Add an OrActivatorBuilder because we have one for And.

src/Avalonia.Styling/Styling/Activators/OrActivatorBuilder.cs

@@ -0,0 +1,38 @@
+#nullable enable
+
+namespace Avalonia.Styling.Activators
+{
+    internal struct OrActivatorBuilder
+    {
+        private IStyleActivator? _single;
+        private OrActivator? _multiple;
+
+        public int Count => _multiple?.Count ?? (_single is object ? 1 : 0);
+
+        public void Add(IStyleActivator? activator)
+        {
+            if (activator == null)
+            {
+                return;
+            }
+
+            if (_single is null && _multiple is null)
+            {
+                _single = activator;
+            }
+            else
+            {
+                if (_multiple is null)
+                {
+                    _multiple = new OrActivator();
+                    _multiple.Add(_single!);
+                    _single = null;
+                }
+
+                _multiple.Add(activator);
+            }
+        }
+
+        public IStyleActivator Get() => _single ?? _multiple!;
+    }
+}

src/Avalonia.Styling/Styling/DescendentSelector.cs

@@ -41,8 +41,7 @@ namespace Avalonia.Styling
         protected override SelectorMatch Evaluate(IStyleable control, bool subscribe)
         {
             var c = (ILogical)control;
-            IStyleActivator? descendentMatch = null;
-            OrActivator? descendantMatches = null;
+            var descendantMatches = new OrActivatorBuilder();
 
             while (c != null)
             {
@@ -54,21 +53,7 @@ namespace Avalonia.Styling
 
                     if (match.Result == SelectorMatchResult.Sometimes)
                     {
-                        if (descendentMatch is null && descendantMatches is null)
-                        {
-                            descendentMatch = match.Activator;
-                        }
-                        else
-                        {
-                            if (descendantMatches is null)
-                            {
-                                descendantMatches = new OrActivator();
-                                descendantMatches.Add(descendentMatch!);
-                                descendentMatch = null;
-                            }
-
-                            descendantMatches.Add(match.Activator!);
-                        }
+                        descendantMatches.Add(match.Activator);
                     }
                     else if (match.IsMatch)
                     {
@@ -77,13 +62,9 @@ namespace Avalonia.Styling
                 }
             }
 
-            if (descendantMatches is object)
-            {
-                return new SelectorMatch(descendantMatches);
-            }
-            else if (descendentMatch is object)
+            if (descendantMatches.Count > 0)
             {
-                return new SelectorMatch(descendentMatch);
+                return new SelectorMatch(descendantMatches.Get());
             }
             else
             {

src/Avalonia.Styling/Styling/OrSelector.cs

@@ -70,8 +70,7 @@ namespace Avalonia.Styling
 
         protected override SelectorMatch Evaluate(IStyleable control, bool subscribe)
         {
-            IStyleActivator? activator = null;
-            OrActivator? activators = null;
+            var activators = new OrActivatorBuilder();
             var neverThisInstance = false;
 
             foreach (var selector in _selectors)
@@ -87,32 +86,14 @@ namespace Avalonia.Styling
                         neverThisInstance = true;
                         break;
                     case SelectorMatchResult.Sometimes:
-                        if (activator is null && activators is null)
-                        {
-                            activator = match.Activator;
-                        }
-                        else
-                        {
-                            if (activators is null)
-                            {
-                                activators = new OrActivator();
-                                activators.Add(activator!);
-                                activator = null;
-                            }
-
-                            activators.Add(match.Activator!);
-                        }
+                        activators.Add(match.Activator!);
                         break;
                 }
             }
 
-            if (activators is object)
+            if (activators.Count > 0)
             {
-                return new SelectorMatch(activators);
-            }
-            else if (activator is object)
-            {
-                return new SelectorMatch(activator);
+                return new SelectorMatch(activators.Get());
             }
             else if (neverThisInstance)
             {