Chrome DevTools frontend reviewer skill: ui-and-widgets

This skill provides review guidance for Chrome DevTools frontend changes matching the ui-and-widgets path group (^front_end/(ui|panels)/.*.(ts|css)$). It focuses specifically on code review verification for UI and widget changes, complementing authoring guidance in ui-widgets and docs/ui_engineering.md, and integrating with the general review checklist in code-review. Every rule below recurred in at least 5 CLs from 2 distinct reviewers in historical reviews; cite the rule id in findings.

Path group ui-and-widgets (17 rules)

UI_MODERN_COMPONENT_AND_ENCAPSULATION_STANDARDS: Use modern DevTools UI primitives (devtools-button, devtools-data-grid, devtools-icon), #private fields, and module entrypoint imports

• Severity: WARNING
• Why: Legacy UI.Fragment, DataGridImpl, LitElement, new Icon(), and TypeScript private fields are superseded by standard <devtools-*> custom elements (imported via module entrypoints such as import * as Buttons from '../../ui/components/buttons/buttons.js' so custom tags register at runtime) and ECMAScript #private fields.
• Look for: UI.Fragment.build, legacy DataGridImpl, extends LitElement, private keyword instead of # in new UI classes, or <devtools-*> tags in Lit templates without importing the corresponding component module entrypoint (e.g. import * as Buttons from '../../ui/components/buttons/buttons.js').
• Do not flag: Overriding inherited protected lifecycle methods on framework base classes (e.g. extending UI.Widget.Widget).

UI_TYPE_SAFETY_AND_EXHAUSTIVENESS: Model multi-state UI workflows with discriminated unions, Platform.assertNever exhaustiveness checks, and strict domain types

• Severity: WARNING
• Why: Representing mutually exclusive UI states with loose optional boolean flags or any types permits impossible states and silently misses new enum variants. Discriminated unions paired with Platform.assertNever in default: branches enforce compile-time exhaustiveness.
• Look for: switch statements over union/enum types missing Platform.assertNever(x) in default:, any or declare field modifiers in UI classes, or multiple boolean flags (isLoading, isError, isEmpty) representing mutually exclusive states.
• Do not flag: Switch statements that intentionally handle only a small subset of a large external protocol enum.

UI_PURE_VIEW_AND_PRESENTER_SEPARATION: Keep UI.Widget view functions pure and stateless by computing presentation data in presenters, passing decoupled ViewInput data structures, and preserving helper topology

• Severity: WARNING
• Why: In DevTools' MVP architecture, DEFAULT_VIEW functions must be pure projections of ViewInput to Lit templates. Querying SDK models, reading global settings, mutating presenter state, or calling host/clipboard APIs inside the view couples rendering to global state and breaks isolated view testing. Additionally, deleting existing standalone/module-level DOM helper functions or async class update helpers and inlining them into performUpdate() or DEFAULT_VIEW during Lit migrations unnecessarily flattens the abstraction topology and inflates diffs.
• Look for: DEFAULT_VIEW or view helper functions calling Common.Settings.Settings.instance(), SDK.*, Host.*, mutating input.* arrays in place, accepting raw SDK/Protocol domain objects instead of decoupled presentation interfaces on ViewInput, or Lit migrations that delete existing standalone/class DOM helper functions and inline their logic into performUpdate() or DEFAULT_VIEW instead of converting them in place to return LitTemplate (or Promise<LitTemplate|null>).
• Do not flag: Legacy UI.Widget subclasses that have not yet migrated to a DEFAULT_VIEW function.

UI_LIT_DECLARATIVE_TEMPLATES_AND_DIRECTIVES: Use Lit.nothing, repeat with keys, classMap/styleMap, and ifDefined in declarative templates while preserving modular helper functions

• Severity: WARNING
• Why: Mutating DOM elements via appendChild, classList.toggle, or innerHTML alongside Lit templates, or returning ''/null instead of Lit.nothing in conditional branches, corrupts Lit's part tracking and creates unnecessary text nodes or wrapper <div>s. Conversely, collapsing existing standalone DOM helper functions (e.g., linkifyIcon, maybeCreateLinkToElementsPanel) or class rendering helpers into a single monolithic DEFAULT_VIEW template harms readability and reviewability compared to converting those helpers in place to return LitTemplate.
• Look for: Ternary branches returning '' or undefined in html templates instead of Lit.nothing, unkeyed .map() on dynamic stateful lists instead of repeat(), ifDefined omissions on optional attributes, multiple Lit.render() calls mutating the same container, or Lit migrations that flatten/inline existing DOM helper functions and class helper methods instead of preserving them in their original locations returning LitTemplate.
• Do not flag: Static arrays of primitive labels where item identity never reorders.

UI_WIDGET_DOM_AND_LISTENER_LIFECYCLE: Attach listeners to external models and managers in wasShown and detach them symmetrically in willHide

• Severity: ERROR
• Why: Subscribing to long-lived SDK TargetManager, Settings, or model events in a widget's constructor without unsubscribing in willHide() (or onDetach() / onunbind()) causes hidden panels to process background updates and leaks detached widget instances.
• Look for: addEventListener, addModelListener, or addChangeListener on singletons, models, or settings inside UI.Widget constructors or wasShown() without a matching remove* call in willHide() using the same function reference.
• Do not flag: Listeners attached to the widget's own internal DOM elements (this.contentElement), which are garbage-collected together with the widget.

UI_ACCESSIBILITY_AND_SEMANTIC_HTML: Use native semantic HTML controls with accurate ARIA attributes, keyboard handlers, and !event.isComposing guards

• Severity: WARNING
• Why: Clickable <div>/<span> elements and nested interactive controls (<button> inside <a> or <button>) break keyboard navigation and screen readers. Interactive elements require semantic tags (<button>, <details>/<summary>, <thead>/<tbody>), synchronized ARIA state (aria-expanded, aria-details for rich tooltips vs aria-describedby for plain text), and !event.isComposing checks on Enter keydown handlers.
• Look for: @click on <div>/<span> without button semantics, <button> nested inside <button>/<x-link>, Enter keydown handlers missing !event.isComposing, or aria-describedby pointing to interactive <devtools-tooltip> elements.
• Do not flag: Delegating click listeners on container elements that handle bubbled events from child buttons.

UI_PERFORMANCE_AND_FLAMECHART_RENDERING: Precompute expensive metrics outside render loops, use Set/Map for O(1) lookups, and debounce high-frequency UI updates

• Severity: WARNING
• Why: Performing linear Array.includes/find scans, sorting, or regex parsing inside FlameChart canvas draw loops, ElementsTreeOutline mutation callbacks, or Lit render passes causes frame drops and UI jank on large traces and DOM trees.
• Look for: .includes() / .find() / .sort() inside canvas draw or event iteration loops, repeated ResizeObserver registrations on individual child items instead of the parent container, or un-debounced filter/mutation handlers.
• Do not flag: One-time initialization passes over bounded small arrays (less than 20 items).

UI_LOCALIZATION_AND_UISTRINGS_HYGIENE: Define user-facing strings in sentence-case UIStrings using complete sentences and i18nTemplate for embedded elements, and prune unused entries

• Severity: WARNING
• Why: Concatenating translated fragments breaks word order in non-English locales, title-Cased strings violate Chromium UX standards, translating English-only experimental strings wastes localization quota, and unused UIStrings entries bloat locale bundles.
• Look for: String concatenation around i18nString(...), HTML/DOM elements interpolated without i18nTemplate, Title Case labels in UIStrings, unlocalized English literals in templates, or unused keys in UIStrings.
• Do not flag: Technical protocol identifiers, MIME types, or strings in UIStringsNotTranslate / i18n.i18n.lockedString(...).

UI_REACTIVE_WIDGET_UPDATE_LIFECYCLE: Schedule widget re-renders asynchronously via this.requestUpdate() and guard property setters with equality checks

• Severity: WARNING
• Why: Calling this.performUpdate() or invoking this.#view(...) synchronously inside property setters causes redundant synchronous DOM renders when multiple properties change in the same tick. Setters should short-circuit if the value is unchanged and call this.requestUpdate().
• Look for: Widget property setters calling this.performUpdate() or this.#view(...) directly, or setters calling this.requestUpdate() without checking if (this.#prop === value) return;.
• Do not flag: Internal test helpers or synchronous initialization paths explicitly documented as requiring immediate flush.

UI_VISUAL_LOGGING_INSTRUMENTATION: Instrument interactive UI elements and containers with kebab-case VisualLogging contexts registered in KnownContextValues

• Severity: WARNING
• Why: DevTools relies on Visual Elements (jslog) telemetry to measure feature usage. Omitting jslog on new buttons/toggles/menus, using localized or camelCase context strings, adding redundant panel prefixes inside already-logged containers, or dropping jslog during Lit refactors breaks analytics.
• Look for: Interactive elements (<button>, <devtools-button>, <input>, menus, drawers) missing jslog or jslogContext, non-kebab-case context names, or new context values missing from KnownContextValues.ts.
• Do not flag: Purely decorative icons or internal wrapper <div>s that represent neither a distinct visual section nor an interactive control.

UI_CSS_SCOPING_AND_FLEX_LAYOUT: Scope light-DOM stylesheets and align elements using flexbox or grid rather than competing margin and position offsets

• Severity: INFO
• Why: Unscoped class or tag selectors in light-DOM widget stylesheets leak across the DevTools document, while combining vertical-align, negative margins, and position: relative offsets creates brittle layouts that break under font scaling or zoom.
• Look for: Unscoped generic selectors in light-DOM .css files (or :host inside @scope blocks instead of :scope), or CSS rules combining vertical-align, negative margin-top, and pixel offsets to center icons and buttons.
• Do not flag: Shadow DOM component stylesheets where :host scoping is native.

UI_VIEW_OUTPUT_AND_CALLBACK_CONTRACTS: Encapsulate imperative DOM actions in ViewOutput callbacks and pass user actions via semantic ViewInput callbacks instead of raw DOM references

• Severity: WARNING
• Why: Exposing raw HTMLElement references on ViewOutput or passing raw MouseEvent/KeyboardEvent objects to presenter callbacks leaks DOM details into presenters. Views should populate narrow functional closures on ViewOutput (e.g. output.focusInput = () => ...) and unpack event values before calling input.onSelect(value).
• Look for: ViewOutput interfaces containing HTMLElement or Element properties, or ViewInput callbacks typed as (event: Event) => void instead of semantic domain values.
• Do not flag: Third-party editor or canvas host containers that require a raw mount HTMLElement.

UI_PANEL_AND_META_REGISTRATION: Follow DevTools panel directory conventions and register settings and views in the appropriate *-meta.ts scope

• Severity: WARNING
• Why: Panel-level UI.Widget views belong directly under front_end/panels/<panel>/ (reserving components/ for reusable leaf components), internal non-user-facing settings must use SettingCategory.NONE or model descriptors rather than appearing in Settings UI, and shared SDK settings belong in sdk-meta.ts.
• Look for: Panel-level UI.Widget views placed inside panels/<panel>/components/, internal state flags registered with user-facing titles in *-meta.ts, or loadView callbacks calling .instance() instead of using the passed Universe context.
• Do not flag: Standalone reusable leaf components inside components/ or genuine user-configurable preferences.

UI_WEAK_DOM_REFERENCES_AND_OBSERVER_CLEANUP: Store DOM node metadata in WeakMap or WeakRef collections and clean up observers when elements detach

• Severity: WARNING
• Why: Retaining DOM Element or SDK.Target references in strong Map/Set collections (or in WeakMap values that hold strong references back to their keys) prevents detached DOM trees from being garbage-collected, while leaving ResizeObserver, MutationObserver, or Lit.Directives.ref callbacks active after unmount leaks memory.
• Look for: Strong Map<Element, ...> / Set<Element> collections holding DOM/model associations, WeakMap values retaining strong references back to their DOM keys, ResizeObserver/IntersectionObserver without .unobserve()/.disconnect() on teardown, or ref(el => ...) callbacks that do not handle el === undefined.
• Do not flag: Module-level WeakMap<Element, ...> globals whose values do not strongly reference the key, or short-lived local maps created and discarded within a single synchronous function call.

UI_SECURITY_AND_OUTPUT_SANITIZATION: Sanitize untrusted strings, URLs, CSS selectors, and Markdown exports before rendering or exporting

• Severity: ERROR
• Why: Interpolating untrusted network URLs, DOM attributes, or user strings into raw HTML snippets, CSS.escape-free attribute selectors, Markdown table exports, or href attributes without protocol validation risks injection, XSS, broken layout formatting, or text spoofing.
• Look for: Raw string interpolation in querySelector('[attr="' + val + '"]') without CSS.escape, href attribute bindings with unvalidated javascript: URLs, Markdown table cell serialization without escaping |*_<>\, or raw HTML snippet generation without HTML escaping.
• Do not flag: Standard Lit html text bindings (${text}), which escape HTML entities automatically.

UI_DECLARATIVE_CHILD_WIDGET_COMPOSITION: Compose child UI.Widget instances declaratively in Lit templates using the two-argument widget(ChildWidget, {...}) directive instead of imperative show() calls or single-argument factory callbacks

• Severity: WARNING
• Why: Imperatively instantiating child widgets and calling child.show(container) inside presenters or views bypasses Lit reconciliation, complicates lifecycle management, and recreates widgets unnecessarily across renders. Similarly, single-argument factory callbacks widget(element => ...) only run on initial creation and fail to propagate updated properties on subsequent re-renders; strongly prefer the two-argument widget(ChildWidget, {...}) form (converting setFoo() methods on the child widget into set foo() setters if needed).
• Look for: new ChildWidget() followed by .show(...) or .appendChild(child.element) inside Lit view functions or migrated performUpdate() methods, or widget(element => ...) factory callbacks configuring child widget properties imperatively instead of <devtools-widget ${widget(ChildWidget, {...})}></devtools-widget> (or ${widget(ChildWidget, {...})}).
• Do not flag: Top-level legacy split widgets or tabbed panes that have not migrated to Lit views.

UI_AWAIT_UPDATE_COMPLETE_POST_RENDER: Await this.updateComplete before executing post-render DOM measurements, focus changes, or scroll adjustments

• Severity: ERROR
• Why: Because this.requestUpdate() schedules rendering asynchronously in a microtask, querying DOM geometry, focusing newly rendered controls, or anchoring popovers immediately after requestUpdate() operates on stale pre-render DOM state.
• Look for: this.requestUpdate() followed synchronously by .focus(), .getBoundingClientRect(), .scrollIntoView(), or child widget measurement without await this.updateComplete.
• Do not flag: State updates that do not interact with rendered DOM nodes after scheduling the update.