Browser Verify UI Changes

Browser-verify UI changes before marking done

This is the single most consequential discipline lesson from canvas-heavy UI work with an agent. It was learned the hard way after an agent shipped a broken chart indicator three times in a row with all gates green:

1. Type-check passed
2. Biome / ESLint passed
3. Unit tests passed
4. e2e (Playwright) passed
5. The chart was still wrong

The bugs only manifested at paint time:

• A chart lib's isWhitespace=true stripping rows from data without warning
• NaN sentinels for off-screen positions silently dropping draws
• Array-index misalignment between primary and overlay series

Synthetic tests literally cannot catch these. The fix has to be at the same layer the bug manifests: read the actual rendered pixels.

When to use

Reach for this skill for every task that:

• Changes a chart, overlay, indicator, or any visible chart element
• Changes a layout, toolbar button, popover, menu, page
• Changes a route or URL behavior the operator interacts with
• Adds a visual state (active, hover, disabled, loading, empty)

Reach for it ESPECIALLY when:

• The task seems "simple enough" to skip verification (this is the trap)
• You've already burned an iteration on a similar bug
• The reference is a screenshot or an existing tool you're matching

The pattern

Treat the edit and the visual verification as one atomic unit. The moment you think "I'm done" is the moment you MUST screenshot and diff before saying so.

// After every UI-affecting commit/edit, run via chrome-devtools MCP:

await mcp__chrome-devtools__navigate_page({ type: "reload" })
await mcp__chrome-devtools__take_screenshot()
// Compare to the operator's reference at the PIXEL LEVEL:
//   - shape, position, color, breaks, alignment
// If anything is off, the task is NOT done.

// For chart state that doesn't show clearly in a screenshot, use
// evaluate_script to interrogate the chart's actual data:
await mcp__chrome-devtools__evaluate_script({
  function: `() => {
    const hook = window.__myDebugHook;
    const series = hook.chart.panes()[0].getSeries()[0];
    return { data: series.data(), options: series.options() };
  }`
})

If browser access is unavailable in the session, the task is in_progress and you say so explicitly — NEVER completed on assumed-correctness.

Why synthetic tests fall short

The agentic-loop rule

Codify in your project's CLAUDE.md (or equivalent) so the agent reads it before every UI task:

**No browser-affecting task is "done" until verified in the browser via
chrome-devtools MCP.** For any change with a user-visible outcome (chart,
UI, route, overlay, popover, etc.), reload the page, exercise the
specific behavior end-to-end through the live app (click the button,
drag the chart, toggle the setting), and screenshot or programmatically
confirm the expected state. tsc + biome + unit + e2e passing is
necessary but not sufficient — synthetic tests don't catch what only
manifests at paint time. Acceptable verification = a chrome-devtools
take_screenshot, take_snapshot, or evaluate_script inspecting actual
chart state. If browser access is unavailable in the session, mark the
task in_progress and say so explicitly rather than completing on
assumed-correctness.

Gotchas

• The rule loaded in context is not a checklist. When this rule first lands, the agent often ignores it on the NEXT iteration because "context I've read" ≠ "checklist I run before declaring done." Make verification a forcing-function step in the task workflow, not just a documented rule.
• "It looks right" isn't verification. Compare against the explicit reference. The operator's screenshot, the Pine source render, the previous-version screenshot — there's always a reference.
• Screenshots aren't always enough. Some bugs (off-by-one OHLC, wrong volume, hidden state) show up in series.data() but not in pixels. Use evaluate_script for those.
• Don't claim "verified" if the dev server might be stale. Force a navigate-reload before screenshotting.
• Browser-verification doesn't replace tests. It catches paint-time bugs the tests can't. Keep the tests; add the verification.

Reference implementation

The rule above was added to a project's CLAUDE.md mid-session after an indicator port shipped wrong three times in a row despite green CI. The exact incident — a "trend fill" implementation that recolored the wrong layer, then drew the fill at the wrong y-range, then connected lines across session gaps — was each time invisible to type-check + unit + e2e, and each time obvious in a screenshot.

The lesson was written up as a long-form memory note feedback_browser_verify_before_done.md in the project's memory store, then codified as a numbered hard rule (#9) in CLAUDE.md so future agent sessions load it as context.

Related skills

• lightweight-charts-integration — the chart-specific paint-time bugs that escape synthetic tests
• react-canvas-race-conditions — another class of bug invisible to unit tests
• read-reference-source-first — its companion discipline ("don't guess; read the source")