Chat Customizations Editor

When changing harness descriptor interfaces or factory functions, verify both core and sessions registrations compile.

Key Interfaces

• IHarnessDescriptor — drives all UI behavior declaratively (hidden sections, button overrides, file filters, agent gating). See spec for full field reference.
• ISectionOverride — per-section button customization (command invocation, root file creation, type labels, file extensions).
• IStorageSourceFilter — controls which storage sources and user roots are visible per harness/type.
• IExternalCustomizationItemProvider / IExternalCustomizationItem — internal interfaces (in customizationHarnessService.ts) for extension-contributed providers that supply items directly. These mirror the proposed extension API types.

Principle: the UI widgets read everything from the descriptor — no harness-specific conditionals in widget code.

Extension API (chatSessionCustomizationProvider)

The proposed API in src/vscode-dts/vscode.proposed.chatSessionCustomizationProvider.d.ts lets extensions register customization providers. Changes to IExternalCustomizationItem or IExternalCustomizationItemProvider must be kept in sync across the full chain:

When adding fields to IExternalCustomizationItem, update all five layers. The proposed API .d.ts is additive-only (new optional fields are backward-compatible and do not require a version bump).

Testing

Component explorer fixtures (see component-fixtures skill): aiCustomizationListWidget.fixture.ts, aiCustomizationManagementEditor.fixture.ts under src/vs/workbench/test/browser/componentFixtures/.

Screenshotting specific tabs

The management editor fixture supports a selectedSection option to render any tab. Each tab has Dark/Light variants auto-generated by defineThemedFixtureGroup.

Available fixture IDs (use with mcp_component-exp_screenshot):

Adding a new tab fixture: Add a variant to the defineThemedFixtureGroup in aiCustomizationManagementEditor.fixture.ts:

MyNewTab: defineComponentFixture({
    labels: { kind: 'screenshot' },
    render: ctx => renderEditor(ctx, {
        harness: CustomizationHarness.VSCode,
        selectedSection: AICustomizationManagementSection.MySection,
    }),
}),

The selectedSection calls editor.selectSectionById() after setInput, which navigates to the specified tab and re-layouts.

Populating test data

Each customization type requires its own mock path in createMockPromptsService:

• Agents — getCustomAgents() returns agent objects
• Skills — findAgentSkills() returns IAgentSkill[]
• Prompts — getPromptSlashCommands() returns IChatPromptSlashCommand[]
• Instructions/Hooks — listPromptFiles() filtered by PromptsType
• MCP Servers — mcpWorkspaceServers/mcpUserServers arrays passed to IMcpWorkbenchService mock
• Plugins — IPluginMarketplaceService.installedPlugins and IAgentPluginService.plugins observables

All test data lives in allFiles (prompt-based items) and the mcpWorkspace/UserServers arrays. Add enough items per category (8+) to invoke scrolling.

Exercising built-in grouping

The list widget regroups items from the default chat extension under a "Built-in" header. Three things must be in place for fixtures to exercise this:

1. Include BUILTIN_STORAGE in the harness descriptor's visible sources
2. Mock IProductService.defaultChatAgent.chatExtensionId (e.g., 'GitHub.copilot-chat')
3. Give mock items extension provenance via extensionId / extensionDisplayName matching that ID

Without all three, built-in regrouping silently doesn't run and the fixture only shows flat lists.

Editor contribution service mocks

The management editor embeds a CodeEditorWidget. Electron-side editor contributions (e.g., AgentFeedbackEditorWidgetContribution) are instantiated automatically and crash if their injected services aren't registered. The fixture must mock at minimum:

• IAgentFeedbackService — needs onDidChangeFeedback, onDidChangeNavigation as Event.None
• ICodeReviewService — needs getReviewState() / getPRReviewState() returning idle observables
• IChatEditingService — needs editingSessionsObs as empty observable
• IAgentSessionsService — needs model.sessions as empty array

These are cross-layer imports from vs/sessions/ — use // eslint-disable-next-line local/code-import-patterns on the import lines.

CI regression gates

Key fixtures have blocksCi: true in their labels. The screenshot-test.yml GitHub Action captures screenshots on every PR to main and fails the CI status check if any blocks-ci-labeled fixture's screenshot changes. This catches layout regressions automatically.

Currently gated fixtures: LocalHarness, McpServersTab, McpServersTabNarrow, AgentsTabNarrow. When adding a new section or layout-critical fixture, add blocksCi: true:

MyFixture: defineComponentFixture({
    labels: { kind: 'screenshot', blocksCi: true },
    render: ctx => renderEditor(ctx, { ... }),
}),

Don't add blocksCi to every fixture — only ones that cover critical layout paths (default view, section with list + footer, narrow viewport). Too many gated fixtures creates noisy CI.

Screenshot stability

Scrollbar fade transitions cause screenshot instability — the scrollbar shifts from visible to invisible fade class ~2 seconds after a programmatic scroll. After calling revealLastItem() or any scroll action, wait for the transition to complete before the fixture's render promise resolves:

await new Promise(resolve => setTimeout(resolve, 2400));
// Then optionally poll until .scrollbar.vertical loses the 'visible' class

Running unit tests

./scripts/test.sh --grep "applyStorageSourceFilter|customizationCounts"
npm run compile-check-ts-native && npm run valid-layers-check

See the sessions skill for sessions-window specific guidance.

Debugging Layout in the Real Product

Component fixtures use mock data and a fixed container size. Layout bugs caused by reflow timing, real data shapes, or narrow window sizes often don't reproduce in fixtures. When a user reports a broken layout, debug in the live Code OSS product.

For launching Code OSS with CDP and connecting agent-browser, see the launch skill. Use --user-data-dir /tmp/code-oss-debug to avoid colliding with an already-running instance from another worktree.

Navigating to the customizations editor

After connecting, use snapshot -i to find the "Open Customizations" button (in the Chat panel header), then click it. To switch sections, use eval with a DOM click since sidebar items aren't interactive refs:

npx agent-browser eval "const items = [...document.querySelectorAll('.section-list-item')]; \
  items.find(el => el.textContent?.includes('MCP'))?.click();"

Inspecting widget layout

agent-browser eval doesn't always print return values. Use document.title as a return channel:

npx agent-browser eval "const w = document.querySelector('.mcp-list-widget'); \
  const lc = w?.querySelector('.mcp-list-container'); \
  const rows = lc?.querySelectorAll('.monaco-list-row'); \
  document.title = 'DBG:rows=' + (rows?.length ?? -1) \
    + ',listH=' + (lc?.offsetHeight ?? -1) \
    + ',seStH=' + (lc?.querySelector('.monaco-scrollable-element')?.style?.height ?? '') \
    + ',wH=' + (w?.offsetHeight ?? -1);"
npx agent-browser eval "document.title" 2>&1

Key diagnostics:

• rows — fewer than expected means list.layout() never received the correct viewport height.
• seStH — empty means the list was never properly laid out.
• listH vs wH — list container height should be widget height minus search bar minus footer.

Common layout issues

Fixture vs real product gaps

Fixtures render at a fixed size (default 900×600) with many mock items. They won't catch:

• Reflow timing — the real product's display:none → visible transition may not have reflowed before layout() fires
• Narrow windows — add narrow fixture variants (e.g., width: 550, height: 400)
• Real data counts — a user with 1 MCP server sees very different layout than a fixture with 12