Allow binding property during ending batch update.

- Allow adding a binding to a cleared property while ending a batch update. Need to check that the existing value isn't a remove sentinel here, otherwise the binding will be lost.
- When a binding is added during the end of a batch update, `_batchUpdate` will be non-null but newly added bindings shouldn't have `BeginBatchUpdate` called on them because no `EndBatchUpdate` will arrive (as we've already called them)
- Add sanity checks to the unit test to make sure that correct notifications are raised

src/Avalonia.Base/ValueStore.cs

@@ -138,7 +138,7 @@ namespace Avalonia
             IObservable<BindingValue<T>> source,
             BindingPriority priority)
         {
-            if (_values.TryGetValue(property, out var slot))
+            if (_values.TryGetValue(property, out var slot) && !IsRemoveSentinel<T>(slot))
             {
                 return BindExisting(slot, property, source, priority);
             }
@@ -338,7 +338,7 @@ namespace Avalonia
         private void AddValue(AvaloniaProperty property, IValue value)
         {
             _values.AddValue(property, value);
-            if (_batchUpdate is object && value is IBatchUpdate batch)
+            if (_batchUpdate?.IsBatchUpdating == true && value is IBatchUpdate batch)
                 batch.BeginBatchUpdate();
             value.Start();
         }
@@ -381,6 +381,8 @@ namespace Avalonia
 
             public BatchUpdate(ValueStore owner) => _owner = owner;
 
+            public bool IsBatchUpdating => _batchUpdateCount > 0;
+
             public void Begin()
             {
                 if (_batchUpdateCount++ == 0)

tests/Avalonia.Base.UnitTests/AvaloniaObjectTests_BatchUpdate.cs

@@ -478,6 +478,7 @@ namespace Avalonia.Base.UnitTests
         {
             var target = new TestClass();
             var raised = 0;
+            var notifications = new List<AvaloniaPropertyChangedEventArgs>();
 
             target.Foo = "foo";
 
@@ -490,11 +491,16 @@ namespace Avalonia.Base.UnitTests
                     target.Bind(TestClass.FooProperty, new TestObservable<string>("bar"));
                     ++raised;
                 }
+
+                notifications.Add(e);
             };
             target.EndBatchUpdate();
 
             Assert.Equal("bar", target.Foo);
             Assert.Equal(1, raised);
+            Assert.Equal(2, notifications.Count);
+            Assert.Equal(null, notifications[0].NewValue);
+            Assert.Equal("bar", notifications[1].NewValue);
         }
 
         public class TestClass : AvaloniaObject