Fix parent selectors with /template/ at end.

Previously the combinator state was lost when traversing nested selectors. To fix it, instead of running the parent selector in `NestingSelector.Evaluate`, return the parent selector with `MovePrevious`. This required `MovePrevious` to be aware of parent styles and because I had to change its signature, I also made it internal as it doesn't need to be a public API.

src/Avalonia.Base/Styling/ChildSelector.cs

@@ -64,7 +64,7 @@ namespace Avalonia.Styling
             }
         }
 
-        protected override Selector? MovePrevious() => null;
+        private protected override (Selector?, IStyle?) MovePrevious(IStyle? nestingParent) => (null, null);
         protected override Selector? MovePreviousOrParent() => _parent;
     }
 }

src/Avalonia.Base/Styling/DescendentSelector.cs

@@ -69,7 +69,7 @@ namespace Avalonia.Styling
             }
         }
 
-        protected override Selector? MovePrevious() => null;
+        private protected override (Selector?, IStyle?) MovePrevious(IStyle? nestingParent) => (null, null);
         protected override Selector? MovePreviousOrParent() => _parent;
     }
 }

src/Avalonia.Base/Styling/NestingSelector.cs

@@ -17,7 +17,7 @@ namespace Avalonia.Styling
         {
             if (parent is Style s && s.Selector is not null)
             {
-                return s.Selector.Match(control, s.Parent, subscribe);
+                return SelectorMatch.AlwaysThisType;
             }
             else if (parent is ControlTheme theme)
             {
@@ -32,7 +32,11 @@ namespace Avalonia.Styling
                 "Nesting selector was specified but cannot determine parent selector.");
         }
 
-        protected override Selector? MovePrevious() => null;
+        private protected override (Selector?, IStyle?) MovePrevious(IStyle? parent)
+        {
+            return parent is Style parentStyle ? (parentStyle.Selector, parentStyle.Parent) : (null, null);
+        }
+
         protected override Selector? MovePreviousOrParent() => null;
     }
 }

src/Avalonia.Base/Styling/NotSelector.cs

@@ -66,7 +66,7 @@ namespace Avalonia.Styling
             }
         }
 
-        protected override Selector? MovePrevious() => _previous;
+        private protected override (Selector?, IStyle?) MovePrevious(IStyle? nestingParent) => (_previous, nestingParent);
         protected override Selector? MovePreviousOrParent() => _previous;
     }
 }

src/Avalonia.Base/Styling/NthChildSelector.cs

@@ -104,7 +104,7 @@ namespace Avalonia.Styling
             return match ? SelectorMatch.AlwaysThisInstance : SelectorMatch.NeverThisInstance;
         }
 
-        protected override Selector? MovePrevious() => _previous;
+        private protected override (Selector?, IStyle?) MovePrevious(IStyle? nestingParent) => (_previous, nestingParent);
         protected override Selector? MovePreviousOrParent() => _previous;
 
         public override string ToString()

src/Avalonia.Base/Styling/OrSelector.cs

@@ -102,7 +102,7 @@ namespace Avalonia.Styling
             }
         }
 
-        protected override Selector? MovePrevious() => null;
+        private protected override (Selector?, IStyle?) MovePrevious(IStyle? nestingParent) => (null, null);
         protected override Selector? MovePreviousOrParent() => null;
 
         internal override void ValidateNestingSelector(bool inControlTheme)

src/Avalonia.Base/Styling/PropertyEqualsSelector.cs

@@ -89,7 +89,7 @@ namespace Avalonia.Styling
             
         }
 
-        protected override Selector? MovePrevious() => _previous;
+        private protected override (Selector?, IStyle?) MovePrevious(IStyle? nestingParent) => (_previous, nestingParent);
         protected override Selector? MovePreviousOrParent() => _previous;
 
         internal static bool Compare(Type propertyType, object? propertyValue, object? value)

src/Avalonia.Base/Styling/Selector.cs

@@ -84,7 +84,13 @@ namespace Avalonia.Styling
         /// <summary>
         /// Moves to the previous selector.
         /// </summary>
-        protected abstract Selector? MovePrevious();
+        /// <param name="nestingParent">
+        /// The parent style, if the selector is on a nested style.
+        /// </param>
+        /// <remarks>
+        /// The previous selector, and its nesting parent.
+        /// </remarks>
+        private protected abstract (Selector?, IStyle?) MovePrevious(IStyle? nestingParent);
 
         /// <summary>
         /// Moves to the previous selector or the parent selector.
@@ -142,14 +148,14 @@ namespace Avalonia.Styling
             ref AndActivatorBuilder activators,
             ref Selector? combinator)
         {
-            var previous = selector.MovePrevious();
+            var (previous, previousParent) = selector.MovePrevious(parent);
 
             // Selectors are stored from right-to-left, so we recurse into the selector in order to
             // reverse this order, because the type selector will be on the left and is our best
             // opportunity to exit early.
             if (previous != null && !previous.IsCombinator)
             {
-                var previousMatch = Match(control, previous, parent, subscribe, ref activators, ref combinator);
+                var previousMatch = Match(control, previous, previousParent, subscribe, ref activators, ref combinator);
 
                 if (previousMatch < SelectorMatchResult.Sometimes)
                 {

src/Avalonia.Base/Styling/TemplateSelector.cs

@@ -48,7 +48,7 @@ namespace Avalonia.Styling
             return _parent.Match(templatedParent, parent, subscribe);
         }
 
-        protected override Selector? MovePrevious() => null;
+        private protected override (Selector?, IStyle?) MovePrevious(IStyle? nestingParent) => (null, null);
         protected override Selector? MovePreviousOrParent() => _parent;
     }
 }

src/Avalonia.Base/Styling/TypeNameAndClassSelector.cs

@@ -139,7 +139,7 @@ namespace Avalonia.Styling
             return Name == null ? SelectorMatch.AlwaysThisType : SelectorMatch.AlwaysThisInstance;
         }
 
-        protected override Selector? MovePrevious() => _previous;
+        private protected override (Selector?, IStyle?) MovePrevious(IStyle? nestingParent) => (_previous, nestingParent);
         protected override Selector? MovePreviousOrParent() => _previous;
 
         private string BuildSelectorString()

tests/Avalonia.Base.UnitTests/Styling/SelectorTests_Nesting.cs

@@ -1,5 +1,6 @@
 using System;
 using Avalonia.Controls;
+using Avalonia.Controls.Templates;
 using Avalonia.Styling;
 using Avalonia.Styling.Activators;
 using Xunit;
@@ -148,6 +149,85 @@ namespace Avalonia.Base.UnitTests.Styling
             control.Classes.Remove("foo");
             Assert.False(sink.Active);
         }
+        
+        [Fact]
+        public void Template_Nesting_OfType_Matches()
+        {
+            var control = new Control1 { Classes = { "foo" } };
+            var button = new Button
+            {
+                Template = new FuncControlTemplate((x, _) => control),
+            };
+
+            button.ApplyTemplate();
+
+            Style nested;
+            var parent = new Style(x => x.OfType<Button>().Template())
+            {
+                Children =
+                {
+                    (nested = new Style(x => x.Nesting().OfType<Control1>())),
+                }
+            };
+
+            // REMOOOOVEEEE
+            var foo = new Style(x => x.OfType<Button>().Template().OfType<Control1>());
+            var match2 = foo.Selector.Match(control, parent);
+            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, match2.Result);
+
+            var match = nested.Selector.Match(control, parent);
+            Assert.Equal(SelectorMatchResult.AlwaysThisInstance, match.Result);
+
+
+        }
+
+        [Fact]
+        public void Template_Nesting_OfType_Class_Matches()
+        {
+            var control = new Control1 { Classes = { "foo" } };
+            var button = new Button
+            {
+                Template = new FuncControlTemplate((x, _) => control),
+            };
+
+            button.ApplyTemplate();
+
+            Style nested;
+            var parent = new Style(x => x.OfType<Button>().Template())
+            {
+                Children =
+                {
+                    (nested = new Style(x => x.Nesting().OfType<Control1>().Class("foo"))),
+                }
+            };
+
+            var match = nested.Selector.Match(control, parent);
+            Assert.Equal(SelectorMatchResult.Sometimes, match.Result);
+        }
+
+        [Fact]
+        public void Class_Template_Nesting_OfType_Matches()
+        {
+            var control = new Control1 { Classes = { "foo" } };
+            var button = new Button
+            {
+                Template = new FuncControlTemplate((x, _) => control),
+            };
+
+            button.ApplyTemplate();
+
+            Style nested;
+            var parent = new Style(x => x.OfType<Button>().Class("bar").Template())
+            {
+                Children =
+                {
+                    (nested = new Style(x => x.Nesting().OfType<Control1>())),
+                }
+            };
+
+            var match = nested.Selector.Match(control, parent);
+            Assert.Equal(SelectorMatchResult.Sometimes, match.Result);
+        }
 
         [Fact]
         public void Or_Nesting_Class_Matches()