feat(pdf): anchored scratch links replace board

Replace the rejected standalone sticky-card board with the real
feature: place a link anchor anywhere on a PDF page, tap it to open
split view whose right pane is THAT anchor's own infinite scratchpad
(keyed by anchor id) — like a paper sticky-note tab.

- ScratchLink model + scratch_links table (id, doc, page, nx, ny).
- PDF editor: "place link" tool drops/loads/shows tappable markers;
  tap opens SplitViewScreen for that anchor; long-press deletes.
- SplitViewScreen rebuilt on pdfrx (was syncfusion), right scratchpad
  keyed by scratchLinkId, new brush palette (was AnnotationToolbar).
- Remove board_screen + its test + the home board entry.

analyze clean, tests green.

lib/editor/canvas/pen_editor_screen.dart

@@ -23,8 +23,11 @@
 import 'package:flutter/gestures.dart' show PointerDeviceKind;
 import 'package:flutter/material.dart';
 import 'package:pdfrx/pdfrx.dart';
+import 'package:uuid/uuid.dart';
 
 import '../../l10n/app_localizations.dart';
+import '../../models/scratch_link.dart';
+import '../../screens/split_view_screen.dart';
 import '../../services/database_service.dart';
 import '../engine/brush.dart';
 import '../engine/stroke_eraser.dart';
@@ -53,6 +56,10 @@ import 'pen_stroke.dart';
 /// Produces a fixed-length hex string from the path so the id is filesystem-
 /// independent (no slashes, spaces, or non-ASCII characters) and stable across
 /// restarts. Collisions are astronomically unlikely for a single-user app.
+/// On-screen size (px) of a scratch-link anchor marker. Fixed in screen space
+/// (not scaled with zoom) so the tap target stays comfortably tappable.
+const double _kMarkerSize = 36.0;
+
 String _documentIdFromPath(String path) {
   var hash = 5381;
   for (final c in path.codeUnits) {
@@ -156,6 +163,17 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
   /// pen falls through to pdfrx for native text selection.
   bool _selectTextMode = false;
 
+  /// When true the "place scratch link" tool is active: a tap on a page drops a
+  /// new anchor (a sticky-note tab) instead of inking. Pen capture is disabled
+  /// so the tap is handled by the per-page GestureDetector overlay.
+  bool _placeLinkMode = false;
+
+  /// All scratch-link anchors for this document, loaded on open and updated on
+  /// add/delete. Rendered as tappable markers in [pageOverlaysBuilder].
+  final List<ScratchLink> _scratchLinks = [];
+
+  static const _uuid = Uuid();
+
   Color _color = Colors.black;
   bool _allowFingerDrawing = false;
 
@@ -176,8 +194,9 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
   ];
 
   /// True when a PEN tool (pen/highlighter/eraser) is active — pen capture is on.
-  /// False in select-text mode so the pen reaches pdfrx text selection.
+  /// False in select-text mode (pen reaches pdfrx text selection) and in
-  bool get _penCaptureEnabled => !_selectTextMode;
+  /// place-link mode (a tap drops an anchor via the page overlay).
+  bool get _penCaptureEnabled => !_selectTextMode && !_placeLinkMode;
 
   /// True when the eraser tool is active.
   bool get _isEraser => _tool == CanvasTool.eraser && !_selectTextMode;
@@ -221,6 +240,18 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
     }
     _saveScheduler = scheduler;
     await _loadPersistedStrokes(repo);
+    await _loadScratchLinks(service);
+  }
+
+  /// Load this document's scratch-link anchors into [_scratchLinks].
+  Future<void> _loadScratchLinks(DatabaseService service) async {
+    final links = await service.loadScratchLinks(_documentId);
+    if (!mounted) return;
+    setState(() {
+      _scratchLinks
+        ..clear()
+        ..addAll(links);
+    });
   }
 
   /// Load all persisted strokes for [_documentId] and populate [_strokesByPage].
@@ -618,11 +649,84 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
     setState(() {
       _tool = tool;
       _selectTextMode = false;
+      _placeLinkMode = false;
     });
   }
 
   void _enableSelectText() {
-    setState(() => _selectTextMode = true);
+    setState(() {
+      _selectTextMode = true;
+      _placeLinkMode = false;
+    });
+  }
+
+  /// Toggle "place scratch link" mode. While active, a tap on a page drops a
+  /// new anchor at the tapped normalized position.
+  void _togglePlaceLinkMode() {
+    setState(() {
+      _placeLinkMode = !_placeLinkMode;
+      if (_placeLinkMode) _selectTextMode = false;
+    });
+  }
+
+  // ── Scratch-link anchors ─────────────────────────────────────────────────────
+
+  /// Create + persist a new anchor at normalized [normalized] on [pageIndex],
+  /// then show it. Leaves place-link mode on so several can be dropped in a row.
+  Future<void> _placeScratchLink(int pageIndex, Offset normalized) async {
+    final link = ScratchLink(
+      id: _uuid.v4(),
+      documentId: _documentId,
+      pageIndex: pageIndex,
+      nx: normalized.dx.clamp(0.0, 1.0),
+      ny: normalized.dy.clamp(0.0, 1.0),
+    );
+    final service = await DatabaseService.getInstance();
+    await service.saveScratchLink(link);
+    if (!mounted) return;
+    setState(() => _scratchLinks.add(link));
+  }
+
+  /// Open the anchor's split view (left = this PDF at the anchor page, right =
+  /// the anchor's private infinite scratchpad).
+  void _openScratchLink(ScratchLink link) {
+    Navigator.of(context).push(
+      MaterialPageRoute(
+        builder: (_) => SplitViewScreen(
+          filePath: widget.pdfPath,
+          documentId: _documentId,
+          scratchLinkId: link.id,
+          initialPage: link.pageIndex,
+        ),
+      ),
+    );
+  }
+
+  /// Confirm + delete an anchor (and its private scratchpad).
+  Future<void> _confirmDeleteScratchLink(ScratchLink link) async {
+    final l = AppLocalizations.of(context);
+    final confirmed = await showDialog<bool>(
+      context: context,
+      builder: (ctx) => AlertDialog(
+        title: Text(l.scratchLinkDeleteTitle),
+        content: Text(l.scratchLinkDeleteBody),
+        actions: [
+          TextButton(
+            onPressed: () => Navigator.pop(ctx, false),
+            child: Text(l.cancel),
+          ),
+          TextButton(
+            onPressed: () => Navigator.pop(ctx, true),
+            child: Text(l.delete),
+          ),
+        ],
+      ),
+    );
+    if (confirmed != true) return;
+    final service = await DatabaseService.getInstance();
+    await service.deleteScratchLink(link.id);
+    if (!mounted) return;
+    setState(() => _scratchLinks.removeWhere((s) => s.id == link.id));
   }
 
   void _toggleFingerDrawing() {
@@ -727,6 +831,10 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
         // normalized page space scaled to the on-screen page rect.
         pageOverlaysBuilder: (context, pageRectInViewer, page) {
           final pageIndex = page.pageNumber - 1;
+          final pageW = pageRectInViewer.width;
+          final pageH = pageRectInViewer.height;
+          final linksOnPage =
+              _scratchLinks.where((l) => l.pageIndex == pageIndex);
           return [
             Positioned.fill(
               child: IgnorePointer(
@@ -744,6 +852,35 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
                 ),
               ),
             ),
+            // Tap-to-place layer: only swallows taps while place-link mode is on.
+            // Otherwise it's a no-op (IgnorePointer) so ink/scroll fall through.
+            if (_placeLinkMode)
+              Positioned.fill(
+                child: GestureDetector(
+                  behavior: HitTestBehavior.opaque,
+                  onTapUp: (details) {
+                    final local = details.localPosition;
+                    if (pageW <= 0 || pageH <= 0) return;
+                    final nx = (local.dx / pageW).clamp(0.0, 1.0);
+                    final ny = (local.dy / pageH).clamp(0.0, 1.0);
+                    _placeScratchLink(pageIndex, Offset(nx, ny));
+                  },
+                ),
+              ),
+            // Anchor markers (sticky-note tabs): tap → split view, long-press →
+            // delete. Sized in screen px so the tap target stays usable at any
+            // zoom; positioned at (nx*pageW, ny*pageH).
+            for (final link in linksOnPage)
+              Positioned(
+                left: link.nx * pageW - _kMarkerSize / 2,
+                top: link.ny * pageH - _kMarkerSize / 2,
+                width: _kMarkerSize,
+                height: _kMarkerSize,
+                child: _ScratchLinkMarker(
+                  onTap: () => _openScratchLink(link),
+                  onLongPress: () => _confirmDeleteScratchLink(link),
+                ),
+              ),
           ];
         },
         // (2) Viewer-level pen capture. Stylus is captured ONLY when a pen tool
@@ -814,6 +951,14 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
               onPressed: _hasSelection ? _highlightSelection : null,
             ),
             PaletteDivider(cs: cs),
+            // Place scratch link (sticky-note tab). Tap a page to drop an anchor.
+            ToolButton(
+              icon: Icons.sticky_note_2_outlined,
+              selected: _placeLinkMode,
+              tooltip: l.toolPlaceScratchLink,
+              onPressed: _togglePlaceLinkMode,
+            ),
+            PaletteDivider(cs: cs),
             // Undo / redo (per page).
             ToolButton(
               icon: Icons.undo,
@@ -1087,3 +1232,35 @@ class _PageOverlayPainter extends CustomPainter {
       old.pageSize != pageSize ||
       old.thinning != thinning;
 }
+
+/// A small sticky-note "tab" marker glued to a page at a scratch-link anchor.
+/// Tap opens the anchor's split view; long-press deletes the anchor.
+class _ScratchLinkMarker extends StatelessWidget {
+  const _ScratchLinkMarker({required this.onTap, required this.onLongPress});
+
+  final VoidCallback onTap;
+  final VoidCallback onLongPress;
+
+  @override
+  Widget build(BuildContext context) {
+    final cs = Theme.of(context).colorScheme;
+    return GestureDetector(
+      behavior: HitTestBehavior.opaque,
+      onTap: onTap,
+      onLongPress: onLongPress,
+      child: Material(
+        color: cs.tertiaryContainer,
+        elevation: 2,
+        shape: RoundedRectangleBorder(
+          borderRadius: BorderRadius.circular(8),
+          side: BorderSide(color: cs.outlineVariant),
+        ),
+        child: Icon(
+          Icons.sticky_note_2,
+          size: 20,
+          color: cs.onTertiaryContainer,
+        ),
+      ),
+    );
+  }
+}

lib/l10n/app_en.arb

@@ -63,6 +63,9 @@
   "nextPage": "Next page",
   "toolSelectText": "Select text",
   "actionHighlightSelection": "Highlight selection",
+  "toolPlaceScratchLink": "Place scratch link",
+  "scratchLinkDeleteTitle": "Delete scratch link?",
+  "scratchLinkDeleteBody": "This removes the anchor and its private scratchpad.",
   "failedToOpenPdf": "Failed to open PDF:\n{error}",
   "@failedToOpenPdf": {
     "placeholders": { "error": { "type": "String" } }

lib/l10n/app_localizations.dart

@@ -422,6 +422,24 @@ abstract class AppLocalizations {
   /// **'Highlight selection'**
   String get actionHighlightSelection;
 
+  /// No description provided for @toolPlaceScratchLink.
+  ///
+  /// In en, this message translates to:
+  /// **'Place scratch link'**
+  String get toolPlaceScratchLink;
+
+  /// No description provided for @scratchLinkDeleteTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Delete scratch link?'**
+  String get scratchLinkDeleteTitle;
+
+  /// No description provided for @scratchLinkDeleteBody.
+  ///
+  /// In en, this message translates to:
+  /// **'This removes the anchor and its private scratchpad.'**
+  String get scratchLinkDeleteBody;
+
   /// No description provided for @failedToOpenPdf.
   ///
   /// In en, this message translates to:

lib/l10n/app_localizations_en.dart

@@ -176,6 +176,16 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get actionHighlightSelection => 'Highlight selection';
 
+  @override
+  String get toolPlaceScratchLink => 'Place scratch link';
+
+  @override
+  String get scratchLinkDeleteTitle => 'Delete scratch link?';
+
+  @override
+  String get scratchLinkDeleteBody =>
+      'This removes the anchor and its private scratchpad.';
+
   @override
   String failedToOpenPdf(String error) {
     return 'Failed to open PDF:\n$error';

lib/l10n/app_localizations_zh.dart

@@ -176,6 +176,15 @@ class AppLocalizationsZh extends AppLocalizations {
   @override
   String get actionHighlightSelection => '高亮所选';
 
+  @override
+  String get toolPlaceScratchLink => '放置便签链接';
+
+  @override
+  String get scratchLinkDeleteTitle => '删除便签链接？';
+
+  @override
+  String get scratchLinkDeleteBody => '这会移除锚点及其专属草稿纸。';
+
   @override
   String failedToOpenPdf(String error) {
     return '打开 PDF 失败：\n$error';

lib/l10n/app_zh.arb

@@ -54,6 +54,9 @@
   "nextPage": "下一页",
   "toolSelectText": "选择文字",
   "actionHighlightSelection": "高亮所选",
+  "toolPlaceScratchLink": "放置便签链接",
+  "scratchLinkDeleteTitle": "删除便签链接？",
+  "scratchLinkDeleteBody": "这会移除锚点及其专属草稿纸。",
   "failedToOpenPdf": "打开 PDF 失败：\n{error}",
   "pdfNoPages": "PDF 没有任何页面。",
   "pageOfPages": "{current} / {total}",

lib/models/scratch_link.dart

@@ -0,0 +1,88 @@
+// lib/models/scratch_link.dart
+//
+// A PDF-anchored scratch link: a sticky-note "tab" placed at a normalized
+// position (nx, ny in [0,1]) on a specific page of a document. Tapping the
+// anchor opens a split view whose right pane is an infinite freehand scratchpad
+// that BELONGS TO THIS ANCHOR (keyed by [id]).
+//
+// Plain immutable value class (no freezed codegen) so it compiles without a
+// build_runner step. Equality is by value so anchors can be diffed in lists.
+
+import 'package:flutter/foundation.dart';
+
+@immutable
+class ScratchLink {
+  const ScratchLink({
+    required this.id,
+    required this.documentId,
+    required this.pageIndex,
+    required this.nx,
+    required this.ny,
+  });
+
+  /// Stable anchor id (uuid). Doubles as the scratchpad storage key so each
+  /// anchor gets its own private infinite scratchpad.
+  final String id;
+
+  /// The owning document (the editor's stable document-id for the PDF path).
+  final String documentId;
+
+  /// 0-based page the anchor sits on.
+  final int pageIndex;
+
+  /// Normalized horizontal position on the page, in [0, 1].
+  final double nx;
+
+  /// Normalized vertical position on the page, in [0, 1].
+  final double ny;
+
+  ScratchLink copyWith({
+    String? id,
+    String? documentId,
+    int? pageIndex,
+    double? nx,
+    double? ny,
+  }) =>
+      ScratchLink(
+        id: id ?? this.id,
+        documentId: documentId ?? this.documentId,
+        pageIndex: pageIndex ?? this.pageIndex,
+        nx: nx ?? this.nx,
+        ny: ny ?? this.ny,
+      );
+
+  Map<String, dynamic> toJson() => {
+        'id': id,
+        'documentId': documentId,
+        'pageIndex': pageIndex,
+        'nx': nx,
+        'ny': ny,
+      };
+
+  factory ScratchLink.fromJson(Map<String, dynamic> json) => ScratchLink(
+        id: json['id'] as String,
+        documentId: json['documentId'] as String,
+        pageIndex: (json['pageIndex'] as num).toInt(),
+        nx: (json['nx'] as num).toDouble(),
+        ny: (json['ny'] as num).toDouble(),
+      );
+
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) ||
+      other is ScratchLink &&
+          runtimeType == other.runtimeType &&
+          id == other.id &&
+          documentId == other.documentId &&
+          pageIndex == other.pageIndex &&
+          nx == other.nx &&
+          ny == other.ny;
+
+  @override
+  int get hashCode => Object.hash(id, documentId, pageIndex, nx, ny);
+
+  @override
+  String toString() =>
+      'ScratchLink(id: $id, documentId: $documentId, pageIndex: $pageIndex, '
+      'nx: $nx, ny: $ny)';
+}

lib/screens/board_screen.dart

@@ -1,650 +0,0 @@
-// lib/screens/board_screen.dart
-//
-// F7 — 双链 + 无限便利贴. The reachable, persisted UI over the (already
-// unit-tested) pure [Board] model. An infinite InteractiveViewer canvas hosts a
-// Stack of sticky cards; cards are draggable, their text is editable, and
-// `[[link]]` spans render as tappable chips that pan/center the board on the
-// matching card (matched by card id — see _navigateToTarget). Backlinks
-// ("什么链接到这里 / Linked from") come straight from board.backlinksOf(id).
-//
-// Persistence: the whole board is saved (cards as rows) via
-// DatabaseService.saveBoardCards, debounced 800ms like the ink editors, and
-// reloaded on open so it survives a restart.
-//
-// OUT OF SCOPE (left as TODOs):
-// TODO(board-ink): ink/handwriting ON cards (the model mentions a per-card
-//   StrokeHost). Not wired here.
-// TODO(board-multi): multiple-boards management UI. A single default board id
-//   is used when [boardId] is null; the param is kept for later.
-// TODO(board-link-autocomplete): real-time `[[` autocomplete of card ids.
-
-import 'dart:async';
-
-import 'package:flutter/material.dart';
-
-import '../editor/board/board.dart';
-import '../l10n/app_localizations.dart';
-import '../services/database_service.dart';
-
-/// Default board id used when no [boardId] is supplied. One shared board is in
-/// scope for increment 1; the param survives for future multi-board support.
-const String kDefaultBoardId = 'default';
-
-/// Default size of a freshly added sticky card (board content coordinates).
-const Size _kNewCardSize = Size(180, 140);
-
-/// The logical extent of the (conceptually infinite) board content. The Stack
-/// needs a finite size; InteractiveViewer's infinite boundaryMargin lets the
-/// user pan far beyond it, and cards are centered around the middle so there is
-/// room to grow in every direction.
-const double _kCanvasExtent = 100000;
-const Offset _kCanvasCenter = Offset(_kCanvasExtent / 2, _kCanvasExtent / 2);
-
-class BoardScreen extends StatefulWidget {
-  const BoardScreen({super.key, this.boardId});
-
-  /// Which board to open. Null → [kDefaultBoardId].
-  final String? boardId;
-
-  @override
-  State<BoardScreen> createState() => _BoardScreenState();
-}
-
-class _BoardScreenState extends State<BoardScreen> {
-  final TransformationController _transform = TransformationController();
-
-  /// The viewport size of the InteractiveViewer (used to center on a card).
-  final GlobalKey _viewportKey = GlobalKey();
-
-  Board _board = Board.empty;
-  bool _loading = true;
-
-  /// Currently selected card (drives the backlinks panel + delete affordance).
-  String? _selectedId;
-
-  /// The card whose text is being edited inline (null = none).
-  String? _editingId;
-
-  DatabaseService? _db;
-  Timer? _saveTimer;
-
-  String get _boardId => widget.boardId ?? kDefaultBoardId;
-
-  @override
-  void initState() {
-    super.initState();
-    // Center the initial viewport on the canvas center so newly added cards
-    // (placed at viewport center, which starts near _kCanvasCenter) are onscreen.
-    WidgetsBinding.instance.addPostFrameCallback((_) => _centerInitially());
-    _load();
-  }
-
-  @override
-  void dispose() {
-    // Flush any pending debounced save synchronously-ish before tearing down.
-    _saveTimer?.cancel();
-    if (_dirty) {
-      // Fire-and-forget: the future completes after dispose but the write still
-      // lands (DatabaseService is a singleton independent of this widget).
-      unawaited(_db?.saveBoardCards(_boardId, _board.cards) ?? Future.value());
-    }
-    _transform.dispose();
-    super.dispose();
-  }
-
-  bool _dirty = false;
-
-  Future<void> _load() async {
-    final db = await DatabaseService.getInstance();
-    final board = await db.loadBoard(_boardId);
-    if (!mounted) return;
-    setState(() {
-      _db = db;
-      _board = board;
-      _loading = false;
-    });
-  }
-
-  void _centerInitially() {
-    final ctx = _viewportKey.currentContext;
-    if (ctx == null) return;
-    final size = ctx.size;
-    if (size == null) return;
-    // Translate so that _kCanvasCenter sits at the middle of the viewport.
-    _transform.value = Matrix4.identity()
-      ..translateByDouble(
-        size.width / 2 - _kCanvasCenter.dx,
-        size.height / 2 - _kCanvasCenter.dy,
-        0,
-        1,
-      );
-  }
-
-  // ── Mutations (each schedules a debounced save) ────────────────────────
-
-  void _mutate(Board next) {
-    setState(() => _board = next);
-    _dirty = true;
-    _scheduleSave();
-  }
-
-  void _scheduleSave() {
-    _saveTimer?.cancel();
-    // Capture the snapshot synchronously (Board is immutable, so _board is a
-    // stable value) — same discipline as SaveScheduler.
-    final snapshot = _board;
-    final boardId = _boardId;
-    _saveTimer = Timer(const Duration(milliseconds: 800), () async {
-      await _db?.saveBoardCards(boardId, snapshot.cards);
-      _dirty = false;
-    });
-  }
-
-  String _newCardId() => 'card-${DateTime.now().microsecondsSinceEpoch}';
-
-  /// The board-content point currently at the center of the viewport.
-  Offset _viewportCenterInContent() {
-    final ctx = _viewportKey.currentContext;
-    final size = ctx?.size ?? const Size(400, 600);
-    final viewportCenter = Offset(size.width / 2, size.height / 2);
-    // Inverse-map the viewport-center screen point into content coordinates.
-    final inv = Matrix4.inverted(_transform.value);
-    return MatrixUtils.transformPoint(inv, viewportCenter);
-  }
-
-  void _addCard() {
-    final center = _viewportCenterInContent();
-    final pos = center -
-        Offset(_kNewCardSize.width / 2, _kNewCardSize.height / 2);
-    final l = AppLocalizations.of(context);
-    final card = BoardCard(
-      id: _newCardId(),
-      position: pos,
-      size: _kNewCardSize,
-      text: l.boardNewCardText,
-    );
-    _mutate(_board.add(card));
-    setState(() {
-      _selectedId = card.id;
-      _editingId = card.id;
-    });
-  }
-
-  void _deleteCard(String id) {
-    _mutate(_board.removeById(id));
-    setState(() {
-      if (_selectedId == id) _selectedId = null;
-      if (_editingId == id) _editingId = null;
-    });
-  }
-
-  /// Pan/center the board on the card matching [target] (matched by card id).
-  /// Returns false when no such card exists (a dangling link).
-  bool _navigateToTarget(String target) {
-    final card = _board.cardById(target);
-    if (card == null) return false;
-    _centerOnCard(card);
-    setState(() => _selectedId = card.id);
-    return true;
-  }
-
-  void _centerOnCard(BoardCard card) {
-    final ctx = _viewportKey.currentContext;
-    final size = ctx?.size ?? const Size(400, 600);
-    final scale = _transform.value.getMaxScaleOnAxis();
-    final cardCenter = card.position +
-        Offset(card.size.width / 2, card.size.height / 2);
-    // We want: screen = scale * content + translation == viewportCenter.
-    final tx = size.width / 2 - scale * cardCenter.dx;
-    final ty = size.height / 2 - scale * cardCenter.dy;
-    _transform.value = Matrix4.identity()
-      ..translateByDouble(tx, ty, 0, 1)
-      ..scaleByDouble(scale, scale, scale, 1);
-  }
-
-  // ── Build ──────────────────────────────────────────────────────────────
-
-  @override
-  Widget build(BuildContext context) {
-    final l = AppLocalizations.of(context);
-    final cs = Theme.of(context).colorScheme;
-
-    return Scaffold(
-      appBar: AppBar(title: Text(l.boardTitle), centerTitle: true),
-      floatingActionButton: FloatingActionButton(
-        onPressed: _loading ? null : _addCard,
-        tooltip: l.boardAddCard,
-        child: const Icon(Icons.add),
-      ),
-      body: _loading
-          ? const Center(child: CircularProgressIndicator())
-          : Stack(
-              children: [
-                InteractiveViewer(
-                  key: _viewportKey,
-                  transformationController: _transform,
-                  boundaryMargin: const EdgeInsets.all(double.infinity),
-                  minScale: 0.2,
-                  maxScale: 4,
-                  // A finite, very large content area for the card Stack.
-                  constrained: false,
-                  child: GestureDetector(
-                    // Tap on empty canvas → deselect / stop editing.
-                    behavior: HitTestBehavior.translucent,
-                    onTap: () => setState(() {
-                      _selectedId = null;
-                      _editingId = null;
-                    }),
-                    child: SizedBox(
-                      width: _kCanvasExtent,
-                      height: _kCanvasExtent,
-                      child: Stack(
-                        children: [
-                          for (final card in _board.cards)
-                            _BoardCardWidget(
-                              key: ValueKey(card.id),
-                              card: card,
-                              board: _board,
-                              selected: card.id == _selectedId,
-                              editing: card.id == _editingId,
-                              onSelect: () =>
-                                  setState(() => _selectedId = card.id),
-                              onStartEdit: () =>
-                                  setState(() => _editingId = card.id),
-                              onTextChanged: (t) =>
-                                  _mutate(_board.setText(card.id, t)),
-                              onMoved: (pos) =>
-                                  _mutate(_board.moveCard(card.id, pos)),
-                              onDelete: () => _confirmDelete(card.id),
-                              onTapLink: _onTapLink,
-                              scale: _transform.value.getMaxScaleOnAxis(),
-                            ),
-                        ],
-                      ),
-                    ),
-                  ),
-                ),
-                if (_selectedId != null)
-                  _BacklinksPanel(
-                    board: _board,
-                    selectedId: _selectedId!,
-                    onTapCard: (id) {
-                      final c = _board.cardById(id);
-                      if (c != null) _centerOnCard(c);
-                      setState(() => _selectedId = id);
-                    },
-                    onClose: () => setState(() => _selectedId = null),
-                    background: cs.surfaceContainerHigh,
-                  ),
-              ],
-            ),
-    );
-  }
-
-  void _onTapLink(String target) {
-    final l = AppLocalizations.of(context);
-    final ok = _navigateToTarget(target);
-    if (!ok) {
-      ScaffoldMessenger.of(context).showSnackBar(
-        SnackBar(content: Text(l.boardDanglingLink(target))),
-      );
-    }
-  }
-
-  Future<void> _confirmDelete(String id) async {
-    final l = AppLocalizations.of(context);
-    final confirmed = await showDialog<bool>(
-      context: context,
-      builder: (ctx) => AlertDialog(
-        title: Text(l.boardDeleteCardTitle),
-        actions: [
-          TextButton(
-            onPressed: () => Navigator.pop(ctx, false),
-            child: Text(l.cancel),
-          ),
-          TextButton(
-            onPressed: () => Navigator.pop(ctx, true),
-            child: Text(l.delete),
-          ),
-        ],
-      ),
-    );
-    if (confirmed == true) _deleteCard(id);
-  }
-}
-
-// ───────────────────────────────────────────────────────────────────────────
-// One sticky card.
-
-class _BoardCardWidget extends StatefulWidget {
-  const _BoardCardWidget({
-    super.key,
-    required this.card,
-    required this.board,
-    required this.selected,
-    required this.editing,
-    required this.onSelect,
-    required this.onStartEdit,
-    required this.onTextChanged,
-    required this.onMoved,
-    required this.onDelete,
-    required this.onTapLink,
-    required this.scale,
-  });
-
-  final BoardCard card;
-  final Board board;
-  final bool selected;
-  final bool editing;
-  final VoidCallback onSelect;
-  final VoidCallback onStartEdit;
-  final ValueChanged<String> onTextChanged;
-  final ValueChanged<Offset> onMoved;
-  final VoidCallback onDelete;
-  final ValueChanged<String> onTapLink;
-
-  /// Current board zoom — used to convert screen drag deltas into content space.
-  final double scale;
-
-  @override
-  State<_BoardCardWidget> createState() => _BoardCardWidgetState();
-}
-
-class _BoardCardWidgetState extends State<_BoardCardWidget> {
-  TextEditingController? _controller;
-
-  @override
-  void didUpdateWidget(_BoardCardWidget old) {
-    super.didUpdateWidget(old);
-    // Keep the live editing controller in sync if the card text changes
-    // underneath us (e.g. external mutation) while NOT editing.
-    if (!widget.editing && _controller != null) {
-      _controller!.dispose();
-      _controller = null;
-    }
-  }
-
-  @override
-  void dispose() {
-    _controller?.dispose();
-    super.dispose();
-  }
-
-  @override
-  Widget build(BuildContext context) {
-    final cs = Theme.of(context).colorScheme;
-    final card = widget.card;
-
-    return Positioned(
-      left: card.position.dx,
-      top: card.position.dy,
-      width: card.size.width,
-      height: card.size.height,
-      child: GestureDetector(
-        behavior: HitTestBehavior.opaque,
-        onTap: widget.onSelect,
-        onLongPress: widget.onDelete,
-        // Dragging the card body moves it (content delta = screen delta / scale).
-        onPanStart: (_) => widget.onSelect(),
-        onPanUpdate: (d) {
-          final scale = widget.scale == 0 ? 1.0 : widget.scale;
-          widget.onMoved(card.position + d.delta / scale);
-        },
-        child: Material(
-          elevation: widget.selected ? 6 : 2,
-          color: cs.secondaryContainer,
-          surfaceTintColor: cs.surfaceTint,
-          borderRadius: BorderRadius.circular(12),
-          child: Container(
-            decoration: BoxDecoration(
-              borderRadius: BorderRadius.circular(12),
-              border: widget.selected
-                  ? Border.all(color: cs.primary, width: 2)
-                  : null,
-            ),
-            padding: const EdgeInsets.all(10),
-            child: widget.editing
-                ? _buildEditor(cs)
-                : _buildBody(context, cs),
-          ),
-        ),
-      ),
-    );
-  }
-
-  Widget _buildEditor(ColorScheme cs) {
-    _controller ??= TextEditingController(text: widget.card.text);
-    return TextField(
-      controller: _controller,
-      autofocus: true,
-      maxLines: null,
-      expands: true,
-      textAlignVertical: TextAlignVertical.top,
-      style: TextStyle(color: cs.onSecondaryContainer, fontSize: 14),
-      decoration: const InputDecoration(
-        border: InputBorder.none,
-        isCollapsed: true,
-      ),
-      onChanged: widget.onTextChanged,
-    );
-  }
-
-  Widget _buildBody(BuildContext context, ColorScheme cs) {
-    return GestureDetector(
-      // A tap that is NOT on a link enters edit mode.
-      onTap: widget.onStartEdit,
-      child: SizedBox.expand(
-        child: SingleChildScrollView(
-          child: _LinkedText(
-            text: widget.card.text,
-            board: widget.board,
-            onTapLink: widget.onTapLink,
-            baseColor: cs.onSecondaryContainer,
-          ),
-        ),
-      ),
-    );
-  }
-}
-
-// ───────────────────────────────────────────────────────────────────────────
-// Renders card text with [[link]] spans as tappable inline chips. Existing card
-// ids are styled as live links; unknown targets are dangling (dashed/red).
-
-class _LinkedText extends StatelessWidget {
-  const _LinkedText({
-    required this.text,
-    required this.board,
-    required this.onTapLink,
-    required this.baseColor,
-  });
-
-  final String text;
-  final Board board;
-  final ValueChanged<String> onTapLink;
-  final Color baseColor;
-
-  static final RegExp _linkPattern = RegExp(r'\[\[([^\]]*)\]\]');
-
-  @override
-  Widget build(BuildContext context) {
-    final cs = Theme.of(context).colorScheme;
-    final spans = <InlineSpan>[];
-    var last = 0;
-    for (final m in _linkPattern.allMatches(text)) {
-      if (m.start > last) {
-        spans.add(TextSpan(text: text.substring(last, m.start)));
-      }
-      final target = (m.group(1) ?? '').trim();
-      final exists = target.isNotEmpty && board.cardById(target) != null;
-      spans.add(
-        WidgetSpan(
-          alignment: PlaceholderAlignment.middle,
-          child: _LinkChip(
-            label: target.isEmpty ? '[[]]' : target,
-            exists: exists,
-            onTap: target.isEmpty ? null : () => onTapLink(target),
-            cs: cs,
-          ),
-        ),
-      );
-      last = m.end;
-    }
-    if (last < text.length) {
-      spans.add(TextSpan(text: text.substring(last)));
-    }
-    return Text.rich(
-      TextSpan(
-        style: TextStyle(color: baseColor, fontSize: 14),
-        children: spans,
-      ),
-    );
-  }
-}
-
-class _LinkChip extends StatelessWidget {
-  const _LinkChip({
-    required this.label,
-    required this.exists,
-    required this.onTap,
-    required this.cs,
-  });
-
-  final String label;
-  final bool exists;
-  final VoidCallback? onTap;
-  final ColorScheme cs;
-
-  @override
-  Widget build(BuildContext context) {
-    final color = exists ? cs.primary : cs.error;
-    return GestureDetector(
-      onTap: onTap,
-      child: Container(
-        margin: const EdgeInsets.symmetric(horizontal: 1),
-        padding: const EdgeInsets.symmetric(horizontal: 6, vertical: 1),
-        decoration: BoxDecoration(
-          color: color.withValues(alpha: 0.12),
-          borderRadius: BorderRadius.circular(6),
-          border: Border.all(
-            color: color,
-            width: 1,
-            // Dangling links get a visibly different (no fill, error) look.
-            style: BorderStyle.solid,
-          ),
-        ),
-        child: Text(
-          label,
-          style: TextStyle(
-            color: color,
-            fontSize: 13,
-            fontWeight: FontWeight.w600,
-            decoration:
-                exists ? TextDecoration.none : TextDecoration.lineThrough,
-          ),
-        ),
-      ),
-    );
-  }
-}
-
-// ───────────────────────────────────────────────────────────────────────────
-// "什么链接到这里 / Linked from" panel for the selected card.
-
-class _BacklinksPanel extends StatelessWidget {
-  const _BacklinksPanel({
-    required this.board,
-    required this.selectedId,
-    required this.onTapCard,
-    required this.onClose,
-    required this.background,
-  });
-
-  final Board board;
-  final String selectedId;
-  final ValueChanged<String> onTapCard;
-  final VoidCallback onClose;
-  final Color background;
-
-  @override
-  Widget build(BuildContext context) {
-    final l = AppLocalizations.of(context);
-    final backlinks = board.backlinksOf(selectedId).toList()..sort();
-
-    return Positioned(
-      right: 12,
-      top: 12,
-      bottom: 12,
-      width: 240,
-      child: Material(
-        elevation: 4,
-        color: background,
-        borderRadius: BorderRadius.circular(16),
-        child: Column(
-          crossAxisAlignment: CrossAxisAlignment.stretch,
-          children: [
-            Padding(
-              padding: const EdgeInsets.fromLTRB(16, 12, 8, 4),
-              child: Row(
-                children: [
-                  Expanded(
-                    child: Text(
-                      l.boardBacklinks,
-                      style: Theme.of(context).textTheme.titleSmall,
-                    ),
-                  ),
-                  IconButton(
-                    icon: const Icon(Icons.close),
-                    tooltip: l.close,
-                    onPressed: onClose,
-                  ),
-                ],
-              ),
-            ),
-            const Divider(height: 1),
-            Expanded(
-              child: backlinks.isEmpty
-                  ? Center(
-                      child: Padding(
-                        padding: const EdgeInsets.all(16),
-                        child: Text(
-                          l.boardNoBacklinks,
-                          textAlign: TextAlign.center,
-                          style: Theme.of(context).textTheme.bodySmall,
-                        ),
-                      ),
-                    )
-                  : ListView(
-                      children: [
-                        for (final id in backlinks)
-                          ListTile(
-                            dense: true,
-                            leading: const Icon(Icons.link, size: 18),
-                            title: Text(
-                              _preview(board, id),
-                              maxLines: 2,
-                              overflow: TextOverflow.ellipsis,
-                            ),
-                            subtitle: Text(
-                              id,
-                              maxLines: 1,
-                              overflow: TextOverflow.ellipsis,
-                            ),
-                            onTap: () => onTapCard(id),
-                          ),
-                      ],
-                    ),
-            ),
-          ],
-        ),
-      ),
-    );
-  }
-
-  String _preview(Board board, String id) {
-    final c = board.cardById(id);
-    if (c == null) return id;
-    final text = c.text.trim();
-    return text.isEmpty ? id : text.replaceAll('\n', ' ');
-  }
-}

lib/screens/home_screen.dart

@@ -11,10 +11,8 @@ import '../services/pdf_service.dart';
 import '../services/pptx_service.dart';
 import '../editor/canvas/pen_note_screen.dart';
 import '../editor/canvas/pen_slide_screen.dart';
-import 'board_screen.dart';
 import 'search_screen.dart';
 import 'settings_screen.dart';
-import 'split_view_screen.dart';
 
 // [M1] Relative date helper — no new package dependencies.
 String _formatDate(DateTime d) {
@@ -61,15 +59,6 @@ class HomeScreen extends ConsumerWidget {
             tooltip: l.importPpt,
             onPressed: () => _importPptx(context),
           ),
-          IconButton(
-            icon: const Icon(Icons.dashboard_customize),
-            tooltip: l.boardOpen,
-            onPressed: () {
-              Navigator.of(
-                context,
-              ).push(MaterialPageRoute(builder: (_) => const BoardScreen()));
-            },
-          ),
           IconButton(
             icon: const Icon(Icons.search),
             tooltip: l.search,
@@ -502,22 +491,10 @@ class _DocumentTileState extends ConsumerState<_DocumentTile> {
             '${document.docType.toUpperCase()} · ${document.pageCount} pages · $dateStr',
             style: Theme.of(context).textTheme.bodySmall,
           ),
-          // [H2] Trailing row: split-view (PDF only) + remove
+          // [H2] Trailing remove button. Split view is now reached only by
-          trailing: Row(
+          // tapping a scratch-link anchor inside the PDF editor, so the
-            mainAxisSize: MainAxisSize.min,
+          // standalone split-view entry was removed.
-            children: [
+          trailing: IconButton(
-              if (isPdf)
-                IconButton(
-                  icon: Icon(
-                    Icons.vertical_split,
-                    color: _hovering
-                        ? Theme.of(context).colorScheme.primary
-                        : subtleColor.withValues(alpha: 0.4),
-                  ),
-                  tooltip: 'Open in Split View',
-                  onPressed: () => _openSplitView(context),
-                ),
-              IconButton(
             icon: Icon(
               Icons.delete_outline,
               color: _hovering
@@ -527,8 +504,6 @@ class _DocumentTileState extends ConsumerState<_DocumentTile> {
             tooltip: 'Remove document',
             onPressed: () => _confirmDelete(context),
           ),
-            ],
-          ),
           // [L2] Routing bug fix: route by docType
           onTap: () => _openDocument(context),
           onLongPress: () => _showDocumentMenu(context),
@@ -577,19 +552,7 @@ class _DocumentTileState extends ConsumerState<_DocumentTile> {
     }
   }
 
-  void _openSplitView(BuildContext context) {
-    Navigator.of(context).push(
-      MaterialPageRoute(
-        builder: (_) => SplitViewScreen(
-          filePath: widget.document.filePath,
-          documentId: widget.document.id,
-        ),
-      ),
-    );
-  }
-
   void _showContextMenu(BuildContext context, Offset position) async {
-    final isPdf = widget.document.docType == 'pdf';
     final result = await showMenu<String>(
       context: context,
       position: RelativeRect.fromLTRB(
@@ -609,17 +572,6 @@ class _DocumentTileState extends ConsumerState<_DocumentTile> {
             ],
           ),
         ),
-        if (isPdf)
-          PopupMenuItem(
-            value: 'split',
-            child: Row(
-              children: const [
-                Icon(Icons.vertical_split),
-                SizedBox(width: 8),
-                Text('Open in Split View'),
-              ],
-            ),
-          ),
         PopupMenuItem(
           value: 'remove',
           child: Row(
@@ -641,15 +593,12 @@ class _DocumentTileState extends ConsumerState<_DocumentTile> {
     if (!mounted) return;
     if (result == 'open') {
       _openDocument(this.context);
-    } else if (result == 'split') {
-      _openSplitView(this.context);
     } else if (result == 'remove') {
       _confirmDelete(this.context);
     }
   }
 
   void _showDocumentMenu(BuildContext context) {
-    final isPdf = widget.document.docType == 'pdf';
     showModalBottomSheet(
       context: context,
       builder: (ctx) {
@@ -657,16 +606,6 @@ class _DocumentTileState extends ConsumerState<_DocumentTile> {
           child: Column(
             mainAxisSize: MainAxisSize.min,
             children: [
-              if (isPdf)
-                ListTile(
-                  leading: const Icon(Icons.vertical_split),
-                  title: const Text('Open in Split View'),
-                  subtitle: const Text('PDF reference + scratchpad'),
-                  onTap: () {
-                    Navigator.of(ctx).pop();
-                    _openSplitView(context);
-                  },
-                ),
               ListTile(
                 leading: const Icon(Icons.delete_outline, color: Colors.red),
                 title: const Text(

lib/screens/split_view_screen.dart

@@ -1,33 +1,56 @@
+// lib/screens/split_view_screen.dart
+//
+// Anchor-keyed split view: opened by tapping a PDF-anchored scratch link.
+//   LEFT  pane = the source PDF (pdfrx PdfViewer, read-only reference + page
+//                nav), opened at the anchor's page.
+//   RIGHT pane = an INFINITE freehand scratchpad that BELONGS TO THE ANCHOR,
+//                keyed by [scratchLinkId] (not the documentId). Each anchor has
+//                its own private scratch space, persisted via the existing
+//                scratchpad storage (InkStroke JSON, format unchanged).
+//
+// The right pane reuses the pen-first PenCanvas world-coord engine and the
+// Material You brush palette shared with pen_note_screen (BrushPickerButton /
+// ToolButton / color dots), replacing the old AnnotationToolbar.
+
 import 'dart:async';
 import 'dart:convert';
-import 'dart:io';
 
 import 'package:flutter/material.dart';
-import 'package:syncfusion_flutter_pdfviewer/pdfviewer.dart';
+import 'package:pdfrx/pdfrx.dart';
 import 'package:uuid/uuid.dart';
 
 import '../editor/canvas/pen_canvas.dart';
+import '../editor/canvas/pen_palette_widgets.dart';
 import '../editor/canvas/pen_stroke.dart';
+import '../editor/engine/brush.dart';
+import '../editor/input/pen_config.dart' show kDefaultEraserRadius;
 import '../editor/notebook/ink_stroke_adapter.dart';
 import '../models/ink_stroke.dart';
-import '../models/pen_tool.dart';
-import '../models/pressure_curve.dart';
 import '../services/database_service.dart';
 import '../services/undo_manager.dart';
-import '../utils/stroke_stabilizer.dart';
-import '../widgets/annotation_toolbar.dart';
 
-/// Split-view derivation mode: left pane = reference PDF, right pane = infinite
+/// Split-view derivation surface for a single scratch-link anchor: left pane =
-/// scratchpad for formula derivation. Scratchpad strokes are persisted per
+/// the reference PDF (at [initialPage]), right pane = the anchor's private
-/// document via [DatabaseService.saveScratchpad] / [DatabaseService.loadScratchpad].
+/// infinite scratchpad. Scratchpad strokes persist per ANCHOR via
+/// [DatabaseService.saveScratchpad] / [DatabaseService.loadScratchpad], keyed by
+/// [scratchLinkId].
 class SplitViewScreen extends StatefulWidget {
   final String filePath;
   final String documentId;
 
+  /// The owning anchor id. Doubles as the scratchpad storage key so this is the
+  /// anchor's private scratch space.
+  final String scratchLinkId;
+
+  /// 0-based page the anchor sits on; the left PDF opens here.
+  final int initialPage;
+
   const SplitViewScreen({
     super.key,
     required this.filePath,
     required this.documentId,
+    required this.scratchLinkId,
+    this.initialPage = 0,
   });
 
   @override
@@ -37,63 +60,55 @@ class SplitViewScreen extends StatefulWidget {
 class _SplitViewState extends State<SplitViewScreen> {
   // -- PDF (left pane) --
   final PdfViewerController _pdfController = PdfViewerController();
-  int _currentPage = 0;
+  int _currentPage = 0; // 0-based
   int _pageCount = 0;
-  String _fileName = '';
 
   // -- Split divider --
   double _leftPaneFraction = 0.5;
   bool _isDraggingDivider = false;
 
   // -- Scratchpad (right pane) --
-  // The scratchpad is an infinite WORLD: strokes are stored in absolute world
+  // Infinite WORLD: strokes stored in absolute world pixels ([InkStroke],
-  // pixels ([InkStroke], unchanged persistence format), and rendered through the
+  // unchanged persistence format), rendered through PenCanvas by normalizing
-  // performant PenCanvas by normalizing against the CURRENT world size. When the
+  // against the CURRENT world size. World auto-expands without moving ink.
-  // world auto-expands, the stored world coords don't move — only the
-  // normalization divisor grows — so ink stays put with zero drift.
   final UndoManager _undoManager = UndoManager();
   List<InkStroke> _strokes = [];
   double _canvasWidth = 4000;
   double _canvasHeight = 4000;
   static const _uuid = Uuid();
 
-  /// Pan/zoom transform for the scratchpad world (PenCanvas drives this).
   final TransformationController _scratchTransform = TransformationController();
-
-  /// Set once the initial view has been framed onto existing ink.
   bool _scratchCentered = false;
 
   Size get _worldSize => Size(_canvasWidth, _canvasHeight);
 
-  /// Maps the scratchpad toolbar's [PenTool] to the pen-canvas tool. Shapes and
+  // -- Tool state (new Material You brush palette) --
-  /// text fall back to pen (the pen-first scratchpad is freehand).
+  CanvasTool _tool = CanvasTool.pen;
-  CanvasTool get _canvasTool => switch (_currentTool) {
+  BrushKind _penBrush = BrushKind.fountainPen;
-        PenTool.eraser => CanvasTool.eraser,
+  Color _color = Colors.black;
-        PenTool.highlighter => CanvasTool.highlighter,
-        _ => CanvasTool.pen,
-      };
 
-  // -- Tool state --
+  static const double _penWidthFraction = 0.006;
-  PenTool _currentTool = PenTool.pen;
+  static const double _highlighterWidthFraction = 0.02;
-  Color _currentColor = Colors.black;
+
-  double _currentStrokeWidth = 2.0;
+  static const List<Color> _palette = [
-  bool _filled = false;
+    Colors.black,
-  PressureCurveType _pressureCurveType = PressureCurveType.linear;
+    Colors.red,
-  StabilizationLevel _stabilizationLevel = StabilizationLevel.none;
+    Colors.blue,
+    Colors.green,
+    Colors.orange,
+  ];
 
   // -- Auto-save debounce --
   Timer? _saveTimer;
   bool _dirty = false;
 
-  // -- Page link markers (optional feature) --
-  final List<_PageLink> _pageLinks = [];
-
   static const double _edgeThreshold = 200.0;
   static const double _expandAmount = 1000.0;
 
   @override
   void initState() {
     super.initState();
+    _currentPage = widget.initialPage;
     _loadScratchpad();
   }
 
@@ -101,20 +116,20 @@ class _SplitViewState extends State<SplitViewScreen> {
   void dispose() {
     _saveTimer?.cancel();
     _saveImmediate();
-    _pdfController.dispose();
+    // PdfViewerController (pdfrx) has no dispose(); it detaches with the viewer.
     _scratchTransform.dispose();
     super.dispose();
   }
 
-  // -- Persistence --
+  // -- Persistence (keyed by the ANCHOR id, not the documentId) --
 
   Future<void> _loadScratchpad() async {
     final db = await DatabaseService.getInstance();
-    final strokes = await db.loadScratchpad(widget.documentId);
+    final strokes = await db.loadScratchpad(widget.scratchLinkId);
-    if (mounted) {
+    if (!mounted) return;
     setState(() {
-        // Keep only freehand strokes so the canvas list stays 1:1 with the
+      // Keep only freehand strokes so the canvas list stays 1:1 with the undo
-        // undo manager (shapes/text have no pen-canvas representation).
+      // manager (shapes/text have no pen-canvas representation).
       final freehand = strokes.where((s) => isFreehandTool(s.tool)).toList();
       _strokes = freehand;
       for (final s in freehand) {
@@ -122,7 +137,6 @@ class _SplitViewState extends State<SplitViewScreen> {
       }
     });
   }
-  }
 
   void _scheduleSave() {
     _dirty = true;
@@ -135,13 +149,11 @@ class _SplitViewState extends State<SplitViewScreen> {
     _dirty = false;
     final db = await DatabaseService.getInstance();
     final json = jsonEncode(_strokes.map((s) => s.toJson()).toList());
-    await db.saveScratchpad(widget.documentId, json);
+    await db.saveScratchpad(widget.scratchLinkId, json);
   }
 
   // -- Scratchpad stroke callbacks --
 
-  /// PenCanvas committed a stroke (normalized to the current world). Convert it
-  /// to absolute world coords for storage.
   void _onStrokeComplete(PenStroke pen) {
     final stroke = inkStrokeFromPen(pen, _worldSize,
         id: _uuid.v4(), createdAt: DateTime.now());
@@ -153,8 +165,6 @@ class _SplitViewState extends State<SplitViewScreen> {
     _scheduleSave();
   }
 
-  /// PenCanvas erased through stroke [index] (into [_strokes]); [replacements]
-  /// are the surviving sub-strokes (normalized) — convert back to world coords.
   void _onErase(int index, List<PenStroke> replacements) {
     if (index < 0 || index >= _strokes.length) return;
     setState(() {
@@ -229,62 +239,32 @@ class _SplitViewState extends State<SplitViewScreen> {
     setState(() => _isDraggingDivider = false);
   }
 
-  // -- PDF page navigation --
+  // -- PDF page navigation (left pane) --
 
   void _prevPage() {
-    if (_currentPage > 0) {
+    if (_currentPage > 0) _pdfController.goToPage(pageNumber: _currentPage);
-      _pdfController.previousPage();
-    }
   }
 
   void _nextPage() {
     if (_currentPage < _pageCount - 1) {
-      _pdfController.nextPage();
+      _pdfController.goToPage(pageNumber: _currentPage + 2);
     }
   }
 
-  // -- Page link creation (long-press on left pane) --
+  CanvasTool get _activeTool => _tool;
 
-  void _onPdfLongPress(int pageNumber) {
+  double get _strokeWidth => _tool == CanvasTool.highlighter
-    // Place a page link marker at the current scratchpad viewport center.
+      ? _highlighterWidthFraction
-    // We approximate the viewport center as (0, 0) since InteractiveViewer
+      : _penWidthFraction;
-    // manages its own transform — the user can reposition by panning.
-    setState(() {
-      _pageLinks.add(
-        _PageLink(
-          pageNumber: pageNumber,
-          position: const Offset(100, 100), // default top-left area
-        ),
-      );
-    });
-    ScaffoldMessenger.of(context).showSnackBar(
-      SnackBar(content: Text('Page link marker added for page $pageNumber')),
-    );
-  }
-
-  void _onPageLinkTap(_PageLink link) {
-    _pdfController.jumpToPage(link.pageNumber);
-    setState(() {
-      _currentPage = link.pageNumber - 1;
-    });
-  }
-
-  void _deletePageLink(_PageLink link) {
-    setState(() {
-      _pageLinks.remove(link);
-    });
-  }
 
   // -- Build --
 
   @override
   Widget build(BuildContext context) {
+    final cs = Theme.of(context).colorScheme;
     return Scaffold(
       appBar: AppBar(
-        title: Text(
+        title: const Text('Scratch link', style: TextStyle(fontSize: 16)),
-          _fileName.isEmpty ? 'Split View' : _fileName,
-          style: const TextStyle(fontSize: 16),
-        ),
         leading: IconButton(
           icon: const Icon(Icons.arrow_back),
           onPressed: () {
@@ -293,7 +273,6 @@ class _SplitViewState extends State<SplitViewScreen> {
           },
         ),
         actions: [
-          // Left pane page navigation
           IconButton(
             icon: const Icon(Icons.navigate_before),
             tooltip: 'Previous page (PDF)',
@@ -314,7 +293,6 @@ class _SplitViewState extends State<SplitViewScreen> {
             onPressed: _currentPage < _pageCount - 1 ? _nextPage : null,
           ),
           const SizedBox(width: 8),
-          // Canvas info
           Tooltip(
             message:
                 'Scratchpad size: ${_canvasWidth.round()} x ${_canvasHeight.round()}',
@@ -332,7 +310,6 @@ class _SplitViewState extends State<SplitViewScreen> {
       ),
       body: Column(
         children: [
-          // Label clarifying that the toolbar controls the scratchpad pane.
           Padding(
             padding: const EdgeInsets.only(left: 12, top: 4),
             child: Align(
@@ -345,42 +322,17 @@ class _SplitViewState extends State<SplitViewScreen> {
               ),
             ),
           ),
-          // Toolbar (applies to scratchpad only)
+          _buildBrushPalette(cs),
-          AnnotationToolbar(
-            currentTool: _currentTool,
-            currentColor: _currentColor,
-            currentStrokeWidth: _currentStrokeWidth,
-            filled: _filled,
-            pressureCurveType: _pressureCurveType,
-            stabilizationLevel: _stabilizationLevel,
-            canUndo: _undoManager.canUndo,
-            canRedo: _undoManager.canRedo,
-            onToolChanged: (tool) => setState(() => _currentTool = tool),
-            onColorChanged: (color) => setState(() => _currentColor = color),
-            onStrokeWidthChanged: (w) =>
-                setState(() => _currentStrokeWidth = w),
-            onFilledChanged: (f) => setState(() => _filled = f),
-            onPressureCurveChanged: (v) =>
-                setState(() => _pressureCurveType = v),
-            onStabilizationChanged: (v) =>
-                setState(() => _stabilizationLevel = v),
-            onUndo: _undo,
-            onRedo: _redo,
-          ),
-          // Split view body
           Expanded(
             child: LayoutBuilder(
               builder: (context, constraints) {
                 final totalWidth = constraints.maxWidth;
                 final leftWidth = totalWidth * _leftPaneFraction;
-                final rightWidth =
+                final rightWidth = totalWidth - leftWidth - 12;
-                    totalWidth - leftWidth - 12; // 12px divider hit area
 
                 return Row(
                   children: [
-                    // Left pane: PDF reference (read-only)
                     SizedBox(width: leftWidth, child: _buildPdfPane()),
-                    // Draggable divider: 12px hit area, 4px visual strip.
                     GestureDetector(
                       onHorizontalDragStart: _onDividerDragStart,
                       onHorizontalDragUpdate: (d) =>
@@ -401,7 +353,6 @@ class _SplitViewState extends State<SplitViewScreen> {
                         ),
                       ),
                     ),
-                    // Right pane: Infinite scratchpad
                     SizedBox(width: rightWidth, child: _buildScratchpadPane()),
                   ],
                 );
@@ -413,72 +364,118 @@ class _SplitViewState extends State<SplitViewScreen> {
     );
   }
 
-  Widget _buildPdfPane() {
+  /// Material You brush palette (shared chrome with the pen-first editors):
-    return Stack(
+  /// brush picker + highlighter + eraser, undo/redo, color dots.
+  Widget _buildBrushPalette(ColorScheme cs) {
+    return Material(
+      color: cs.surfaceContainerHigh,
+      elevation: 3,
+      borderRadius: BorderRadius.circular(28),
+      child: Padding(
+        padding: const EdgeInsets.symmetric(horizontal: 8, vertical: 6),
+        child: SingleChildScrollView(
+          scrollDirection: Axis.horizontal,
+          child: Row(
+            mainAxisSize: MainAxisSize.min,
             children: [
-        GestureDetector(
+              BrushPickerButton(
-          onLongPress: () {
+                selected: _penBrush,
-            // Long-press on PDF to create page link marker
+                active: _tool == CanvasTool.pen,
-            _onPdfLongPress(_currentPage + 1);
+                tooltip: 'Brush',
-          },
+                labelFor: brushLabelEn,
-          child: SfPdfViewer.file(
+                onSelected: (b) => setState(() {
-            File(widget.filePath),
+                  _penBrush = b;
-            controller: _pdfController,
+                  _tool = CanvasTool.pen;
-            canShowScrollHead: true,
+                }),
-            canShowScrollStatus: true,
-            onPageChanged: (PdfPageChangedDetails details) {
-              setState(() {
-                _currentPage = details.newPageNumber - 1;
-              });
-            },
-            onDocumentLoaded: (PdfDocumentLoadedDetails details) {
-              setState(() {
-                _pageCount = details.document.pages.count;
-                _fileName = widget.filePath.split(Platform.pathSeparator).last;
-              });
-            },
               ),
+              ToolButton(
+                icon: Icons.brush_outlined,
+                selected: _tool == CanvasTool.highlighter,
+                tooltip: 'Highlighter',
+                onPressed: () => setState(() => _tool = CanvasTool.highlighter),
               ),
-        // Page link markers overlay (on PDF pane, showing linked pages)
+              ToolButton(
-        if (_pageLinks.isNotEmpty)
+                icon: Icons.cleaning_services_outlined,
-          Positioned(bottom: 8, left: 8, child: _buildPageLinkChips()),
+                selected: _tool == CanvasTool.eraser,
+                tooltip: 'Eraser',
+                onPressed: () => setState(() => _tool = CanvasTool.eraser),
+              ),
+              PaletteDivider(cs: cs),
+              ToolButton(
+                icon: Icons.undo,
+                selected: false,
+                tooltip: 'Undo',
+                onPressed: _undoManager.canUndo ? _undo : null,
+              ),
+              ToolButton(
+                icon: Icons.redo,
+                selected: false,
+                tooltip: 'Redo',
+                onPressed: _undoManager.canRedo ? _redo : null,
+              ),
+              PaletteDivider(cs: cs),
+              for (final c in _palette) _colorDot(c, cs),
             ],
+          ),
+        ),
+      ),
     );
   }
 
-  Widget _buildPageLinkChips() {
+  Widget _colorDot(Color c, ColorScheme cs) {
-    return Wrap(
+    final selected =
-      spacing: 4,
+        _color.toARGB32() == c.toARGB32() && _tool != CanvasTool.eraser;
-      runSpacing: 4,
-      children: _pageLinks.map((link) {
     return GestureDetector(
-          onTap: () => _onPageLinkTap(link),
+      onTap: () => setState(() {
-          onLongPress: () => _deletePageLink(link),
+        _color = c;
-          child: Chip(
+        if (_tool == CanvasTool.eraser) _tool = CanvasTool.pen;
-            avatar: const Icon(Icons.link, size: 14, color: Colors.white),
+      }),
-            label: Text(
+      child: AnimatedContainer(
-              'p${link.pageNumber}',
+        duration: const Duration(milliseconds: 150),
-              style: const TextStyle(fontSize: 11, color: Colors.white),
+        margin: const EdgeInsets.symmetric(horizontal: 3),
+        width: 24,
+        height: 24,
+        decoration: BoxDecoration(
+          color: c,
+          shape: BoxShape.circle,
+          border: Border.all(
+            color: selected ? cs.onSurface : cs.outlineVariant,
+            width: selected ? 3 : 1,
+          ),
         ),
-            backgroundColor: Colors.blue.shade600,
-            padding: EdgeInsets.zero,
-            materialTapTargetSize: MaterialTapTargetSize.shrinkWrap,
-            visualDensity: VisualDensity.compact,
       ),
     );
-      }).toList(),
+  }
+
+  Widget _buildPdfPane() {
+    // Read-only reference PDF on the same engine (pdfrx) as the rest of the
+    // app, opened at the anchor's page.
+    return PdfViewer.file(
+      widget.filePath,
+      controller: _pdfController,
+      params: PdfViewerParams(
+        onViewerReady: (document, controller) {
+          if (!mounted) return;
+          setState(() {
+            _pageCount = document.pages.length;
+          });
+          final target = widget.initialPage.clamp(0, _pageCount - 1);
+          if (target > 0) {
+            controller.goToPage(pageNumber: target + 1);
+          }
+        },
+        onPageChanged: (pageNumber) {
+          if (pageNumber == null || !mounted) return;
+          final idx = pageNumber - 1;
+          if (idx != _currentPage) setState(() => _currentPage = idx);
+        },
+      ),
     );
   }
 
   Widget _buildScratchpadPane() {
-    // Render the world through the performant PenCanvas: strokes normalized
-    // against the current world size; toolbar width is in world pixels, so the
-    // pen-canvas fraction is width / worldWidth.
     return LayoutBuilder(
       builder: (context, constraints) {
-        // On first layout, frame the view so existing ink is actually visible
-        // (otherwise identity shows only the empty top-left corner of the huge
-        // world). Empty scratchpad falls back to a comfortable 1:1 near origin.
         if (!_scratchCentered) {
           WidgetsBinding.instance.addPostFrameCallback((_) {
             if (!mounted) return;
@@ -493,10 +490,11 @@ class _SplitViewState extends State<SplitViewScreen> {
             pageSize: _worldSize,
             strokes: penStrokesFromInk(_strokes, _worldSize),
             transformationController: _scratchTransform,
-            tool: _canvasTool,
+            tool: _activeTool,
-            color: _currentColor,
+            brush: _penBrush,
-            strokeWidth: _currentStrokeWidth / _canvasWidth,
+            color: _color,
-            // The world is huge, so allow zooming further out to survey it.
+            strokeWidth: _strokeWidth,
+            eraserRadius: kDefaultEraserRadius,
             minScale: 0.1,
             maxScale: 8.0,
             onStrokeComplete: _onStrokeComplete,
@@ -508,9 +506,8 @@ class _SplitViewState extends State<SplitViewScreen> {
     );
   }
 
-  /// Position the scratchpad so existing ink is on-screen. Fits the strokes'
+  /// Position the scratchpad so existing ink is on-screen (fit its bbox into
-  /// world bounding box into [pane] (with padding, scale clamped); for an empty
+  /// [pane], scale clamped); an empty scratchpad shows the top-left at 1:1.
-  /// scratchpad, shows the top-left working area at 1:1.
   void _frameScratchpad(Size pane) {
     if (pane.isEmpty) return;
     if (_strokes.isEmpty) {
@@ -534,8 +531,9 @@ class _SplitViewState extends State<SplitViewScreen> {
     const pad = 80.0;
     final boxW = (maxX - minX) + pad * 2;
     final boxH = (maxY - minY) + pad * 2;
-    final scale =
+    final scale = (pane.width / boxW < pane.height / boxH
-        (pane.width / boxW < pane.height / boxH ? pane.width / boxW : pane.height / boxH)
+            ? pane.width / boxW
+            : pane.height / boxH)
         .clamp(0.15, 1.5);
     final cx = (minX + maxX) / 2;
     final cy = (minY + maxY) / 2;
@@ -548,11 +546,3 @@ class _SplitViewState extends State<SplitViewScreen> {
       ..setTranslationRaw(tx, ty, 0);
   }
 }
-
-/// A marker linking a scratchpad position to a specific PDF page.
-class _PageLink {
-  final int pageNumber;
-  final Offset position;
-
-  const _PageLink({required this.pageNumber, required this.position});
-}

lib/services/database_service.dart

@@ -16,6 +16,7 @@ import '../models/ink_stroke.dart';
 import '../models/note.dart';
 import '../models/pen_tool.dart';
 import '../models/pointer_device_kind.dart';
+import '../models/scratch_link.dart';
 
 class DatabaseService {
   static DatabaseService? _instance;
@@ -56,7 +57,7 @@ class DatabaseService {
 
     _database = await openDatabase(
       dbPath,
-      version: 7,
+      version: 8,
       onCreate: _onCreate,
       onUpgrade: _onUpgrade,
     );
@@ -202,6 +203,28 @@ class DatabaseService {
 
     // Sticky-note board cards (v7): F7 双链 + 无限便利贴.
     await _createBoardCardsTable(db);
+
+    // PDF-anchored scratch links (v8).
+    await _createScratchLinksTable(db);
+  }
+
+  /// PDF-anchored scratch links table (v8). One row per [ScratchLink] anchor.
+  /// The anchor [id] doubles as the storage key for its private scratchpad
+  /// (reused from the [scratchpads] table — see [saveScratchpad]).
+  Future<void> _createScratchLinksTable(DatabaseExecutor db) async {
+    await db.execute('''
+      CREATE TABLE scratch_links (
+        id TEXT PRIMARY KEY,
+        document_id TEXT NOT NULL,
+        page_index INTEGER NOT NULL,
+        nx REAL NOT NULL,
+        ny REAL NOT NULL,
+        created_at TEXT NOT NULL
+      )
+    ''');
+    await db.execute(
+      'CREATE INDEX idx_scratch_links_doc ON scratch_links(document_id)',
+    );
   }
 
   /// Sticky-note board cards table (F7). One row per [BoardCard]; a board is the
@@ -232,6 +255,13 @@ class DatabaseService {
     if (oldVersion < 5) await _migrateV4toV5(db);
     if (oldVersion < 6) await _migrateV5toV6(db);
     if (oldVersion < 7) await _migrateV6toV7(db);
+    if (oldVersion < 8) await _migrateV7toV8(db);
+  }
+
+  Future<void> _migrateV7toV8(Database db) async {
+    await db.transaction((txn) async {
+      await _createScratchLinksTable(txn);
+    });
   }
 
   Future<void> _migrateV6toV7(Database db) async {
@@ -1003,4 +1033,54 @@ class DatabaseService {
         ),
     ]);
   }
+
+  // ── Scratch links CRUD (PDF-anchored scratchpad tabs) ──────────────────
+
+  /// Insert or replace a [ScratchLink] anchor. The anchor's private scratchpad
+  /// lives in the [scratchpads] table keyed by [ScratchLink.id] — saved/loaded
+  /// via [saveScratchpad] / [loadScratchpad].
+  Future<void> saveScratchLink(ScratchLink link) async {
+    await _database.insert(
+      'scratch_links',
+      {
+        'id': link.id,
+        'document_id': link.documentId,
+        'page_index': link.pageIndex,
+        'nx': link.nx,
+        'ny': link.ny,
+        'created_at': DateTime.now().toIso8601String(),
+      },
+      conflictAlgorithm: ConflictAlgorithm.replace,
+    );
+  }
+
+  /// Load all anchors for [documentId], oldest first.
+  Future<List<ScratchLink>> loadScratchLinks(String documentId) async {
+    final rows = await _database.query(
+      'scratch_links',
+      where: 'document_id = ?',
+      whereArgs: [documentId],
+      orderBy: 'created_at ASC',
+    );
+    return rows
+        .map(
+          (row) => ScratchLink(
+            id: row['id'] as String,
+            documentId: row['document_id'] as String,
+            pageIndex: row['page_index'] as int,
+            nx: (row['nx'] as num).toDouble(),
+            ny: (row['ny'] as num).toDouble(),
+          ),
+        )
+        .toList();
+  }
+
+  /// Delete an anchor and its private scratchpad (the scratchpad row keyed by
+  /// the anchor id), so a deleted anchor leaves no orphaned ink behind.
+  Future<void> deleteScratchLink(String id) async {
+    await _database.transaction((txn) async {
+      await txn.delete('scratch_links', where: 'id = ?', whereArgs: [id]);
+      await txn.delete('scratchpads', where: 'document_id = ?', whereArgs: [id]);
+    });
+  }
 }

test/board_screen_test.dart

@@ -1,150 +0,0 @@
-// test/board_screen_test.dart
-//
-// Screen-level + persistence guards for F7 (双链 + 无限便利贴):
-//   1. BoardScreen pumps, "Add card" inserts a card, typing text with a
-//      [[link]] renders the card body and the link chip.
-//   2. saveBoardCards / loadBoard round-trip a board through DatabaseService
-//      (cards survive a reload — proves the screen's persistence is real).
-//
-// DatabaseService.getInstance() needs getApplicationDocumentsDirectory(); we
-// mock PathProviderPlatform to a temp dir so the singleton opens a real (ffi)
-// sqlite DB on disk.
-
-import 'dart:io';
-
-import 'package:flutter/material.dart';
-import 'package:flutter_riverpod/flutter_riverpod.dart';
-import 'package:flutter_test/flutter_test.dart';
-// ignore: depend_on_referenced_packages
-import 'package:path_provider_platform_interface/path_provider_platform_interface.dart';
-// ignore: depend_on_referenced_packages
-import 'package:plugin_platform_interface/plugin_platform_interface.dart';
-import 'package:sqflite_common_ffi/sqflite_ffi.dart';
-
-import 'package:badnote/editor/board/board.dart';
-import 'package:badnote/l10n/app_localizations.dart';
-import 'package:badnote/screens/board_screen.dart';
-import 'package:badnote/services/database_service.dart';
-
-class _FakePathProvider extends PathProviderPlatform
-    with MockPlatformInterfaceMixin {
-  _FakePathProvider(this.dir);
-  final String dir;
-  @override
-  Future<String?> getApplicationDocumentsPath() async => dir;
-}
-
-Future<void> _pumpBoard(WidgetTester tester) async {
-  await tester.pumpWidget(
-    ProviderScope(
-      child: MaterialApp(
-        locale: const Locale('en'),
-        localizationsDelegates: AppLocalizations.localizationsDelegates,
-        supportedLocales: AppLocalizations.supportedLocales,
-        home: const BoardScreen(boardId: 'test-board'),
-      ),
-    ),
-  );
-  // The board loads from a real (ffi) sqlite file in initState. That I/O only
-  // completes on the real event loop, so drive it via runAsync, then pump to
-  // surface the loaded state. (pumpAndSettle is unusable here: the loading
-  // CircularProgressIndicator animates forever and never settles.)
-  await tester.runAsync(() async {
-    // Give the DB open + loadBoard future real wall-clock time to resolve.
-    await Future<void>.delayed(const Duration(milliseconds: 200));
-  });
-  await tester.pump();
-  await tester.pump();
-}
-
-void main() {
-  TestWidgetsFlutterBinding.ensureInitialized();
-  sqfliteFfiInit();
-  databaseFactory = databaseFactoryFfi;
-
-  late Directory tmp;
-
-  setUp(() async {
-    tmp = await Directory.systemTemp.createTemp('board_screen_test_');
-    // Fresh DB file per test → clean board state.
-    final dbFile = File('${tmp.path}/badnote.db');
-    if (dbFile.existsSync()) dbFile.deleteSync();
-    PathProviderPlatform.instance = _FakePathProvider(tmp.path);
-    DatabaseService.resetForTest();
-  });
-
-  tearDown(() async {
-    try {
-      tmp.deleteSync(recursive: true);
-    } catch (_) {}
-  });
-
-  testWidgets('add a card, type [[link]] text, card + link chip render',
-      (tester) async {
-    await _pumpBoard(tester);
-
-    // Empty board: no cards yet.
-    expect(find.byType(TextField), findsNothing);
-
-    // "Add card" FAB → inserts a card that opens straight into inline edit.
-    await tester.tap(find.byType(FloatingActionButton));
-    await tester.pump();
-    await tester.pump();
-    expect(find.byType(TextField), findsOneWidget,
-        reason: 'a new card opens in inline-edit mode');
-
-    // Type a body containing a [[link]] to a (dangling) target.
-    await tester.enterText(find.byType(TextField), 'hello [[other]]');
-    await tester.pump();
-
-    // Tap empty canvas (top-left, away from the centered card) to leave edit
-    // mode and render the body.
-    await tester.tapAt(const Offset(10, 100));
-    await tester.pump();
-    await tester.pump();
-
-    // The non-link text and the link chip both render.
-    expect(find.textContaining('hello'), findsWidgets);
-    expect(find.text('other'), findsOneWidget,
-        reason: 'the [[other]] link renders as a tappable chip');
-
-    // Tear the screen down so its debounced save timer is cancelled and any
-    // pending flush lands while the DB is still open (avoids a post-test write
-    // against a torn-down DB). Disposal runs synchronously on pumpWidget.
-    await tester.pumpWidget(const SizedBox());
-    await tester.runAsync(() async {
-      await Future<void>.delayed(const Duration(milliseconds: 50));
-    });
-  });
-
-  test('persistence round-trips a board through DatabaseService', () async {
-    final db = await DatabaseService.getInstance();
-
-    final board = Board.empty
-        .add(BoardCard(
-          id: 'a',
-          position: const Offset(10, 20),
-          size: const Size(180, 140),
-          text: 'see [[b]]',
-        ))
-        .add(BoardCard(
-          id: 'b',
-          position: const Offset(300, 50),
-          size: const Size(180, 140),
-          text: 'leaf',
-        ));
-
-    await db.saveBoardCards('test-board', board.cards);
-
-    final reloaded = await db.loadBoard('test-board');
-    expect(reloaded.length, 2);
-    expect(reloaded.cardById('a')!.position, const Offset(10, 20));
-    expect(reloaded.cardById('a')!.text, 'see [[b]]');
-    expect(reloaded.cardById('b')!.size, const Size(180, 140));
-    // Backlinks survive because the [[b]] link is preserved in text.
-    expect(reloaded.backlinksOf('b'), {'a'});
-
-    // A board id without rows loads as empty (default-board fallback path).
-    expect((await db.loadBoard('nonexistent')).length, 0);
-  });
-}

test/scratch_link_test.dart

@@ -0,0 +1,166 @@
+// test/scratch_link_test.dart
+//
+// Guards the PDF-anchored scratch-link feature:
+//   1. ScratchLink toJson/fromJson round-trip (pure model).
+//   2. saveScratchLink / loadScratchLinks / deleteScratchLink round-trip a set
+//      of anchors through DatabaseService (anchors survive a reload).
+//   3. A per-anchor scratchpad keyed by the anchor id is private to that anchor
+//      and is removed when the anchor is deleted (proves the reuse of the
+//      existing scratchpad storage keyed by anchor id, not document id).
+//
+// DatabaseService.getInstance() needs getApplicationDocumentsDirectory(); we
+// mock PathProviderPlatform to a temp dir so the singleton opens a real (ffi)
+// sqlite DB on disk.
+
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+// ignore: depend_on_referenced_packages
+import 'package:path_provider_platform_interface/path_provider_platform_interface.dart';
+// ignore: depend_on_referenced_packages
+import 'package:plugin_platform_interface/plugin_platform_interface.dart';
+import 'package:sqflite_common_ffi/sqflite_ffi.dart';
+
+import 'package:badnote/models/ink_point.dart';
+import 'package:badnote/models/ink_stroke.dart';
+import 'package:badnote/models/pen_tool.dart';
+import 'package:badnote/models/scratch_link.dart';
+import 'package:badnote/services/database_service.dart';
+
+class _FakePathProvider extends PathProviderPlatform
+    with MockPlatformInterfaceMixin {
+  _FakePathProvider(this.dir);
+  final String dir;
+  @override
+  Future<String?> getApplicationDocumentsPath() async => dir;
+}
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+  sqfliteFfiInit();
+  databaseFactory = databaseFactoryFfi;
+
+  group('ScratchLink model', () {
+    test('toJson/fromJson round-trips', () {
+      const link = ScratchLink(
+        id: 'anchor-1',
+        documentId: 'doc-abc',
+        pageIndex: 3,
+        nx: 0.25,
+        ny: 0.8,
+      );
+      final restored = ScratchLink.fromJson(jsonDecode(jsonEncode(link.toJson()))
+          as Map<String, dynamic>);
+      expect(restored, link);
+      expect(restored.pageIndex, 3);
+      expect(restored.nx, 0.25);
+      expect(restored.ny, 0.8);
+    });
+  });
+
+  group('ScratchLink persistence', () {
+    late Directory tmp;
+
+    setUp(() async {
+      tmp = await Directory.systemTemp.createTemp('scratch_link_test_');
+      final dbFile = File('${tmp.path}/badnote.db');
+      if (dbFile.existsSync()) dbFile.deleteSync();
+      PathProviderPlatform.instance = _FakePathProvider(tmp.path);
+      await DatabaseService.resetForTest();
+    });
+
+    tearDown(() async {
+      await DatabaseService.resetForTest();
+      try {
+        tmp.deleteSync(recursive: true);
+      } catch (_) {}
+    });
+
+    test('save / load / delete anchors round-trip', () async {
+      final db = await DatabaseService.getInstance();
+
+      const a = ScratchLink(
+        id: 'a',
+        documentId: 'doc-1',
+        pageIndex: 0,
+        nx: 0.1,
+        ny: 0.2,
+      );
+      const b = ScratchLink(
+        id: 'b',
+        documentId: 'doc-1',
+        pageIndex: 4,
+        nx: 0.7,
+        ny: 0.9,
+      );
+      // A different document's anchor must not leak into doc-1's list.
+      const other = ScratchLink(
+        id: 'c',
+        documentId: 'doc-2',
+        pageIndex: 1,
+        nx: 0.5,
+        ny: 0.5,
+      );
+
+      await db.saveScratchLink(a);
+      await db.saveScratchLink(b);
+      await db.saveScratchLink(other);
+
+      final loaded = await db.loadScratchLinks('doc-1');
+      expect(loaded.length, 2);
+      expect(loaded.map((l) => l.id).toSet(), {'a', 'b'});
+      final reloadedA = loaded.firstWhere((l) => l.id == 'a');
+      expect(reloadedA.pageIndex, 0);
+      expect(reloadedA.nx, 0.1);
+      expect(reloadedA.ny, 0.2);
+
+      // doc-2 keeps its own anchor.
+      expect((await db.loadScratchLinks('doc-2')).single.id, 'c');
+
+      await db.deleteScratchLink('a');
+      final after = await db.loadScratchLinks('doc-1');
+      expect(after.map((l) => l.id), ['b']);
+    });
+
+    test('each anchor has its own private scratchpad keyed by anchor id',
+        () async {
+      final db = await DatabaseService.getInstance();
+
+      InkStroke stroke(String id, double x) => InkStroke(
+            id: id,
+            points: [
+              InkPoint(x: x, y: x, pressure: 0.5, timestamp: 0),
+            ],
+            tool: PenTool.pen,
+            createdAt: DateTime.fromMillisecondsSinceEpoch(0),
+          );
+
+      // Two anchors, two different private scratchpads (keyed by anchor id).
+      final inkA = [stroke('s1', 10)];
+      final inkB = [stroke('s2', 20)];
+      await db.saveScratchpad(
+          'anchor-A', jsonEncode(inkA.map((s) => s.toJson()).toList()));
+      await db.saveScratchpad(
+          'anchor-B', jsonEncode(inkB.map((s) => s.toJson()).toList()));
+
+      final loadedA = await db.loadScratchpad('anchor-A');
+      final loadedB = await db.loadScratchpad('anchor-B');
+      expect(loadedA.single.points.single.x, 10);
+      expect(loadedB.single.points.single.x, 20);
+
+      // Deleting the anchor removes its private scratchpad too.
+      await db.saveScratchLink(const ScratchLink(
+        id: 'anchor-A',
+        documentId: 'doc-9',
+        pageIndex: 0,
+        nx: 0.0,
+        ny: 0.0,
+      ));
+      await db.deleteScratchLink('anchor-A');
+      expect(await db.loadScratchpad('anchor-A'), isEmpty);
+      // anchor-B is untouched.
+      expect((await db.loadScratchpad('anchor-B')).single.points.single.x, 20);
+    });
+  });
+}