feat: show real neighboring pages during back and forward swipes

app/lib/application/actors/fetch_actor.dart

@@ -31,6 +31,15 @@ class SubmitForm extends FetchCommand {
   final Map<String, String> fields;
 }
 
+/// Parses a page's source without touching the network; used to prepare the
+/// neighboring pages that swipes reveal.
+class ParseSource extends FetchCommand {
+  const ParseSource({required this.url, required this.source});
+
+  final String url;
+  final String source;
+}
+
 sealed class FetchResult {
   const FetchResult();
 }
@@ -94,6 +103,7 @@ class FetchHandler with Handler<FetchCommand, FetchResult> {
         await _fetchPage(url, etag, lastModified, cachedSource),
       SubmitForm(:final url, :final method, :final fields) =>
         await _submitForm(url, method, fields),
+      ParseSource(:final url, :final source) => _parse(source, url),
     };
   }
 

app/lib/application/controllers/browser_controller.dart

@@ -1,5 +1,3 @@
-import 'dart:async';
-
 import 'package:actors/actors.dart';
 import 'package:drift/drift.dart' show Value;
 import 'package:signals/signals.dart';
@@ -47,9 +45,10 @@ class BrowserController {
   /// Pages opened this session, so links can show the visited color.
   final visitedUrls = signal(<String>{});
 
-  /// Titles of the stack's neighboring pages, shown while swiping.
-  final backTitle = signal<String?>(null);
-  final forwardTitle = signal<String?>(null);
+  /// The parsed pages next to the current one in the stack, ready to show
+  /// while swiping between them.
+  final backPage = signal<MewsPage?>(null);
+  final forwardPage = signal<MewsPage?>(null);
 
   bool get canGoBack => _stackIndex > 0;
   bool get canGoForward => _stackIndex >= 0 && _stack.length - 1 > _stackIndex;
@@ -69,24 +68,36 @@ class BrowserController {
     _stack.removeRange(_stackIndex + 1, _stack.length);
     _stack.add(url);
     _stackIndex++;
-    _updatePeeks();
+    _updateNeighbors();
     await _loadCurrent();
   }
 
   Future<void> back() async {
     if (!canGoBack) return;
     _stackIndex--;
-    _updatePeeks();
+    // The neighbor was parsed for the swipe already; show it right away so
+    // the transition lands on content instead of a blank page while
+    // _loadCurrent revalidates it against the server.
+    _showParsedNeighbor(backPage.value);
+    _updateNeighbors();
     await _loadCurrent();
   }
 
   Future<void> forward() async {
     if (!canGoForward) return;
     _stackIndex++;
-    _updatePeeks();
+    _showParsedNeighbor(forwardPage.value);
+    _updateNeighbors();
     await _loadCurrent();
   }
 
+  void _showParsedNeighbor(MewsPage? page) {
+    if (page == null) return;
+    currentPage.value = page;
+    error.value = null;
+    visitedUrls.value = {...visitedUrls.value, page.url};
+  }
+
   Future<void> reload() => _loadCurrent();
 
   Future<void> submitForm(String action, String method, Map<String, String> fields) async {
@@ -99,7 +110,7 @@ class BrowserController {
         _stack.removeRange(_stackIndex + 1, _stack.length);
         _stack.add(page.url);
         _stackIndex++;
-        _updatePeeks();
+        _updateNeighbors();
         _show(page);
       case _:
         error.value = const LoadFailed();
@@ -112,21 +123,34 @@ class BrowserController {
     if (url == null) return;
     _stack.add(url);
     _stackIndex = 0;
-    _updatePeeks();
+    _updateNeighbors();
     await _loadCurrent();
   }
 
-  /// Refreshes the neighboring-page titles from the cache; a page without a
-  /// cached title falls back to its URL.
-  void _updatePeeks() {
-    Future<String?> peek(int index) async {
-      if (index < 0 || index >= _stack.length) return null;
-      final url = _stack[index];
-      return (await _db.cachedPage(url))?.title ?? url;
-    }
+  /// Parses the stack's neighboring pages in the fetch actor so swipes can
+  /// show the real content. Results arriving after another navigation are
+  /// dropped, so rapid navigation never shows a stale neighbor.
+  void _updateNeighbors() {
+    _loadNeighbor(_stackIndex - 1, backPage);
+    _loadNeighbor(_stackIndex + 1, forwardPage);
+  }
 
-    unawaited(peek(_stackIndex - 1).then((t) => backTitle.value = t));
-    unawaited(peek(_stackIndex + 1).then((t) => forwardTitle.value = t));
+  Future<void> _loadNeighbor(int index, Signal<MewsPage?> target) async {
+    final expectedIndex = _stackIndex;
+    if (index < 0 || index >= _stack.length) {
+      target.value = null;
+      return;
+    }
+    final url = _stack[index];
+    final cached = await _db.cachedPage(url);
+    if (_stackIndex != expectedIndex) return;
+    if (cached == null) {
+      target.value = null;
+      return;
+    }
+    final result = await _fetch.send(ParseSource(url: url, source: cached.source));
+    if (_stackIndex != expectedIndex) return;
+    target.value = result is FetchPageOk ? result.page : null;
   }
 
   Future<void> _loadCurrent() async {