Keep a touch pan with the scrollable that started it

- The browser backend remembers the element that scrolled for the pan and
  routes the rest of the gesture from it, so a list scrolled into the start
  point no longer takes the gesture over
- The pin falls back to the point when it stops handling, which is how the
  gesture reaches an outer scrollable once the inner one runs out of room
- The pin is dropped on a pointer press or cancel

Author: al6uiz

src/MewUI.Platform.Browser/BrowserWindowBackend.cs

@@ -9,6 +9,8 @@ internal sealed class BrowserWindowBackend : IWindowBackend
     private bool _disposed;
     private bool _shown;
     private bool _mouseCaptured;
+    // Element that scrolled for the active touch pan, kept so the gesture cannot change owner.
+    private UIElement? _panTarget;
     private readonly BrowserWindowSurface _surface = new();
 
     internal BrowserWindowBackend(BrowserPlatformHost host, Window window)
@@ -86,6 +88,7 @@ internal bool PointerButton(double x, double y, double screenX, double screenY,
         bool isDown, int clickCount, ModifierKeys modifiers, PointerType pointerType)
     {
         if (!_shown || _disposed) return false;
+        _panTarget = null;
         var mappedButton = button switch
         {
             1 => MouseButton.Middle,
@@ -147,15 +150,26 @@ internal void PointerPan(double x, double y, double screenX, double screenY,
 
         // A notch is worth ScrollWheelStep DIPs, and notches may be fractional, so dividing the
         // finger delta by the step makes the content track the finger one to one.
-        WindowInputRouter.MouseWheel(
-            Window,
-            new Point(x, y),
-            new Point(screenX, screenY),
-            new Vector(deltaXDip / step, deltaYDip / step),
-            leftDown: false,
-            rightDown: false,
-            middleDown: false,
-            modifiers);
+        var delta = new Vector(deltaXDip / step, deltaYDip / step);
+        var point = new Point(x, y);
+        var screenPoint = new Point(screenX, screenY);
+
+        // The gesture stays with whatever first scrolled for it. Hit-testing the start point again
+        // every move would hand the gesture to any other scrollable that the scrolling brought under
+        // it. A pinned target that stops handling (it ran out of room, or it was virtualized away)
+        // falls back to the point, which is also how the gesture reaches an outer scrollable.
+        var handler = WindowInputRouter.MouseWheel(
+            Window, point, screenPoint, delta, false, false, false, modifiers, _panTarget);
+        if (handler == null && _panTarget != null)
+        {
+            handler = WindowInputRouter.MouseWheel(
+                Window, point, screenPoint, delta, false, false, false, modifiers, routeFrom: null);
+        }
+
+        if (handler != null)
+        {
+            _panTarget = handler;
+        }
     }
 
     internal void PointerLeave()
@@ -170,6 +184,7 @@ internal void PointerCancel()
     {
         if (_disposed) return;
         _mouseCaptured = false;
+        _panTarget = null;
         Window.ReleaseMouseCapture();
         WindowInputRouter.UpdateMouseOver(Window, null);
     }

src/MewUI.Platform.Browser/Registration.cs

@@ -32,7 +32,7 @@ public static bool RenderFrame(double cssWidth, double cssHeight, double deviceP
     public static bool PointerMove(double x, double y, double screenX, double screenY, int buttons, ModifierKeys modifiers)
         => BrowserPlatformHost.Active?.PointerMove(x, y, screenX, screenY, buttons, modifiers) == true;
 
-    /// <param name="pointerType">0 for a mouse, 1 for touch, 2 for a pen.</param>
+    /// <summary>Routes a pointer press or release; pointerType is 0 for a mouse, 1 for touch, 2 for a pen.</summary>
     public static bool PointerButton(double x, double y, double screenX, double screenY, int button, int buttons,
         bool isDown, int clickCount, ModifierKeys modifiers, int pointerType)
         => BrowserPlatformHost.Active?.PointerButton(