a commit, they swap. The "alternate" is what was current before this commit — we use it to compute "what changed?". Watch out: after the next commit, your saved reference points to stale data.

PerformedWork flag (0x01) A bit React sets on a fiber when it actually executed the render function. The most reliable signal that "yes, this component re-rendered" — used by bippy and adopted in our v2.0.3.

Render reason Why a component re-rendered. Three categories: props changed, state changed, context changed. Our Performance tab surfaces this per render.

StrictMode A React debugging tool that intentionally double-mounts components in development to surface bugs caused by impure rendering or missing effect cleanup. Has no effect in production builds.

Performance

LCP (Largest Contentful Paint) Core Web Vital. Time from page navigation to the largest element in the viewport finishing render. Target: <2.5s. Surfaced in our Performance tab.

FCP (First Contentful Paint) Core Web Vital. Time to first DOM content render. Target: <1.8s.

TTFB (Time to First Byte) Time from navigation to first response byte. Target: <800ms.

INP (Interaction to Next Paint) Core Web Vital that replaced FID in 2024. Worst observed input-response latency during the page's lifetime. Target: <200ms. Coming to our Performance tab in v2.1.

FID (First Input Delay) Deprecated Core Web Vital (replaced by INP), but still measured for backward compat. Time from first user interaction to the browser processing it.

CLS (Cumulative Layout Shift) Core Web Vital. Sum of all unexpected layout shifts during page lifetime. Target: <0.1. We have a dedicated CLS tab.

Jank Perceptible lag in the UI. Usually caused by long synchronous JavaScript work blocking the main thread. Our v2.0.3 release was specifically about eliminating jank caused by the extension itself.

Extension architecture

Page world The JavaScript realm of the inspected page. Our inject script runs here to access React internals. Cannot directly call chrome.* APIs.

Content script A script that runs in an isolated JavaScript realm but with access to the page's DOM. Runs at document_start for us. Bridges the page world ↔ the extension.

Service worker (SW) The background script in Manifest V3. Routes messages, holds short-lived state. Dies after 30 seconds of idle.

DevTools panel The React UI we render in the DevTools strip. Has access to chrome.devtools.* APIs. Lives as long as DevTools is open.

Manifest V3 (MV3) The current Chrome extension API version. Replaces persistent background pages with ephemeral service workers. Stricter CSP. Required for new extensions since 2024.

State management

Redux store The single source of truth for state in Redux apps. We probe for it via __REDUX_DEVTOOLS_EXTENSION__, the same hook the official Redux DevTools uses.

Selector A function that extracts a slice of Redux state. useSelector(state => state.user.name). Selectors should return referentially-stable values when nothing relevant changed — otherwise components re-render unnecessarily.

shallowEqual A comparison function from react-redux that checks if two objects have the same top-level keys with the same values. Used as the second arg to useSelector to prevent re-renders on irrelevant state changes.

createSelector A memoization wrapper from reselect (and now Redux Toolkit). Caches selector output keyed by input identity.

Time-travel debugging Replaying past Redux actions to see how state evolved. Planned for our Redux tab in v2.2.

AI Analysis

Snapshot The bundle of data we send to the AI for analysis. Built by snapshot-builder.ts. Includes component render counts, error patterns, Redux state structure — but NOT raw component source code.

Token optimizer Logic in token-optimizer.ts that trims the snapshot to fit within LLM context limits while preserving signal.

Subscription key A user-provided key that unlocks unlimited AI Analysis. Validated server-side by our Cloudflare Worker against a SHA-256 hash list — we never store the raw key.

Free tier 3 AI Analysis calls per 5-minute window without a subscription key.

Process / project

OpenSpec change A multi-file design proposal under openspec/changes/<name>/. Required for non-trivial architectural changes per our HARNESS.md. Includes proposal.md, design.md, tasks.md, and specs/ slices.

Sisyphus plan A maintainer-facing implementation plan under .sisyphus/plans/ (gitignored). Tracks work in progress. The "boulder" file points at the active plan.

Lane / change type Risk classification per the harness: tiny, normal, or high-risk lane × user-feature, bug-fix, infra, refactor, docs, or dependency-bump type. Determines required validation + review.

Review Gate A required step in the harness: a fresh review agent must verify each acceptance criterion before a change can be archived. Bypassable only for tiny docs lane changes.

MCP (Model Context Protocol) Anthropic's open standard for LLM-tool communication. Coming to React Debugger in v2.1 — see openspec/changes/mcp-server-v1/.

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