# Subtitle Preview Scale Implementation Plan

> **For Claude:** REQUIRED SUB-SKILL: Use superpowers:executing-plans to implement this plan task-by-task.

**Goal:** Scale subtitle text inside the compact preview screen so it looks proportional, without changing the real subtitle size values used elsewhere in the flow.

**Architecture:** Keep `subtitleDefaults.fontSize` as the source of truth and derive a preview-only font size in `src/components/UploadScreen.tsx`. Lock the behavior with upload-screen tests that prove the preview is scaled down while upload/export still use the real font size.

**Tech Stack:** React 19, TypeScript, Tailwind CSS v4 utilities, Vitest, React Testing Library

---

### Task 1: Lock preview-only scaling with a failing test

**Files:**
- Modify: `src/components/UploadScreen.test.tsx`
- Reference: `src/components/UploadScreen.tsx`

**Step 1: Write the failing test**

Update the existing preview test so it expects scaled preview rendering rather than raw font size rendering:

```tsx
it('scales the subtitle preview while preserving the real subtitle size value', () => {
  renderUploadScreen();

  const sizeSlider = screen.getByLabelText(/subtitle initial size/i);
  const positionSlider = screen.getByLabelText(/subtitle initial position/i);
  const preview = screen.getByTestId('upload-subtitle-preview');

  expect(sizeSlider).toHaveValue('24');
  expect(positionSlider).toHaveValue('10');
  expect(preview).toHaveStyle({ fontSize: '12px', bottom: '10%' });

  fireEvent.change(sizeSlider, { target: { value: '32' } });
  fireEvent.change(positionSlider, { target: { value: '18' } });

  expect(preview).toHaveStyle({ fontSize: '16px', bottom: '18%' });
});
```

**Step 2: Run test to verify it fails**

Run:

```bash
node ./node_modules/vitest/vitest.mjs run src/components/UploadScreen.test.tsx
```

Expected: FAIL because the preview currently renders the raw font size values.

**Step 3: Write minimal implementation**

Add a preview-only derived font size and use it for the preview text.

**Step 4: Run test to verify it passes**

Run:

```bash
node ./node_modules/vitest/vitest.mjs run src/components/UploadScreen.test.tsx
```

Expected: PASS for the new preview-scaling expectation.

**Step 5: Commit**

```bash
git add src/components/UploadScreen.tsx src/components/UploadScreen.test.tsx
git commit -m "test: lock subtitle preview scaling"
```

### Task 2: Keep export behavior unchanged

**Files:**
- Modify: `src/components/UploadScreen.tsx`
- Reference: `src/components/UploadScreen.test.tsx`

**Step 1: Write the failing test**

Use the existing upload-confirmation test as the regression check for preserving the real font size value.

**Step 2: Run test to verify it fails**

Run:

```bash
node ./node_modules/vitest/vitest.mjs run src/components/UploadScreen.test.tsx
```

Expected: FAIL only if the implementation accidentally changes uploaded subtitle defaults instead of preview-only rendering.

**Step 3: Write minimal implementation**

Ensure only the preview rendering uses the derived font size. Keep:

- slider state values unchanged
- upload callback values unchanged
- bottom offset behavior unchanged

**Step 4: Run test to verify it passes**

Run:

```bash
node ./node_modules/vitest/vitest.mjs run src/components/UploadScreen.test.tsx
```

Expected: PASS for both the preview mapping test and the upload-confirmation regression test.

**Step 5: Commit**

```bash
git add src/components/UploadScreen.tsx src/components/UploadScreen.test.tsx
git commit -m "fix: scale subtitle preview text"
```

### Task 3: Full verification

**Files:**
- Modify: `src/components/UploadScreen.tsx`
- Modify: `src/components/UploadScreen.test.tsx`

**Step 1: Write the failing test**

If any final regression appears, add the smallest targeted test first.

**Step 2: Run test to verify it fails**

Run:

```bash
node ./node_modules/vitest/vitest.mjs run src/components/UploadScreen.test.tsx src/App.test.tsx
```

Expected: FAIL only if a real regression remains.

**Step 3: Write minimal implementation**

Apply only the smallest visual or logic adjustment needed to restore parity.

**Step 4: Run test to verify it passes**

Run:

```bash
npm.cmd test
npm.cmd run lint
npm.cmd run build
```

Expected:

- all tests PASS
- lint PASS
- build PASS

**Step 5: Commit**

```bash
git add src/components/UploadScreen.tsx src/components/UploadScreen.test.tsx docs/plans/2026-03-21-subtitle-preview-scale-design.md docs/plans/2026-03-21-subtitle-preview-scale.md
git commit -m "test: verify subtitle preview scaling"
```