Check enabled state as well as visibility.

WPF and Avalonia's `IsEnabled` properties are slightly different.

In WPF if reflects both the enabled state of the actual control and the effectively enabled state which comes from ancestor controls. In Avalonia that effectively enabled state is exposed on a different property: `IsEffectivelyEnabled`. When I ported the tab navigation code from WPF, I didn't take that into account.

WPF's visibility property however doesn't reflect the state of a control's owners and so tab navigation for invisible controls works correctly. Take advantage of this fact by changing any checks for `IsVisible` to also check `IsEnabled`.

src/Avalonia.Input/Navigation/TabNavigation.cs

@@ -234,7 +234,7 @@ namespace Avalonia.Input.Navigation
             // Return the first visible element.
             var uiElement = e as InputElement;
 
-            if (uiElement is null || uiElement.IsVisible)
+            if (uiElement is null || IsVisibleAndEnabled(uiElement))
             {
                 if (e is IVisual elementAsVisual)
                 {
@@ -245,7 +245,7 @@ namespace Avalonia.Input.Navigation
                     {
                         if (children[i] is InputElement ie)
                         {
-                            if (ie.IsVisible)
+                            if (IsVisibleAndEnabled(ie))
                                 return ie;
                             else
                             {
@@ -270,7 +270,7 @@ namespace Avalonia.Input.Navigation
             // Return the last visible element.
             var uiElement = e as InputElement;
 
-            if (uiElement == null || uiElement.IsVisible)
+            if (uiElement == null || IsVisibleAndEnabled(uiElement))
             {
                 var elementAsVisual = e as IVisual;
 
@@ -283,7 +283,7 @@ namespace Avalonia.Input.Navigation
                     {
                         if (children[i] is InputElement ie)
                         {
-                            if (ie.IsVisible)
+                            if (IsVisibleAndEnabled(ie))
                                 return ie;
                             else
                             {
@@ -600,7 +600,7 @@ namespace Avalonia.Input.Navigation
                     var vchild = children[i];
                     if (vchild == elementAsVisual)
                         break;
-                    if (vchild.IsVisible == true && vchild is IInputElement ie)
+                    if (vchild is IInputElement ie && IsVisibleAndEnabled(ie))
                         prev = ie;
                 }
                 return prev;
@@ -668,5 +668,6 @@ namespace Avalonia.Input.Navigation
         }
 
         private static bool IsTabStopOrGroup(IInputElement e) => IsTabStop(e) || IsGroup(e);
+        private static bool IsVisibleAndEnabled(IInputElement e) => e.IsVisible && e.IsEnabled;
     }
 }