Transitioner: Short-circuit SelectionChanged event when not originating from Transitioner (#3619)

* Short-circuit SelectionChanged event when not originating from Transitioner

* Add test for short circuit SelectionChanged event

* Test for SelectionChanged fix

---------

Co-authored-by: Michel Michels <[email protected]>

Author: nicolaihenriksen

src/MaterialDesignThemes.Wpf/Transitions/Transitioner.cs

@@ -62,6 +62,7 @@ public Transitioner()
         CommandBindings.Add(new CommandBinding(MoveFirstCommand, MoveFirstHandler));
         CommandBindings.Add(new CommandBinding(MoveLastCommand, MoveLastHandler));
         AddHandler(TransitionerSlide.InTransitionFinished, new RoutedEventHandler(IsTransitionFinishedHandler));
+        AddHandler(SelectionChangedEvent, new RoutedEventHandler(SelectionChangedEventHandler));
         Loaded += (sender, args) =>
         {
             if (SelectedIndex != -1)
@@ -108,6 +109,15 @@ private void IsTransitionFinishedHandler(object sender, RoutedEventArgs routedEv
         }
     }
 
+    private static void SelectionChangedEventHandler(object sender, RoutedEventArgs e)
+    {
+        if (e.OriginalSource is not Transitioner)
+        {
+            // This event is bubbling up from a child Selector element. Swallow the event to avoid confusion with the SelectionChanged event on the Transitioner itself.
+            e.Handled = true;
+        }
+    }
+
     private void MoveNextHandler(object sender, ExecutedRoutedEventArgs executedRoutedEventArgs)
     {
         if (AutoApplyTransitionOrigins)

tests/MaterialDesignThemes.Wpf.Tests/TransitionerTests.cs

@@ -1,5 +1,4 @@
 using MaterialDesignThemes.Wpf.Transitions;
-using Xunit;
 
 namespace MaterialDesignThemes.Wpf.Tests;
 
@@ -52,4 +51,37 @@ public void WhenMovePrevious_ItCanRetreatMultipleSlides()
         //Assert
         Assert.Equal(0, transitioner.SelectedIndex);
     }
+
+    [StaFact]
+    public void ShortCircuitIssue3268()
+    {
+        //Arrange
+        Grid child1 = new();
+        ListBox lb = new();
+        lb.Items.Add(new Label());
+        lb.Items.Add(new Label());
+        child1.Children.Add(lb);
+
+        UserControl child2 = new();
+
+        Transitioner transitioner = new();
+        transitioner.Items.Add(child1);
+        transitioner.Items.Add(child2);
+
+        int selectionChangedCounter = 0;
+        transitioner.SelectionChanged += (s, e) =>
+        {
+            selectionChangedCounter++;
+        };
+        Transitioner.MoveNextCommand.Execute(0, transitioner);
+
+        //Act
+        Assert.NotNull(transitioner.SelectedItem);
+        Assert.True(transitioner.SelectedItem == child1);
+        lb.SelectedItem = lb.Items[1];
+
+        //Assert
+        Assert.Equal(1, selectionChangedCounter);
+        Assert.Equal(0, transitioner.SelectedIndex);
+    }
 }