Clear mouse capture when control removed.

When a captured control is removed from the visual tree, clear the mouse capture.

Note that our capture logic needs to be improved - it shouldn't be possible to capture a non-visible or non-enabled control. This doesn't address that but it does fix the crash in #1203.

Fixes #1203

src/Avalonia.Input/MouseDevice.cs

@@ -20,6 +20,8 @@ namespace Avalonia.Input
         private int _clickCount;
         private Rect _lastClickRect;
         private uint _lastClickTime;
+        private IInputElement _captured;
+        private IDisposable _capturedSubscription;
        
         /// <summary>
         /// Gets the control that is currently capturing by the mouse, if any.
@@ -31,8 +33,23 @@ namespace Avalonia.Input
         /// </remarks>
         public IInputElement Captured
         {
-            get;
-            protected set;
+            get => _captured;
+            protected set
+            {
+                _capturedSubscription?.Dispose();
+                _capturedSubscription = null;
+
+                if (value != null)
+                {
+                    _capturedSubscription = Observable.FromEventPattern<VisualTreeAttachmentEventArgs>(
+                        x => value.DetachedFromVisualTree += x,
+                        x => value.DetachedFromVisualTree -= x)
+                        .Take(1)
+                        .Subscribe(_ => Captured = null);
+                }
+
+                _captured = value;
+            }
         }
         
         /// <summary>
@@ -55,6 +72,7 @@ namespace Avalonia.Input
         /// </remarks>
         public virtual void Capture(IInputElement control)
         {
+            // TODO: Check visibility and enabled state before setting capture.
             Captured = control;
         }
 

tests/Avalonia.Input.UnitTests/MouseDeviceTests.cs

@@ -1,6 +1,5 @@
 using Avalonia.Controls;
 using Avalonia.Input.Raw;
-using Avalonia.Layout;
 using Avalonia.Rendering;
 using Avalonia.UnitTests;
 using Avalonia.VisualTree;
@@ -12,6 +11,24 @@ namespace Avalonia.Input.UnitTests
 {
     public class MouseDeviceTests
     {
+        [Fact]
+        public void Capture_Is_Cleared_When_Control_Removed()
+        {
+            Canvas control;
+            var root = new TestRoot
+            {
+                Child = control = new Canvas(),
+            };
+            var target = new MouseDevice();
+
+            target.Capture(control);
+            Assert.Same(control, target.Captured);
+
+            root.Child = null;
+
+            Assert.Null(target.Captured);
+        }
+
         [Fact]
         public void MouseMove_Should_Update_PointerOver()
         {