---
name: component-fixtures
description: Use when creating or updating component fixtures for screenshot testing or their shared infrastructure, or when designing UI components to be fixture-friendly. Covers fixture file structure, theming, service setup, CSS scoping, async rendering, validation, and common pitfalls.
---
# Component Fixtures
Component fixtures render isolated UI components for visual screenshot testing via the component explorer. Fixtures live in `src/vs/workbench/test/browser/componentFixtures/` and are auto-discovered by the Vite dev server using the glob `src/**/*.fixture.ts`.
Use tools `mcp_component-exp_`* to list and screenshot fixtures. If you cannot see these tools, inform the user to them on.
## Running Fixtures Locally
1. Start the component explorer server: run the **Component Explorer Server** task
2. Use the `mcp_component-exp_list_fixtures` tool to see all available fixtures and their URLs
3. Use the `mcp_component-exp_screenshot` tool to capture screenshots programmatically
## File Structure
Each fixture file exports a default `defineThemedFixtureGroup(...)`. The file must end with `.fixture.ts`.
```
src/vs/workbench/test/browser/componentFixtures/
fixtureUtils.ts # Shared helpers (DO NOT import @vscode/component-explorer elsewhere)
myComponent.fixture.ts # Your fixture file
```
## Basic Pattern
```typescript
import { ComponentFixtureContext, createEditorServices, defineComponentFixture, defineThemedFixtureGroup } from './fixtureUtils.js';
export default defineThemedFixtureGroup({ path: 'myFeature/' }, {
Default: defineComponentFixture({ render: renderMyComponent }),
AnotherVariant: defineComponentFixture({ render: renderMyComponent }),
});
function renderMyComponent({ container, disposableStore, theme }: ComponentFixtureContext): void {
container.style.width = '400px';
const instantiationService = createEditorServices(disposableStore, {
colorTheme: theme,
additionalServices: (reg) => {
// Register additional services the component needs
reg.define(IMyService, MyServiceImpl);
reg.defineInstance(IMockService, mockInstance);
},
});
const widget = disposableStore.add(
instantiationService.createInstance(MyWidget, /* constructor args */)
);
container.appendChild(widget.domNode);
}
```
Key points:
- **`defineThemedFixtureGroup`** automatically creates Dark and Light variants for each fixture
- **`defineComponentFixture`** wraps your render function with theme setup, async readiness, and disposable management
- **`createEditorServices`** provides a `TestInstantiationService` with base editor services pre-registered
- Always register created widgets with `disposableStore.add(...)` to prevent leaks
- Pass `colorTheme: theme` to `createEditorServices` so theme colors render correctly
### File icon themes
Fixtures use Seti file icons by default. Select another built-in theme, or disable file icons, on the individual fixture:
```typescript
defineComponentFixture({ fileIconTheme: 'vs-minimal', render: renderMyComponent });
defineComponentFixture({ fileIconTheme: 'none', render: renderMyComponent });
```
When the rendered component reads `IThemeService`, pass the selected theme from `ComponentFixtureContext` to `createEditorServices`:
```typescript
function renderMyComponent({ disposableStore, theme, fileIconTheme }: ComponentFixtureContext): void {
const instantiationService = createEditorServices(disposableStore, {
colorTheme: theme,
fileIconTheme,
});
}
```
## Utilities from fixtureUtils.ts
| Export | Purpose |
|---|---|
| `defineComponentFixture` | Creates Dark/Light themed fixture variants from a render function |
| `defineThemedFixtureGroup` | Groups multiple themed fixtures into a named fixture group |
| `createEditorServices` | Creates `TestInstantiationService` with all base editor services |
| `registerWorkbenchServices` | Registers additional workbench services (context menu, label, etc.) |
| `createTextModel` | Creates a text model via `ModelService` for editor fixtures |
| `setupTheme` | Applies theme CSS to a container (called automatically by `defineComponentFixture`) |
| `darkTheme` / `lightTheme` | Pre-loaded `ColorThemeData` instances |
**Important:** Only `fixtureUtils.ts` may import from `@vscode/component-explorer`. All fixture files must go through the helpers in `fixtureUtils.ts`.
## CSS Scoping
Fixtures share the document and workbench stylesheets (`isolation: 'none'`). The helpers scope theme and file-icon styles, but DOM, CSS, and process-wide registrations are not isolated automatically.
### Shared theme initialization
`fixtureUtilsCss.ts` loads the shared workbench color and size registrations before generating and caching theme CSS, including the agent tokens in `vs/workbench/common/agentsColors.ts` and `agentsSizes.ts`. Keep required product-wide registrations in this shared bootstrap, not in individual fixtures: discovering another fixture must not change an existing fixture's typography or colors. Reuse lightweight registration modules from the owning shared layer rather than importing product entry points or higher-layer contributions.
### Matching production CSS selectors
Many VS Code components have CSS rules scoped to deep ancestor selectors (e.g., `.interactive-session .interactive-input-part > .widget-container .my-element`). In fixtures, you must recreate the required ancestor DOM structure for these selectors to match:
```typescript
function render({ container }: ComponentFixtureContext): void {
container.classList.add('interactive-session');
// Recreate ancestor structure that CSS selectors expect
const inputPart = dom.$('.interactive-input-part');
const widgetContainer = dom.$('.widget-container');
inputPart.appendChild(widgetContainer);
container.appendChild(inputPart);
widgetContainer.appendChild(myWidget.domNode);
}
```
**Design recommendation for new components:** Avoid deeply nested CSS selectors that require specific ancestor elements. Use self-contained class names (e.g., `.my-widget .my-element` rather than `.parent-view .parent-part > .wrapper .my-element`). This makes components easier to fixture and reuse.
## Services
### Using createEditorServices
`createEditorServices` pre-registers these services: `IAccessibilityService`, `IKeybindingService`, `IClipboardService`, `IOpenerService`, `INotificationService`, `IDialogService`, `IUndoRedoService`, `ILanguageService`, `IConfigurationService`, `IStorageService`, `IThemeService`, `IModelService`, `ICodeEditorService`, `IContextKeyService`, `ICommandService`, `ITelemetryService`, `IHoverService`, `IUserInteractionService`, and more.
### Additional services
Register extra services via `additionalServices`:
```typescript
createEditorServices(disposableStore, {
additionalServices: (reg) => {
// Class-based (instantiated by DI):
reg.define(IMyService, MyServiceImpl);
// Instance-based (pre-constructed):
reg.defineInstance(IMyService, myMockInstance);
},
});
```
### Mocking services
Use the `mock<T>()` helper from `base/test/common/mock.js` to create mock service instances:
```typescript
import { mock } from '../../../../base/test/common/mock.js';
const myService = new class extends mock<IMyService>() {
override someMethod(): string { return 'test'; }
override onSomeEvent = Event.None;
};
reg.defineInstance(IMyService, myService);
```
For mock view models or data objects:
```typescript
const element = new class extends mock<IChatRequestViewModel>() { }();
```
## Async Rendering
The component explorer awaits the render promise, then waits two animation frames. Those frames are not a readiness guarantee for image decoding, native resize observers, or delayed layout.
The render promise must resolve only when the fixture represents its named state:
- Await assets that affect the screenshot. An editor's `setInput()` may return before its main image or thumbnails have loaded; wait for the expected image count and await `decode()`, failing on missing or broken images.
- Finish layout-changing actions before positioning the viewport. Wait for content height, scroll height, scroll position, and finite animations to settle, then scroll and settle again. Assert that an "offscreen" header is actually offscreen and a "visible" header is not covered by sticky content.
- Account for transient visuals. A stable height does not mean a stable screenshot. Headless renders never flash auto-hiding scrollbars on scroll or resize, so do not wait for or hide them; use `ScrollbarVisibility.Visible` when a scrollbar is part of the intended state.
- Supply deterministic display data through existing services/options, such as a single custom thinking phrase. A seeded random generator alone does not make labels independent of render order or async callback order.
- Use bounded, condition-based waits that throw on failure, not fixed sleeps or extra frames that silently accept an incomplete state.
Design fixtures as **state + local inputs + an explicit readiness condition**. Supply local account icons instead of letting a mock fall back to a live avatar URL, and await decoding; `waitForFixtureCondition` polls native browser frames while preserving the virtual clock. To wait for DOM state, such as an element, class, attribute or text appearing, use `waitForFixtureDomCondition`, which re-checks the condition after each DOM mutation; a condition that never holds fails at the explorer's fixture timeout. Prefer shared DOM spinners over animated SVG backgrounds: ancestor CSS cannot stop animations inside an image.
Fixtures that depend on native browser image decoding or resize/scroll callbacks can use `virtualTime: { enabled: false }` and explicitly await those operations. Virtual JavaScript time does not advance the browser's image decoder or layout pipeline.
For async fixtures with paint-order-sensitive edges, `deferPaint: true` keeps the headless fixture transparent until rendering has finished, without changing its layout or interactive preview. It prevents intermediate paints from affecting the final rasterization; it does **not** replace awaiting readiness.
### Validating fixtures
Do not add automated tests whose subject is fixtures or fixture infrastructure (including fixture-only unit, Playwright, and screenshot-stability tests). Validate fixture changes with the existing Component Explorer rendering, screenshot, stability, and error tools, plus direct inspection or temporary browser probes. Ordinary tests for production behavior belong at the owning production API.
Validate at the same readiness boundary CI uses: capture immediately after the headless `renderFixture()` resolves, after idle callbacks, and after remounting following another fixture. Compare exact hashes on the same platform and separately inspect the intended state; do not commit the temporary probes as fixture tests.
### Pitfall: DOM reparenting causes flickering
Avoid moving rendered widgets between DOM parents after initial render. This causes:
- Layout recalculation (the widget jumps as `position: absolute` coordinates become invalid)
- Focus loss (blur events can trigger hide logic in widgets like QuickInput)
- Screenshot instability (the component explorer may capture an intermediate layout state)
**Bad pattern — reparenting a widget after async wait:**
```typescript
async function render({ container }: ComponentFixtureContext): Promise<void> {
const host = document.createElement('div');
container.appendChild(host);
// ... create widget inside host ...
await waitForWidget();
container.appendChild(widget); // BAD: reparenting causes flicker
host.remove();
}
```
**Better pattern — render in-place with the correct DOM structure from the start:**
```typescript
function render({ container }: ComponentFixtureContext): void {
// Set up the correct DOM structure first, then create the widget inside it
const widget = createWidget(container);
container.appendChild(widget.domNode);
}
```
If the component absolutely requires async setup (e.g., QuickInput which renders internally), minimize DOM manipulation after the widget appears by structuring the host container to match the final layout from the beginning.
## Adapting Existing Components for Fixtures
Existing components often need small changes to become fixturable. When writing a fixture reveals friction, fix the component — don't work around it in the fixture. Common adaptations:
### Decouple CSS from ancestor context
If a component's CSS only works inside a deeply nested selector like `.workbench .sidebar .my-view .my-widget`, refactor the CSS to be self-contained. Move the styles so they're scoped to the component's own root class:
```css
/* Before: requires specific ancestors */
.workbench .sidebar .my-view .my-widget .header { font-weight: bold; }
/* After: self-contained */
.my-widget .header { font-weight: bold; }
```
If the component shares styles with its parent (e.g., inheriting background color), use CSS custom properties rather than relying on ancestor selectors.
### Extract hard-coded service dependencies
If a component reaches into singletons or global state instead of using DI, refactor it to accept services through the constructor:
```typescript
// Before: hard to mock in fixtures
class MyWidget {
private readonly config = getSomeGlobalConfig();
}
// After: injectable and testable
class MyWidget {
constructor(@IConfigurationService private readonly configService: IConfigurationService) { }
}
```
### Add options to control auto-focus and animation
Components that auto-focus on creation or run animations cause flaky screenshots. Add an options parameter:
```typescript
interface IMyWidgetOptions {
shouldAutoFocus?: boolean;
}
```
The fixture passes `shouldAutoFocus: false`. The production call site keeps the default behavior.
### Expose internal state for "already completed" rendering
Many components have lifecycle states (loading → active → completed). If the component can only reach the "completed" state through user interaction, add support for initializing directly into that state via constructor data:
```typescript
// The fixture can pass pre-filled data to render the summary/completed state
// without simulating the full user interaction flow.
const carousel: IChatQuestionCarousel = {
questions,
allowSkip: true,
kind: 'questionCarousel',
isUsed: true, // Already completed
data: { 'q1': 'answer' }, // Pre-filled answers
};
```
### Make DOM node accessible
If a component builds its DOM internally and doesn't expose the root element, add a public `readonly domNode: HTMLElement` property so fixtures can append it to the container.
### Keep test-only operations out of the production API
When a fixture or test must drive state that production reaches only through user input or services (for example focusing, selecting, or collapsing rows, or locating a rendered row), don't add public methods for it: other code will start calling them and bypass the real flows. Make the minimum `protected` and implement the operations in a test-only subclass under `test/` (such as `TestSessionsList extends SessionsList`), which product code cannot import.
## Writing Fixture-Friendly Components
When designing new UI components, follow these practices to make them easy to fixture:
### 1. Accept a container element in the constructor
```typescript
// Good: container is passed in
class MyWidget {
constructor(container: HTMLElement, @IFoo foo: IFoo) {
this.domNode = dom.append(container, dom.$('.my-widget'));
}
}
// Also good: widget creates its own domNode for the caller to place
class MyWidget {
readonly domNode: HTMLElement;
constructor(@IFoo foo: IFoo) {
this.domNode = dom.$('.my-widget');
}
}
```
### 2. Use dependency injection for all services
All external dependencies should come through DI so fixtures can provide test implementations:
```typescript
// Good: services injected
constructor(@IThemeService private readonly themeService: IThemeService) { }
// Bad: reaching into globals
constructor() { this.theme = getGlobalTheme(); }
```
### 3. Keep CSS selectors shallow
```css
/* Good: self-contained, easy to fixture */
.my-widget .my-header { ... }
.my-widget .my-list-item { ... }
/* Bad: requires deep ancestor chain */
.workbench .sidebar .my-view .my-widget .my-header { ... }
```
### 4. Avoid reading from layout/window services during construction
Components that measure the window or read layout dimensions during construction are hard to fixture because the shadow DOM container has different dimensions than the workbench:
```typescript
// Prefer: use CSS for sizing, or accept dimensions as parameters
container.style.width = '400px';
container.style.height = '300px';
// Avoid: reading from layoutService during construction
const width = this.layoutService.mainContainerDimension.width;
```
### 5. Support disabling auto-focus in fixtures
Auto-focus can interfere with screenshot stability. Provide options to disable it:
```typescript
interface IMyWidgetOptions {
shouldAutoFocus?: boolean; // Fixtures pass false
}
```
### 6. Expose the DOM node
The fixture needs to append the widget's DOM to the container. Expose it as a public `readonly domNode: HTMLElement`.
### 7. Make hover and focus states renderable
Fixtures cannot trigger CSS `:hover`, and DOM focus is only deterministic through `ctx.focus()`. Keep interaction-dependent visuals reachable from state:
- Pair every `:hover` selector a visual depends on with a `.hovered` class, e.g. `.monaco-list-row:is(:hover, .hovered)`; `:is()` keeps specificity unchanged. List widgets already style `.monaco-list-row.hovered` like a hovered row.
- Drive focus and selection through the component (list rows get `.focused` and `.selected` from the list traits), from a test-only subclass when production has no such API (see [Keep test-only operations out of the production API](#keep-test-only-operations-out-of-the-production-api)). Then call `ctx.focus()` when the state needs DOM focus.
- When JavaScript also reacts to hover (for example `mouseover` listeners), dispatch a `mouseover` on the element as well as adding `.hovered`.
`src/vs/sessions/contrib/sessions/test/browser/sessionsListFixtureUtils.ts` renders `interaction: { hovered, focused, selected }` this way.
## Multiple Fixture Variants
Create variants to show different states of the same component:
```typescript
export default defineThemedFixtureGroup({
// Different data states
Empty: defineComponentFixture({ render: (ctx) => renderWidget(ctx, { items: [] }) }),
WithItems: defineComponentFixture({ render: (ctx) => renderWidget(ctx, { items: sampleItems }) }),
// Different configurations
ReadOnly: defineComponentFixture({ render: (ctx) => renderWidget(ctx, { readonly: true }) }),
Editable: defineComponentFixture({ render: (ctx) => renderWidget(ctx, { readonly: false }) }),
// Lifecycle states
Loading: defineComponentFixture({ render: (ctx) => renderWidget(ctx, { state: 'loading' }) }),
Completed: defineComponentFixture({ render: (ctx) => renderWidget(ctx, { state: 'done' }) }),
});
```
## Learnings
Update this section with insights from your fixture development experience!
* Do not copy the component to the fixture and modify it there. Always adapt the original component to be fixture-friendly, then render it in the fixture. This ensures the fixture tests the real component code and lifecycle, rather than a modified version that may hide bugs.
* **Don't recompose child widgets in fixtures.** Never manually instantiate and add a sub-widget (e.g., a toolbar content widget) that the parent component is supposed to create. Instead, configure the parent correctly (e.g., set the right editor option, register the right provider) so the child appears through the normal code path. Manually recomposing hides integration bugs and doesn't test the real widget lifecycle.
* **Describe state as data for service-heavy components.** When a component reads many services, give it one harness that takes plain state and renders it through the real state-owning services and production menus, instead of per-fixture mocks and DOM hacks. Fixtures then only list data, and a new feature adds a state field to the harness. The sessions list harness (`sessionsListFixtureUtils.ts`) follows this pattern; its real toolbars exposed header bugs that stubbed menus had hidden.
* **Never vary process-wide registrations per fixture.** The explorer UI mounts all fixtures of a folder at once, and they share command, menu, and other global registries. Register shared actions once, the same for every fixture, and vary per-fixture presentation through the fixture's own services, such as its menu service. Screenshot runs render fixtures one at a time and don't catch mount-order races, so preview the whole folder (`___explorer?fixture=<folder>`) to check.