[AnimatedIcon] Fix issue where setting source property would not resi…

…ze AnimatedIcon properly (#6676)

* Add test and fix for animated icon not updating when setting source property

* Fix test failing downlevel

dev/AnimatedIcon/APITests/AnimatedIconTests.cs

@@ -427,5 +427,36 @@ void AnimatedIcon_LayoutUpdated(object sender, object e)
                 layoutUpdatedEvent.Set();
             }
         }
+
+        [TestMethod]
+        public void ChangingSourcePropertyChangesRenderSize()
+        {
+            AnimatedIcon icon = null;
+            RunOnUIThread.Execute(() =>
+            {
+                icon = new AnimatedIcon();
+                Content = new StackPanel() {
+                    Children = { icon }
+                };
+                Content.UpdateLayout();
+            });
+
+            IdleSynchronizer.Wait();
+
+            RunOnUIThread.Execute(() =>
+            {
+                // Icon height will be zero if the source property is not set.
+                Verify.IsTrue(Math.Abs(icon.ActualHeight) < 0.1);
+                icon.Source = new AnimatedChevronDownSmallVisualSource();
+            });
+
+            IdleSynchronizer.Wait();
+
+            RunOnUIThread.Execute(() =>
+            {
+                // Icon will have a height if the AnimatedIcon also updated the visual tree to rerender.
+                Verify.IsTrue(Math.Abs(icon.ActualHeight) > 10);
+            });
+        }
     }
 }

dev/AnimatedIcon/AnimatedIcon.cpp

@@ -476,6 +476,7 @@ void AnimatedIcon::OnSourcePropertyChanged(const winrt::DependencyPropertyChange
     {
         SetRootPanelChildToFallbackIcon();
     }
+    InvalidateMeasure();
 }
 
 void AnimatedIcon::UpdateMirrorTransform()