Added a couple of TabIndex tests.

tests/Avalonia.Input.UnitTests/KeyboardNavigationTests_Tab.cs

@@ -1,3 +1,4 @@
+using System.Collections.Generic;
 using Avalonia.Controls;
 using Xunit;
 
@@ -1132,5 +1133,97 @@ namespace Avalonia.Input.UnitTests
 
             Assert.Null(result);
         }
+
+        [Fact]
+        public void Respects_TabIndex_Moving_Forwards()
+        {
+            Button start;
+
+            var top = new StackPanel
+            {
+                [KeyboardNavigation.TabNavigationProperty] = KeyboardNavigationMode.Cycle,
+                Children =
+                {
+                    new StackPanel
+                    {
+                        Children =
+                        {
+                            new Button { Name = "Button1", TabIndex = 5 },
+                            (start = new Button { Name = "Button2", TabIndex = 2 }),
+                            new Button { Name = "Button3", TabIndex = 1 },
+                        }
+                    },
+                    new StackPanel
+                    {
+                        Children =
+                        {
+                            new Button { Name = "Button4", TabIndex = 3  },
+                            new Button { Name = "Button5", TabIndex = 6  },
+                            new Button { Name = "Button6", TabIndex = 4  },
+                        }
+                    },
+                }
+            };
+
+            var result = new List<string>();
+            var current = (IInputElement)start;
+
+            do
+            {
+                result.Add(((IControl)current).Name);
+                current = KeyboardNavigationHandler.GetNext(current, NavigationDirection.Next);
+            } while (current is object && current != start);
+
+            Assert.Equal(new[]
+            {
+                "Button2", "Button4", "Button6", "Button1", "Button5", "Button3"
+            }, result);
+        }
+
+        [Fact]
+        public void Respects_TabIndex_Moving_Backwards()
+        {
+            Button start;
+
+            var top = new StackPanel
+            {
+                [KeyboardNavigation.TabNavigationProperty] = KeyboardNavigationMode.Cycle,
+                Children =
+                {
+                    new StackPanel
+                    {
+                        Children =
+                        {
+                            new Button { Name = "Button1", TabIndex = 5 },
+                            (start = new Button { Name = "Button2", TabIndex = 2 }),
+                            new Button { Name = "Button3", TabIndex = 1 },
+                        }
+                    },
+                    new StackPanel
+                    {
+                        Children =
+                        {
+                            new Button { Name = "Button4", TabIndex = 3  },
+                            new Button { Name = "Button5", TabIndex = 6  },
+                            new Button { Name = "Button6", TabIndex = 4  },
+                        }
+                    },
+                }
+            };
+
+            var result = new List<string>();
+            var current = (IInputElement)start;
+
+            do
+            {
+                result.Add(((IControl)current).Name);
+                current = KeyboardNavigationHandler.GetNext(current, NavigationDirection.Previous);
+            } while (current is object && current != start);
+
+            Assert.Equal(new[]
+            {
+                "Button2", "Button3", "Button5", "Button1", "Button6", "Button4"
+            }, result);
+        }
     }
 }