[AI-3824] app: Scroll the chat while a text selection is dragged

src/Capacitor.App/Views/ChatTabView.axaml.cs

@@ -29,6 +29,10 @@ public partial class ChatTabView : UserControl {
     bool _readerGesture;
     int _listInputVersion;
     bool _thumbDragging;
+    bool _dragSelecting;
+    bool _edgeScrollQueued;
+    Point _dragInScroll;
+    MarkdownView? _dragView;
     double? _thumbFraction;
     double _smoothRemaining;
     bool _smoothFrameQueued;
@@ -49,7 +53,9 @@ public ChatTabView() {
         // Press and release both: a click's command runs on the release, in a later dispatcher turn
         // than the press, by which time a flag armed on the press alone has been released.
         ChatItems.AddHandler(PointerPressedEvent, OnReaderGesture, RoutingStrategies.Tunnel, handledEventsToo: true);
+        ChatItems.AddHandler(PointerMovedEvent, OnSelectionDrag, RoutingStrategies.Bubble, handledEventsToo: true);
         ChatItems.AddHandler(PointerReleasedEvent, OnReaderGesture, RoutingStrategies.Tunnel, handledEventsToo: true);
+        ChatItems.AddHandler(PointerReleasedEvent, OnSelectionDragEnded, RoutingStrategies.Tunnel, handledEventsToo: true);
         ChatItems.AddHandler(PointerWheelChangedEvent, OnReaderGesture, RoutingStrategies.Tunnel, handledEventsToo: true);
         ChatItems.AddHandler(PointerWheelChangedEvent, OnChatWheelChanged, RoutingStrategies.Tunnel);
         ChatItems.AddHandler(KeyDownEvent, OnReaderGesture, RoutingStrategies.Tunnel, handledEventsToo: true);
@@ -95,6 +101,8 @@ protected override void OnDataContextChanged(EventArgs e) {
         _thumbDragging = false;
         _thumbFraction = null;
         _readerGesture = false;
+        _dragSelecting = false;
+        _dragView = null;
         _followTail = true;
         base.OnDataContextChanged(e);
     }
@@ -104,6 +112,8 @@ protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e
         CancelSmoothScroll();
         _thumbDragging = false;
         _thumbFraction = null;
+        _dragSelecting = false;
+        _dragView = null;
         _attachments?.Dispose();
         _attachments = null;
         base.OnDetachedFromVisualTree(e);
@@ -385,6 +395,62 @@ void OnScrollChanged(object? sender, ScrollChangedEventArgs e) {
         if (_followTail && !atBottom) scroll.ScrollToEnd();
     }
 
+    void OnSelectionDrag(object? sender, PointerEventArgs e) {
+        EnsureChatScroll();
+        if (_scroll is null) return;
+        if (!e.GetCurrentPoint(ChatItems).Properties.IsLeftButtonPressed || OriginatesFromTextBox(e)) {
+            _dragSelecting = false;
+            return;
+        }
+        if (e.Source is not Visual visual) return;
+        var view = visual.GetSelfAndVisualAncestors().OfType<MarkdownView>().FirstOrDefault();
+        if (!_dragSelecting) {
+            if (view is not { IsSelecting: true }) return;
+            _dragView = view;
+            _dragSelecting = true;
+        } else if (view is { IsSelecting: true }) {
+            _dragView = view;
+        }
+        if (_dragView is { HasSelection: true }) _followTail = false;
+        _dragInScroll = e.GetPosition(_scroll);
+        QueueEdgeScroll();
+    }
+
+    void OnSelectionDragEnded(object? sender, RoutedEventArgs e) => _dragSelecting = false;
+
+    void QueueEdgeScroll() {
+        if (_edgeScrollQueued || !_dragSelecting || _scroll is null) return;
+        if (TopLevel.GetTopLevel(this) is not { } top) return;
+        _edgeScrollQueued = true;
+        top.RequestAnimationFrame(_ => {
+            _edgeScrollQueued = false;
+            if (_dragSelecting) StepEdgeScroll();
+        });
+    }
+
+    void StepEdgeScroll() {
+        if (_scroll is null || !_dragSelecting) return;
+        const double edge = 28;
+        const double step = 18;
+        var y = _dragInScroll.Y;
+        var height = _scroll.Viewport.Height;
+        var delta = y < edge ? -step : y > height - edge ? step : 0.0;
+        if (Math.Abs(delta) < 0.01) return;
+        var max = Math.Max(0, _scroll.Extent.Height - _scroll.Viewport.Height);
+        var next = Math.Clamp(_scroll.Offset.Y + delta, 0, max);
+        if (Math.Abs(next - _scroll.Offset.Y) > 0.5) {
+            var wasGesture = _readerGesture;
+            _readerGesture = true;
+            _scroll.Offset = new Vector(_scroll.Offset.X, next);
+            if (!wasGesture) Dispatcher.UIThread.Post(() => _readerGesture = false, DispatcherPriority.Background);
+        }
+        if (_dragView?.TranslatePoint(default, _scroll) is { } origin) {
+            _dragView.ExtendSelection(new Point(_dragInScroll.X - origin.X, _dragInScroll.Y - origin.Y));
+            if (_dragView.HasSelection) _followTail = false;
+        }
+        QueueEdgeScroll();
+    }
+
     static bool OriginatesFromTextBox(RoutedEventArgs e) =>
         e.Source is Visual visual && (visual is TextBox || visual.GetVisualAncestors().OfType<TextBox>().Any());
 

src/Capacitor.App/Views/MarkViewSelection.cs

@@ -32,6 +32,20 @@ public static void Attach(Control host, MarkdownViewer viewer, Func<Task> copy)
         host.AddHandler(InputElement.KeyDownEvent, (_, e) => OnKeyDown(copy, e), RoutingStrategies.Tunnel);
     }
 
+    /// Continues a drag at a point in the viewer's coordinates. Used when the chat scrolls under a
+    /// pointer that is sitting on the viewport edge, so the selection grows without a new move.
+    public static void Extend(MarkdownViewer viewer, Point pointInViewer) {
+        if (LayerOf(viewer) is not Visual layer) return;
+        if (Anchor(layer) is null) return;
+        IsDragging(viewer) = true;
+        var point = viewer.TranslatePoint(pointInViewer, layer) ?? pointInViewer;
+        if (HitTestOffset(layer, point) is not null) MoveFocus(layer, point);
+        else if (Nearest(layer, point) is { } inside) MoveFocus(layer, inside);
+    }
+
+    public static bool IsSelecting(MarkdownViewer viewer) =>
+        IsDragging(viewer) && LayerOf(viewer) is { } layer && Anchor(layer) is not null;
+
     public static bool HasSelection(MarkdownViewer viewer) =>
         LayerOf(viewer) is { } layer && Anchor(layer) is { } anchor && Focus(layer) is { } focus && anchor != focus;
 

src/Capacitor.App/Views/MarkdownView.cs

@@ -62,6 +62,12 @@ public MarkdownView() {
         Content = _viewer;
     }
 
+    internal bool IsSelecting => Content is MarkdownViewer && MarkViewSelection.IsSelecting(_viewer);
+
+    internal void ExtendSelection(Point pointInView) {
+        if (Content is MarkdownViewer) MarkViewSelection.Extend(_viewer, pointInView);
+    }
+
     public bool HasSelection => Content is SelectableTextBlock plain
         ? !string.IsNullOrEmpty(plain.SelectedText)
         : MarkViewSelection.HasSelection(_viewer);