feat: accept edge swipes across the whole margin beside the text column

app/lib/presentation/reader/reader_screen.dart

@@ -97,6 +97,11 @@ class _ReaderScreenState extends State<ReaderScreen> {
                       dark: Theme.of(context).brightness == Brightness.dark,
                     ).background;
 
+              // The 40em column plus its padding: swipes are accepted
+              // anywhere in the margin beside it.
+              final contentWidth =
+                  42 * 17.0 * deps.settings.fontScale.value;
+
               return SwipeNavigator(
                 canGoBack: browser.canGoBack,
                 canGoForward: browser.canGoForward,
@@ -105,6 +110,7 @@ class _ReaderScreenState extends State<ReaderScreen> {
                 backTitle: browser.backPage.value?.title,
                 forwardTitle: browser.forwardPage.value?.title,
                 background: peekBackground,
+                contentWidth: page == null ? null : contentWidth,
                 onBack: browser.back,
                 onForward: browser.forward,
                 child: Stack(

app/lib/presentation/reader/swipe_navigator.dart

@@ -21,6 +21,7 @@ class SwipeNavigator extends StatefulWidget {
     this.forwardView,
     this.backTitle,
     this.forwardTitle,
+    this.contentWidth,
     super.key,
   });
 
@@ -35,6 +36,11 @@ class SwipeNavigator extends StatefulWidget {
   final String? backTitle;
   final String? forwardTitle;
 
+  /// Width of the content column; swipes are accepted anywhere in the empty
+  /// margin beside it, plus a base zone at each screen edge. Null keeps only
+  /// the base edge zone.
+  final double? contentWidth;
+
   final Color background;
   final VoidCallback onBack;
   final VoidCallback onForward;
@@ -73,10 +79,16 @@ class _SwipeNavigatorState extends State<SwipeNavigator>
 
   bool get _settledAnimating => _settling != null && _settle.isAnimating;
 
-  /// How close to a screen edge a drag must begin to navigate; horizontal
+  /// Base zone at each screen edge that always swipes; horizontal drags
-  /// drags elsewhere belong to text selection.
+  /// elsewhere belong to text selection.
   static const _edgeZone = 40.0;
 
+  /// The swipe zone: the whole margin beside the content column, with the
+  /// base zone as the floor.
+  double _stripWidth(double width) => widget.contentWidth == null
+      ? _edgeZone
+      : math.max(_edgeZone, (width - widget.contentWidth!) / 2);
+
   void _onDragStart(DragStartDetails details) {
     if (_settledAnimating) return;
     setState(() => _dragging = true);
@@ -172,23 +184,23 @@ class _SwipeNavigatorState extends State<SwipeNavigator>
               offset: Offset(dx, 0),
               child: widget.child,
             ),
-            // Only horizontal drags that begin at a screen edge navigate;
+            // Horizontal drags in the strips navigate; elsewhere they select
-            // the strips are translucent, so taps, vertical scrolling and
+            // text. The strips are translucent, so taps, vertical scrolling
-            // text selection still pass through everywhere.
+            // and mid-page selection still pass through everywhere.
-            _edgeStrip(fromLeft: true),
+            _edgeStrip(fromLeft: true, width: _stripWidth(_width)),
-            _edgeStrip(fromLeft: false),
+            _edgeStrip(fromLeft: false, width: _stripWidth(_width)),
           ],
         );
       },
     );
   }
 
-  Widget _edgeStrip({required bool fromLeft}) => Positioned(
+  Widget _edgeStrip({required bool fromLeft, required double width}) => Positioned(
         left: fromLeft ? 0 : null,
         right: fromLeft ? null : 0,
         top: 0,
         bottom: 0,
-        width: _edgeZone,
+        width: width,
         child: GestureDetector(
           behavior: HitTestBehavior.translucent,
           onHorizontalDragStart: _onDragStart,

app/test/swipe_navigator_test.dart

@@ -20,8 +20,10 @@ SwipeNavigator _navigator({
   required VoidCallback onForward,
   bool canGoBack = true,
   bool canGoForward = true,
+  double? contentWidth,
 }) =>
     SwipeNavigator(
+      contentWidth: contentWidth,
       canGoBack: canGoBack,
       canGoForward: canGoForward,
       backTitle: 'Previous page',
@@ -101,6 +103,47 @@ void main() {
     expect(find.text('Previous page'), findsNothing);
   });
 
+  testWidgets('swipes anywhere in the margin beside the column',
+      (tester) async {
+    var back = 0;
+    await tester.pumpWidget(_wrap(_navigator(
+      onBack: () => back++,
+      onForward: () {},
+      // A 40em column leaves 200px margins on an 800px surface.
+      contentWidth: 400,
+    )));
+
+    final center = tester.getCenter(find.byType(SwipeNavigator));
+    // Deep inside the left margin, far past the base edge zone.
+    final gesture = await tester.startGesture(Offset(180, center.dy));
+    await gesture.moveBy(const Offset(300, 0));
+    await tester.pump();
+    expect(find.text('Previous page'), findsOneWidget);
+    await gesture.up();
+    await tester.pumpAndSettle();
+    expect(back, 1);
+  });
+
+  testWidgets('drags over the content column still belong to selection',
+      (tester) async {
+    var back = 0;
+    await tester.pumpWidget(_wrap(_navigator(
+      onBack: () => back++,
+      onForward: () {},
+      contentWidth: 400,
+    )));
+
+    final center = tester.getCenter(find.byType(SwipeNavigator));
+    // Inside the column: no swipe, nothing peeks.
+    final gesture = await tester.startGesture(Offset(400, center.dy));
+    await gesture.moveBy(const Offset(300, 0));
+    await tester.pump();
+    expect(find.text('Previous page'), findsNothing);
+    await gesture.up();
+    await tester.pumpAndSettle();
+    expect(back, 0);
+  });
+
   testWidgets('directions without pages rubber-band and never commit',
       (tester) async {
     var back = 0;