docs/adr/013-client-branches-coverage-threshold.md

@@ -0,0 +1,92 @@
+# ADR 013 — Client-Project Branch Coverage Threshold
+
+**Status:** Accepted  
+**Date:** 2026-05-14  
+**Issues:** [#556 — threshold drop](https://git.raddatz.cloud/marcel/familienarchiv/issues/556) · [#496 — long-tail-grind tracking](https://git.raddatz.cloud/marcel/familienarchiv/issues/496)
+
+---
+
+## Context
+
+The browser-mode component test suite (`vitest.client-coverage.config.ts`) enforces Istanbul coverage thresholds across `lines`, `functions`, `branches`, and `statements`. The `branches` metric was set to 80%, but the codebase sits at **75%** — below the gate — causing every CI run of `unit-tests` and `coverage-flake-probe` to fail on this check alone, even when all tests are green.
+
+**Measured baseline (2026-05-14, branch `feat/issue-553-birpc-async-mock-factory`, head `2e6cc346`):**
+
+```
+branches:   75%  (below the 80% gate — reason for this ADR)
+lines:      ≥ 80%
+functions:  ≥ 80%
+statements: ≥ 80%
+```
+
+Reproducer:
+
+```bash
+cd frontend && npm ci && npx vitest run -c vitest.client-coverage.config.ts --coverage
+```
+
+### The long-tail-grind problem
+
+In Istanbul's branch accounting, when a child component gains test coverage its branches are added to the parent's denominator. A child moving from 40% → 80% coverage can drag a parent from 78% → 72% because more branches in the call graph become reachable and must be covered. This is not a bug — it is how branch accounting works — but it means that on a large SvelteKit application the denominator grows with every coverage improvement, making an arbitrary 80% ceiling a constant grind. Per #496, the expected cost to reach 80% branches from 75% is 30–100+ commits with no guarantee of stability.
+
+### Why this layer is different
+
+The 80% branch floor used for backend unit/integration tests is appropriate for Java service code and permission logic. Browser-mode component coverage measures Svelte template branches: conditional class bindings, `{#if}` blocks, empty/loaded/error state guards. These branches have a fundamentally different accounting model and a higher inherent denominator. This ADR **only** lowers the browser-mode component gate; the backend test coverage gates are unaffected.
+
+### Security-relevant uncovered components
+
+The following auth/permission-boundary components currently have low or zero branch coverage. When ratchet-up work begins (see below), these are the highest-priority targets:
+
+- `src/routes/login/+page.svelte`
+- `src/routes/forgot-password/+page.svelte`
+- `src/routes/reset-password/+page.svelte`
+- `src/routes/register/+page.svelte`
+
+Note: the 75% figure already reflects the absence of coverage on these files. Lowering the gate does not create this gap — it makes the existing state legible.
+
+---
+
+## Decision
+
+Drop the `branches` threshold from `80` → `75` in `frontend/vitest.client-coverage.config.ts`. Leave `lines`, `functions`, and `statements` at `80`.
+
+The 75% figure matches the measured current state, allowing CI to pass while deliberate coverage improvement work (tracked in #496) continues without blocking other PRs. The asymmetry in the thresholds block is intentional and documented with an inline comment pointing here.
+
+---
+
+## Ratchet Rule
+
+The branches threshold ratchets **up by 3 percentage points** when the rolling 3-PR-average client-project branches figure on `main` stays at or above `threshold + 3pp` for ≥ 30 consecutive days. Direction is **up-only** — never lower the floor below 75 without a new ADR superseding this one. Manual today (verify before any `vitest.client-coverage.config.ts` edit); a future automation issue may codify the check.
+
+Concretely:
+- When `main` sustains ≥ 78% branches across 3 consecutive PRs for 30 days → raise gate to 78%
+- When `main` sustains ≥ 81% branches across 3 consecutive PRs for 30 days → raise gate back to 80%
+
+---
+
+## Non-goals
+
+- **Not** raising actual branch coverage — that is #496's job, tracked separately.
+- **Not** touching the server-project coverage configuration (`vitest.config.ts`) — only the client project hits the long-tail-grind pattern.
+- **Not** removing or relaxing any existing test files, `skipIf` guards, or axe-playwright accessibility runs.
+
+---
+
+## Consequences
+
+**Easier:**
+- CI unblocked — `unit-tests` and `coverage-flake-probe` jobs pass when all tests are green
+- The ratchet rule creates a concrete, observable path back to 80%
+
+**Harder:**
+- The gate now has near-zero headroom — any branch regression that drops below 75% will fail CI immediately
+- The 75% floor must not be treated as a permanent ceiling; the ratchet discipline requires active attention
+
+---
+
+## References
+
+- [#496 — Branch coverage long-tail grind](https://git.raddatz.cloud/marcel/familienarchiv/issues/496)
+- [#556 — This threshold drop](https://git.raddatz.cloud/marcel/familienarchiv/issues/556)
+- [ADR 012 — Browser-Mode Test Mocking Strategy](./012-browser-test-mocking-strategy.md)
+- `frontend/vitest.client-coverage.config.ts` — thresholds block (lines 44–51)

frontend/vitest.client-coverage.config.ts

@@ -44,7 +44,8 @@ export default defineConfig({
 			thresholds: {
 				lines: 80,
 				functions: 80,
-				branches: 80,
+				// branches at 75 — long-tail parent/child accounting grind; see docs/adr/013-client-branches-coverage-threshold.md and #496
+				branches: 75,
 				statements: 80
 			}
 		}