# Phase 12: Filled Document Preview - Research

**Researched:** 2026-03-21
**Domain:** PDF preview generation, React modal with react-pdf, Send-button gating with staleness detection
**Confidence:** HIGH

<phase_requirements>
## Phase Requirements

| ID | Description | Research Support |
|----|-------------|-----------------|
| PREV-01 | Agent sees a live filled preview of the fully-prepared document (text filled, signatures embedded) before sending to client | Preview route reuses `preparePdf()` in preview mode writing to a versioned path; PreviewModal uses react-pdf `Document` with `ArrayBuffer` file prop; Send-button gating via `previewToken` state; staleness detection by resetting token on field/text change |
</phase_requirements>

## Summary

Phase 12 adds a "Preview" button to the document prepare page that generates a temporary filled PDF — with all text, agent signatures, agent initials, checkboxes, and date stamps embedded — and displays it in a modal using the already-installed `react-pdf` library. The Send button becomes available only after the agent has generated at least one preview and is re-disabled the moment any field or text-fill value changes (staleness detection).

The preview PDF is written to a unique versioned path (e.g., `{docId}_preview_{timestamp}.pdf`) in the uploads directory, completely separate from `_prepared.pdf`. This preserves the integrity of the final prepared document: the prepare route that actually sends to clients always writes to `_prepared.pdf` and is unchanged. The preview route is a thin POST endpoint that calls the existing `preparePdf()` utility with `destPath` pointing to the versioned preview path, then streams the bytes back to the client.

The client side is straightforward: the `PreparePanel` component maintains a `previewToken` (a string or null) that represents whether the current field/text state has a valid preview. The Send button is enabled only when `previewToken !== null`. Any user action that changes fields (via `FieldPlacer.persistFields`) or text fill values fires a `onFieldsChanged` callback into `PreparePanel`, resetting the token to null and re-disabling Send.

**Primary recommendation:** Add a POST `/api/documents/[id]/preview` route that calls `preparePdf()` to a versioned path and returns the bytes as `application/pdf`; render in a `PreviewModal` using `react-pdf` `<Document file={arrayBuffer}>` pattern; gate the Send button on `previewToken` state in `PreparePanel`.

## Standard Stack

### Core
| Library | Version | Purpose | Why Standard |
|---------|---------|---------|--------------|
| `@cantoo/pdf-lib` | 2.6.3 | Generates the preview PDF | Already used in `preparePdf()` — zero new dependency |
| `react-pdf` | 10.4.1 | Renders the preview PDF in the modal | Already installed; `Document` accepts `ArrayBuffer` |
| `pdfjs-dist` | 5.4.296 | Powers react-pdf rendering | Already installed; worker already configured in `PdfViewer.tsx` |

### Supporting
| Library | Version | Purpose | When to Use |
|---------|---------|---------|-------------|
| Next.js Route Handler | 16.2.0 | POST `/api/documents/[id]/preview` returning PDF bytes | Follows exact pattern of `/api/documents/[id]/file` route |
| Node `fs/promises` | built-in | Write preview file, read bytes back | Same pattern as existing prepare route |
| `path` | built-in | Path construction + traversal guard | Same pattern as every other file-serving route |

### Alternatives Considered
| Instead of | Could Use | Tradeoff |
|------------|-----------|----------|
| Returning `ArrayBuffer` from preview route | Generating a presigned URL to preview file | URL approach requires a second route + cleanup; ArrayBuffer is simpler since file is ephemeral |
| `ArrayBuffer` as react-pdf `file` prop | Blob URL via `URL.createObjectURL` | Both work; `ArrayBuffer` is simpler — no cleanup of blob URLs needed, and react-pdf accepts `ArrayBuffer` directly per its type definitions |
| In-memory PDF (never write to disk) | Write-then-read pattern | Write-to-disk is required so the preview path is distinct from `_prepared.pdf`; reusing `preparePdf()` as-is avoids reimplementing the atomic write logic |

**Installation:**
```bash
# No new packages needed — all dependencies already installed
```

## Architecture Patterns

### Recommended Project Structure
```
src/
├── app/api/documents/[id]/preview/
│   └── route.ts                    # POST — generates preview PDF, returns bytes
├── app/portal/(protected)/documents/[docId]/_components/
│   ├── PreparePanel.tsx            # Modified: Preview button + Send gating + staleness
│   ├── PreviewModal.tsx            # NEW: Modal wrapping react-pdf Document/Page
│   └── PdfViewer.tsx               # Existing: exposes onFieldsChanged callback (new prop)
```

### Pattern 1: Preview Route — Reuse preparePdf with Versioned Path

**What:** A POST route that calls the existing `preparePdf()` utility with a timestamp-versioned destination path and returns the resulting PDF bytes directly in the response body.

**When to use:** Agent clicks the Preview button. Route is auth-guarded (session check), reads the same source PDF and field data the prepare route uses, and does NOT update any document DB columns.

**Example:**
```typescript
// Source: adapted from /src/app/api/documents/[id]/prepare/route.ts
// File: src/app/api/documents/[id]/preview/route.ts
import { auth } from '@/lib/auth';
import { db } from '@/lib/db';
import { documents, users } from '@/lib/db/schema';
import { eq } from 'drizzle-orm';
import { preparePdf } from '@/lib/pdf/prepare-document';
import path from 'node:path';
import { readFile, unlink } from 'node:fs/promises';

const UPLOADS_DIR = path.join(process.cwd(), 'uploads');

export async function POST(
  req: Request,
  { params }: { params: Promise<{ id: string }> }
) {
  const session = await auth();
  if (!session?.user?.id) return new Response('Unauthorized', { status: 401 });

  const { id } = await params;
  const body = await req.json() as { textFillData?: Record<string, string> };

  const doc = await db.query.documents.findFirst({ where: eq(documents.id, id) });
  if (!doc) return Response.json({ error: 'Not found' }, { status: 404 });
  if (!doc.filePath) return Response.json({ error: 'No source file' }, { status: 422 });

  const srcPath = path.join(UPLOADS_DIR, doc.filePath);

  // Versioned preview path — never overwrites _prepared.pdf
  const timestamp = Date.now();
  const previewRelPath = doc.filePath.replace(/\.pdf$/, `_preview_${timestamp}.pdf`);
  const previewPath = path.join(UPLOADS_DIR, previewRelPath);

  // Path traversal guard
  if (!previewPath.startsWith(UPLOADS_DIR)) {
    return new Response('Forbidden', { status: 403 });
  }

  const sigFields = (doc.signatureFields as import('@/lib/db/schema').SignatureFieldData[]) ?? [];
  const textFields = body.textFillData ?? {};

  const agentUser = await db.query.users.findFirst({
    where: eq(users.id, session.user.id),
    columns: { agentSignatureData: true, agentInitialsData: true },
  });
  const agentSignatureData = agentUser?.agentSignatureData ?? null;
  const agentInitialsData = agentUser?.agentInitialsData ?? null;

  // Same 422 guards as prepare route
  const { getFieldType } = await import('@/lib/db/schema');
  const hasAgentSigFields = sigFields.some(f => getFieldType(f) === 'agent-signature');
  if (hasAgentSigFields && !agentSignatureData) {
    return Response.json({ error: 'agent-signature-missing', message: 'No agent signature saved.' }, { status: 422 });
  }
  const hasAgentInitialsFields = sigFields.some(f => getFieldType(f) === 'agent-initials');
  if (hasAgentInitialsFields && !agentInitialsData) {
    return Response.json({ error: 'agent-initials-missing', message: 'No agent initials saved.' }, { status: 422 });
  }

  await preparePdf(srcPath, previewPath, textFields, sigFields, agentSignatureData, agentInitialsData);

  // Read bytes and stream back; then clean up the preview file
  const pdfBytes = await readFile(previewPath);
  // Fire-and-forget cleanup — preview files are ephemeral
  unlink(previewPath).catch(() => {});

  return new Response(pdfBytes, {
    headers: { 'Content-Type': 'application/pdf' },
  });
}
```

### Pattern 2: PreviewModal — react-pdf with ArrayBuffer

**What:** A client component modal that receives the PDF as `ArrayBuffer` from a `fetch()` response and passes it directly to `react-pdf`'s `<Document>` component.

**When to use:** After the preview API call succeeds, `PreparePanel` stores the `ArrayBuffer` in state and renders `PreviewModal`. The `file` prop of react-pdf's `Document` accepts `ArrayBuffer` directly — confirmed in `node_modules/react-pdf/dist/shared/types.d.ts`: `export type File = string | ArrayBuffer | Blob | Source | null;`.

**Example:**
```typescript
// Source: react-pdf Document.d.ts (installed v10.4.1)
// File: src/app/portal/(protected)/documents/[docId]/_components/PreviewModal.tsx
'use client';
import { useState } from 'react';
import { Document, Page, pdfjs } from 'react-pdf';
import 'react-pdf/dist/Page/AnnotationLayer.css';
import 'react-pdf/dist/Page/TextLayer.css';

// Worker must be configured here too (each 'use client' module is independent)
pdfjs.GlobalWorkerOptions.workerSrc = new URL(
  'pdfjs-dist/build/pdf.worker.min.mjs',
  import.meta.url,
).toString();

interface PreviewModalProps {
  pdfBytes: ArrayBuffer;
  onClose: () => void;
}

export function PreviewModal({ pdfBytes, onClose }: PreviewModalProps) {
  const [numPages, setNumPages] = useState(0);
  const [pageNumber, setPageNumber] = useState(1);

  // CRITICAL: pdfBytes must be memoized by the parent (passed as stable reference)
  // react-pdf uses === comparison to detect file changes; new ArrayBuffer each render causes reload loop
  return (
    <div style={{ position: 'fixed', inset: 0, background: 'rgba(0,0,0,0.7)', zIndex: 50, display: 'flex', alignItems: 'center', justifyContent: 'center' }}>
      <div style={{ background: '#fff', borderRadius: 8, maxWidth: 900, width: '90vw', maxHeight: '90vh', overflow: 'auto', padding: 16 }}>
        <div style={{ display: 'flex', justifyContent: 'space-between', marginBottom: 8 }}>
          <div style={{ display: 'flex', gap: 8 }}>
            <button onClick={() => setPageNumber(p => Math.max(1, p - 1))} disabled={pageNumber <= 1}>Prev</button>
            <span>{pageNumber} / {numPages || '?'}</span>
            <button onClick={() => setPageNumber(p => Math.min(numPages, p + 1))} disabled={pageNumber >= numPages}>Next</button>
          </div>
          <button onClick={onClose}>Close</button>
        </div>
        <Document
          file={pdfBytes}
          onLoadSuccess={({ numPages }) => setNumPages(numPages)}
        >
          <Page pageNumber={pageNumber} />
        </Document>
      </div>
    </div>
  );
}
```

### Pattern 3: Send Button Gating and Staleness Detection

**What:** `PreparePanel` tracks a `previewToken` (string | null). Token is set to a timestamp string after a successful preview fetch. Token is reset to null whenever the agent changes any text fill or adds/removes fields. Send is disabled while `previewToken === null`.

**When to use:** All Send-button gating lives in `PreparePanel`. FieldPlacer needs an `onFieldsChanged` callback prop so it can notify PreparePanel when fields are persisted.

**Example:**
```typescript
// In PreparePanel.tsx
const [previewToken, setPreviewToken] = useState<string | null>(null);
const [previewBytes, setPreviewBytes] = useState<ArrayBuffer | null>(null);
const [showPreview, setShowPreview] = useState(false);

// When text fill changes:
function handleTextFillChange(data: Record<string, string>) {
  setTextFillData(data);
  setPreviewToken(null);  // Stale — re-preview required
}

// When fields change (callback from FieldPlacer):
function handleFieldsChanged() {
  setPreviewToken(null);  // Stale
}

async function handlePreview() {
  setLoading(true);
  const res = await fetch(`/api/documents/${docId}/preview`, {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ textFillData }),
  });
  if (res.ok) {
    const bytes = await res.arrayBuffer();
    setPreviewBytes(bytes);
    setPreviewToken(Date.now().toString());  // Mark as fresh
    setShowPreview(true);
  } else {
    // Show error
  }
  setLoading(false);
}

// Send button is disabled until preview is fresh:
<button onClick={handlePreview}>Preview</button>
{showPreview && previewBytes && (
  <PreviewModal pdfBytes={previewBytes} onClose={() => setShowPreview(false)} />
)}
<button
  onClick={handlePrepareAndSend}
  disabled={loading || previewToken === null || parseEmails(recipients).length === 0}
>
  Prepare and Send
</button>
```

### Pattern 4: FieldPlacer onFieldsChanged Callback

**What:** `FieldPlacer` currently calls `persistFields()` internally after every drag/drop/delete/resize. Adding an `onFieldsChanged?: () => void` prop and calling it after every `persistFields()` call lets `PreparePanel` reset the stale token.

**Example:**
```typescript
// FieldPlacer.tsx — add prop and call after persistFields
interface FieldPlacerProps {
  docId: string;
  pageInfo: PageInfo | null;
  currentPage: number;
  children: React.ReactNode;
  readOnly?: boolean;
  onFieldsChanged?: () => void;  // NEW
}

// Inside handleDragEnd, handleZonePointerUp after persistFields():
persistFields(docId, next);
onFieldsChanged?.();
```

### Anti-Patterns to Avoid

- **Overwriting `_prepared.pdf` with preview**: The preview MUST use a different path. The prepare route writes to `_prepared.pdf`; the preview route must use a timestamp-versioned path.
- **Storing ArrayBuffer in a ref instead of state**: `react-pdf` uses `===` comparison on the `file` prop. If you store bytes in a ref and pass `ref.current` to `Document`, React will not detect the change. Use `useState<ArrayBuffer | null>`.
- **Not memoizing the `file` prop**: If `previewBytes` is recreated on every render (e.g., `new Uint8Array(...)` inline), react-pdf reloads the PDF on every render. Store the `ArrayBuffer` returned by `res.arrayBuffer()` directly in state and pass that state variable — it is stable between renders.
- **Configuring the pdfjs worker more than once**: If PreviewModal is its own component (not inside PdfViewer), it needs its own `pdfjs.GlobalWorkerOptions.workerSrc` assignment. This is fine — the global is idempotent when set to the same value.
- **Leaking preview files**: The preview route should delete the file after reading it (`unlink` after `readFile`). Fire-and-forget is acceptable since the file is ephemeral. Do not leave preview files accumulating in uploads/.
- **Re-enabling Send without a fresh preview when fields are locked**: The `readOnly` guard in FieldPlacer prevents field changes after status is not Draft. But PreparePanel's `currentStatus !== 'Draft'` branch already returns early, so this is not reachable in practice.

## Don't Hand-Roll

| Problem | Don't Build | Use Instead | Why |
|---------|-------------|-------------|-----|
| PDF rendering in modal | Custom canvas renderer | `react-pdf` `<Document>` + `<Page>` | Already installed; handles PDF.js worker, page scaling, canvas rendering |
| PDF bytes generation | Custom PDF manipulation | Existing `preparePdf()` in `lib/pdf/prepare-document.ts` | Already handles all field types, atomic write, magic-byte verification |
| Byte streaming from API | Custom file serving | Standard `new Response(pdfBytes, { headers: ... })` | Same pattern as `/api/documents/[id]/file` route |
| Staleness detection | Complex hash diffing of fields | Simple token reset on any change | Field positions change frequently; any mutation invalidates preview |

**Key insight:** Every required building block already exists in the codebase. Phase 12 is primarily wiring: a new POST route, a new modal component, and state management in PreparePanel.

## Common Pitfalls

### Pitfall 1: ArrayBuffer Equality and react-pdf Reload Loop
**What goes wrong:** `react-pdf` re-fetches/re-renders the PDF every time the `file` prop reference changes (strict `===` comparison). If you do `file={new Uint8Array(previewBytes)}` or rebuild the bytes object on each render, the PDF continuously reloads.
**Why it happens:** react-pdf uses a memoized source hook internally that compares the `file` prop by reference.
**How to avoid:** Store the `ArrayBuffer` from `res.arrayBuffer()` directly in React state (`useState<ArrayBuffer | null>`). Pass the state variable directly — never wrap it in `new Uint8Array()` or `{ data: ... }` on every render.
**Warning signs:** PDF shows "Loading PDF..." repeatedly after first render; network tab shows repeated preview API calls.

### Pitfall 2: pdfjs Worker Not Configured in PreviewModal
**What goes wrong:** `react-pdf` throws "Setting up fake worker failed" or shows "Failed to load PDF file."
**Why it happens:** The worker is configured in `PdfViewer.tsx` but that assignment is module-scoped to that file's execution context. A new component that imports `react-pdf` independently may execute before PdfViewer.
**How to avoid:** Set `pdfjs.GlobalWorkerOptions.workerSrc` at the top of PreviewModal.tsx exactly as done in PdfViewer.tsx (same `new URL('pdfjs-dist/build/pdf.worker.min.mjs', import.meta.url)` pattern).
**Warning signs:** Modal opens but shows error state immediately.

### Pitfall 3: Preview File Path Collision with Prepared File
**What goes wrong:** Preview overwrites `_prepared.pdf`, corrupting the final prepared document.
**Why it happens:** Copying the path logic from the prepare route without changing the suffix.
**How to avoid:** Use a timestamp suffix: `doc.filePath.replace(/\.pdf$/, `_preview_${Date.now()}.pdf`)`. Never use `_prepared.pdf` as the preview destination.
**Warning signs:** Prepared document shows "draft preview" content (e.g., "Sign Here" placeholder) after agent sends.

### Pitfall 4: Preview File Not Cleaned Up
**What goes wrong:** Preview PDFs accumulate in `uploads/` directory indefinitely.
**Why it happens:** Route writes the file for streaming but doesn't delete it after reading.
**How to avoid:** After `readFile(previewPath)` succeeds, call `unlink(previewPath).catch(() => {})` (fire-and-forget). The preview file is ephemeral — delete it immediately after reading bytes.
**Warning signs:** `uploads/` grows with `*_preview_*.pdf` files.

### Pitfall 5: Send Button Gating Not Propagated Through FieldPlacer Changes
**What goes wrong:** Agent adds or moves a field, then clicks Send without re-previewing — the staleness token is not reset.
**Why it happens:** FieldPlacer calls `persistFields()` internally and has no way to notify PreparePanel that fields changed.
**How to avoid:** Add `onFieldsChanged?: () => void` to `FieldPlacerProps`. Call it after every `persistFields()` invocation (inside `handleDragEnd`, `handleZonePointerUp`, and the delete button handler). In PreparePanel, pass `onFieldsChanged={() => setPreviewToken(null)}` to FieldPlacer.
**Warning signs:** Manual test — place a new field after previewing; Send button stays enabled when it should be disabled.

### Pitfall 6: Deployment Target — Vercel Serverless vs. Home Docker
**What goes wrong:** The write-then-read-then-unlink pattern silently fails on Vercel's serverless runtime (ephemeral/read-only filesystem outside /tmp).
**Why it happens:** Vercel's serverless functions have no persistent writable filesystem outside `/tmp`. `uploads/` is a Docker volume on the home server, not accessible on Vercel.
**How to avoid:** The STATE.md already flags this: "Deployment target (Vercel serverless vs. self-hosted container) must be confirmed before implementing preview route." Based on the entire architecture (uploads/, Docker-volume-based file storage, home server deployment), this app runs on a self-hosted Docker container — not Vercel. The write-to-disk pattern is correct for this environment.
**Warning signs:** If ever deploying to Vercel, the preview route would need to use `/tmp` as the scratch directory, not `uploads/`.

### Pitfall 7: 422 Guards Mirror Mismatch
**What goes wrong:** Preview route and prepare route get out of sync on 422 guards (e.g., prepare route gets a new guard added in a future phase but preview doesn't).
**How to avoid:** The preview route should duplicate the same guards as the prepare route for agent-signature and agent-initials fields. Both must block if the relevant field type is present but the signature data is missing.

## Code Examples

Verified patterns from official sources:

### react-pdf ArrayBuffer file prop (verified from installed v10.4.1 types)
```typescript
// Source: node_modules/react-pdf/dist/shared/types.d.ts
// export type File = string | ArrayBuffer | Blob | Source | null;

// Correct: pass ArrayBuffer directly from res.arrayBuffer()
const bytes = await res.arrayBuffer();
setPreviewBytes(bytes); // store in state

// In render:
<Document file={previewBytes}> // previewBytes is stable (state reference)
  <Page pageNumber={pageNumber} />
</Document>
```

### Next.js Route Handler returning PDF bytes (verified from existing codebase)
```typescript
// Source: /src/app/api/documents/[id]/file/route.ts (existing)
const buffer = await readFile(filePath);
return new Response(buffer, {
  headers: { 'Content-Type': 'application/pdf' },
});
```

### pdfjs worker configuration (verified from PdfViewer.tsx)
```typescript
// Source: /src/app/portal/(protected)/documents/[docId]/_components/PdfViewer.tsx
pdfjs.GlobalWorkerOptions.workerSrc = new URL(
  'pdfjs-dist/build/pdf.worker.min.mjs',
  import.meta.url,
).toString();
```

### Versioned preview path pattern
```typescript
// Avoid collision with _prepared.pdf; use timestamp for uniqueness
const timestamp = Date.now();
const previewRelPath = doc.filePath.replace(/\.pdf$/, `_preview_${timestamp}.pdf`);
const previewPath = path.join(UPLOADS_DIR, previewRelPath);

// Path traversal guard (required on every file operation)
if (!previewPath.startsWith(UPLOADS_DIR)) {
  return new Response('Forbidden', { status: 403 });
}
```

### Ephemeral file cleanup
```typescript
// Read bytes first, then fire-and-forget delete
const pdfBytes = await readFile(previewPath);
unlink(previewPath).catch(() => {}); // ephemeral — delete after read
return new Response(pdfBytes, { headers: { 'Content-Type': 'application/pdf' } });
```

### Staleness token pattern
```typescript
// In PreparePanel state:
const [previewToken, setPreviewToken] = useState<string | null>(null);

// Set on successful preview:
setPreviewToken(Date.now().toString());

// Reset on any state change:
setPreviewToken(null);

// Gate the Send button:
disabled={loading || previewToken === null || parseEmails(recipients).length === 0}
```

## State of the Art

| Old Approach | Current Approach | When Changed | Impact |
|--------------|------------------|--------------|--------|
| Generating preview as a separate pipeline | Reuse existing `preparePdf()` | Always (by design) | Zero duplication; preview is guaranteed to match the actual prepare output |
| Blob URL for PDF bytes | `ArrayBuffer` direct to react-pdf | react-pdf v5+ | No blob URL creation/revocation needed; cleaner lifecycle |

**Deprecated/outdated:**
- **`pdfjs-dist` legacy build**: An earlier decision used the legacy build for server-side text extraction in Phase 13. Phase 12 uses the standard (non-legacy) build for client-side rendering — no change required since react-pdf already bundles the correct worker.

## Open Questions

1. **Preview file cleanup if route errors mid-stream**
   - What we know: `unlink` is called after `readFile`, so if `preparePdf` fails, no file is written and nothing to clean up. If `readFile` fails (unusual), the file would linger.
   - What's unclear: Whether a try/finally cleanup wrapper is warranted.
   - Recommendation: Wrap in try/finally: `try { await preparePdf(...); const bytes = await readFile(previewPath); return new Response(bytes, ...); } finally { unlink(previewPath).catch(() => {}); }` — ensures cleanup even on unexpected errors.

2. **Scroll-to-page in PreviewModal**
   - What we know: The existing PdfViewer shows one page at a time with Prev/Next buttons. PreviewModal can use the same pattern.
   - What's unclear: Whether the agent needs scroll-all-pages vs. page-by-page navigation in the modal.
   - Recommendation: Use the same page-by-page Prev/Next pattern from PdfViewer — no need for scroll-all-pages in a modal.

3. **Modal scroll on small screens**
   - What we know: The modal will contain a multi-page PDF.
   - Recommendation: Use `overflow-y: auto` on the modal container and `max-height: 90vh` — same approach as the sticky PreparePanel.

## Sources

### Primary (HIGH confidence)
- `node_modules/react-pdf/dist/shared/types.d.ts` — `File` type definition confirming `ArrayBuffer` is accepted
- `node_modules/react-pdf/dist/Document.d.ts` — `DocumentProps.file` prop docs
- `/src/lib/pdf/prepare-document.ts` — Existing `preparePdf()` function signature and behavior
- `/src/app/api/documents/[id]/prepare/route.ts` — Prepare route pattern (auth, guards, path construction)
- `/src/app/api/documents/[id]/file/route.ts` — PDF byte-streaming pattern
- `/src/app/portal/(protected)/documents/[docId]/_components/PdfViewer.tsx` — Worker configuration pattern
- `/src/app/portal/(protected)/documents/[docId]/_components/PreparePanel.tsx` — Current Send button logic
- `/src/app/portal/(protected)/documents/[docId]/_components/FieldPlacer.tsx` — `persistFields` call sites

### Secondary (MEDIUM confidence)
- `.planning/STATE.md` — Phase 12 deployment concern (Vercel vs. home Docker server)
- `.planning/ROADMAP.md` — Phase 12 plan descriptions

### Tertiary (LOW confidence)
- None

## Metadata

**Confidence breakdown:**
- Standard stack: HIGH — all dependencies already installed; type definitions verified from installed packages
- Architecture: HIGH — patterns lifted directly from existing codebase; `preparePdf()` reuse is confirmed
- Pitfalls: HIGH — ArrayBuffer equality trap verified from react-pdf type docs; file cleanup and path collision are verifiable from existing code patterns; deployment concern documented in STATE.md

**Research date:** 2026-03-21
**Valid until:** 2026-04-20 (stable libraries — react-pdf, pdf-lib are unlikely to change in 30 days)
