docs(adr): 012 — browser-mode test mocking strategy

Documents why vi.mock(module, factory) races with birpc teardown for
dynamically-imported modules, the libLoader injection pattern used to fix
#535, and the residual exceptions ($app/*, $env/*) that are safe to keep
as vi.mock because they are resolved statically before any test runs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/adr/012-browser-test-mocking-strategy.md

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+# ADR 012 — Browser-Mode Test Mocking Strategy
+
+**Status:** Accepted  
+**Date:** 2026-05-11  
+**Issue:** [#535 — Unit & Component Tests job exits 1 from vitest-browser teardown race](https://git.raddatz.cloud/marcel/familienarchiv/issues/535)
+
+---
+
+## Context
+
+Vitest browser-mode tests (the `client` project, run with `@vitest/browser-playwright` / Chromium) use a different module resolution path than Node-environment tests. When a spec calls `vi.mock('some-module', factory)`, vitest registers a `ManualMockedModule`. At runtime, every time Chromium requests that module, a playwright route handler intercepts the request and calls the Node worker over **birpc** (`resolveManualMock`) to evaluate the factory and return the module body.
+
+This is safe for modules that are imported **statically** at spec module-eval time (e.g. `$app/navigation`, `$env/static/public`): those requests resolve before the first test runs and well before any teardown occurs.
+
+It is **unsafe** for modules that are imported **dynamically** (e.g. inside an `async onMount`, inside a lazy-loaded chunk): Chromium may fetch the module after the worker's birpc channel has already closed, producing:
+
+```
+Error: [birpc] rpc is closed, cannot call "resolveManualMock"
+  ❯ ManualMockedModule.factory   node_modules/@vitest/browser/dist/index.js:3221:34
+```
+
+This raises an unhandled rejection that exits the vitest process with code 1, even though every test in the run reported green.
+
+`pdfjs-dist` and `pdfjs-dist/build/pdf.worker.min.mjs?url` are loaded via `await Promise.all([import('pdfjs-dist'), import('pdfjs-dist/build/pdf.worker.min.mjs?url')])` inside `usePdfRenderer.svelte.ts::init()`, which is called from `onMount`. These dynamic imports triggered the race.
+
+---
+
+## Decision
+
+**Prefer prop injection over `vi.mock(module, factory)` for any module that is loaded dynamically in browser-mode specs.**
+
+### The libLoader pattern (for external rendering libraries)
+
+When a component depends on a large external library loaded via dynamic import, extract the import into an injectable loader function with a production default:
+
+```typescript
+// usePdfRenderer.svelte.ts
+type LibLoader = () => Promise<readonly [typeof import('pdfjs-dist'), { default: string }]>;
+
+const defaultLibLoader: LibLoader = () =>
+  Promise.all([import('pdfjs-dist'), import('pdfjs-dist/build/pdf.worker.min.mjs?url')]);
+
+export function createPdfRenderer(libLoader: LibLoader = defaultLibLoader) { ... }
+```
+
+The component threads the loader as an optional prop:
+
+```svelte
+<!-- PdfViewer.svelte -->
+let { url, ..., libLoader = undefined } = $props();
+const renderer = untrack(() => createPdfRenderer(libLoader));
+```
+
+Tests supply a synchronous fake — no `vi.mock` needed:
+
+```typescript
+const fakePdfjs = { GlobalWorkerOptions: ..., getDocument: vi.fn(), TextLayer: class {} };
+const fakeLoader = vi.fn().mockResolvedValue([fakePdfjs, { default: '' }] as const);
+render(PdfViewer, { url: '...', libLoader: fakeLoader });
+```
+
+### The test-host pattern (for component behaviour)
+
+For components that fetch data or call services, the `*.test-host.svelte` pattern threads the dependency as a prop rather than mocking the module. See `PersonMentionEditor.test-host.svelte` for the canonical example.
+
+---
+
+## Residual exceptions
+
+The following `vi.mock(module, factory)` calls in browser specs are **acceptable** because the mocked modules are loaded statically at spec module-eval time and cannot produce a teardown race:
+
+| Module | Why it stays as vi.mock |
+|--------|------------------------|
+| `$app/navigation` | SvelteKit virtual module — no DI seam |
+| `$app/forms` | SvelteKit virtual module — no DI seam |
+| `$app/state` | SvelteKit virtual module — no DI seam |
+| `$app/stores` | SvelteKit virtual module — no DI seam |
+| `$env/static/public` | Vite env virtual module — no DI seam |
+
+These modules are resolved at static import time (before any test runs). Their `vi.mock` factories are served by birpc synchronously during module graph resolution, not after worker teardown.
+
+---
+
+## Consequences
+
+- New browser-mode specs that need to stub an external library **must not** use `vi.mock(externalLib, factory)`. Add a loader/factory parameter to the underlying hook or service instead.
+- The CI `unit-tests` job includes a permanent grep guard that fails the build if `rpc is closed` appears in any coverage run log. This catches regressions before they reach the acceptance criterion.
+- Acceptance criterion for #535: 60 consecutive green `workflow_dispatch` CI runs against `main` after the fix is merged, with zero `rpc is closed` lines in any log.