test: add preserveMarkup unit coverage for markup helpers

docs/plans/2026-03-08-coverage-99-plus-design.md

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+# Coverage 99+ Design
+
+**Goal:** Raise unit-test coverage from the current post-fix level to 99%+ without changing production behavior.
+
+**Context:** The recent `preserveMarkup` work added substantial new logic under `src/Truncate/engines/markup.tsx` and `src/Truncate/markup/render.tsx`. CI focused on browser E2E coverage, so Codecov correctly reported a project-level drop. A first recovery pass already moved local coverage to `98.41%`, leaving only a handful of defensive branches uncovered.
+
+**Recommended Approach:** Add focused unit tests only, centered in `test/Markup.spec.tsx`, and avoid any production refactor. This keeps the fix surgical, preserves behavior, and directly addresses the Codecov signal instead of muting it.
+
+## Options Considered
+
+### Option A: Add focused helper and engine tests
+- Keep all changes in tests
+- Target the remaining uncovered branches directly
+- Fastest path to 99%+
+
+**Trade-off:** A few tests will exercise fairly defensive branches, so they are slightly more synthetic than top-level component tests.
+
+### Option B: Refactor helper internals for easier direct testing
+- Could make each branch easier to target
+- Might reduce test setup complexity
+
+**Trade-off:** Changes production code purely for coverage, which is unnecessary for this CI repair.
+
+### Option C: Relax or reshape coverage rules
+- Fast to make CI green
+
+**Trade-off:** Hides a real test gap and weakens the project signal. Rejected.
+
+## Design
+
+### Scope
+- Modify only `test/Markup.spec.tsx`
+- Do not modify `src/**` unless a genuine bug is discovered
+- Keep existing E2E coverage unchanged
+
+### Test strategy
+- Cover the remaining helper branches in `render.tsx`
+- Cover the remaining defensive cleanup branches in `markup.tsx`
+- Reuse the current measurement sandbox so tests stay close to the real `preserveMarkup` execution path
+
+### Success criteria
+- `pnpm vitest run test/Markup.spec.tsx` passes
+- `pnpm coverage` reports total coverage at `99%+`
+- No production behavior changes

docs/plans/2026-03-08-coverage-99-plus.md

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+# Coverage 99+ Implementation Plan
+
+> **For Claude:** REQUIRED SUB-SKILL: Use superpowers:executing-plans to implement this plan task-by-task.
+
+**Goal:** Raise project unit-test coverage to 99%+ by adding focused tests for the remaining uncovered `preserveMarkup` helper branches.
+
+**Architecture:** Keep production code unchanged and extend the existing focused spec `test/Markup.spec.tsx`. Use narrowly scoped assertions that target the remaining uncovered defensive branches in the snapshot/render helpers and markup truncation engine, then verify with the existing `vitest` coverage workflow.
+
+**Tech Stack:** React, TypeScript, Vitest, Testing Library, Sinon, Happy DOM
+
+---
+
+### Task 1: Cover remaining render helper branches
+
+**Files:**
+- Modify: `test/Markup.spec.tsx`
+- Reference: `src/Truncate/markup/render.tsx`
+
+**Step 1: Write the failing tests**
+- Add tests for `sliceSnapshotNodes` with zero or fractional remaining width.
+- Add tests for `trimLeadingWhitespace` when the first node is a line break.
+- Add tests for any still-uncovered whitespace handling branches.
+
+**Step 2: Run test to verify it fails or expands coverage needfully**
+Run: `pnpm vitest run test/Markup.spec.tsx`
+Expected: Existing suite stays green or a new assertion fails for the uncovered branch.
+
+**Step 3: Write minimal test-only implementation**
+- Adjust only test fixtures/assertions until the branch is exercised correctly.
+
+**Step 4: Run test to verify it passes**
+Run: `pnpm vitest run test/Markup.spec.tsx`
+Expected: PASS
+
+### Task 2: Cover remaining markup engine defensive branches
+
+**Files:**
+- Modify: `test/Markup.spec.tsx`
+- Reference: `src/Truncate/engines/markup.tsx`
+
+**Step 1: Write the failing tests**
+- Add a test that forces recursive cleanup of whitespace-only text and empty elements.
+- Add a test that exercises unsupported child-node cleanup in the truncation clone path.
+
+**Step 2: Run test to verify it fails or expands coverage needfully**
+Run: `pnpm vitest run test/Markup.spec.tsx`
+Expected: Targeted assertions prove the branch is now reached.
+
+**Step 3: Write minimal test-only implementation**
+- Keep the change isolated to fixture setup and expectations in `test/Markup.spec.tsx`.
+
+**Step 4: Run test to verify it passes**
+Run: `pnpm vitest run test/Markup.spec.tsx`
+Expected: PASS
+
+### Task 3: Verify project coverage
+
+**Files:**
+- Verify: `test/Markup.spec.tsx`
+
+**Step 1: Run focused tests**
+Run: `pnpm vitest run test/Markup.spec.tsx`
+Expected: PASS
+
+**Step 2: Run full coverage**
+Run: `pnpm coverage`
+Expected: Total coverage reaches at least `99%`
+
+**Step 3: Review result**
+- Confirm no production files changed.
+- Confirm only test/docs plan files were touched.