Refactored how we switch control themes.

Instead of simply wiping all control themes and styles that are applied to a control, we can now just remove the `ValueFrame`s which relate to the control theme that was changed.

To do this, added `ValueFrame.FramePriority` which encodes both the  `BindingPriority` and source of the frame (style, control theme, templated parent control theme).

src/Avalonia.Base/Layout/Layoutable.cs

@@ -1,5 +1,6 @@
 using System;
 using Avalonia.Logging;
+using Avalonia.Styling;
 using Avalonia.VisualTree;
 
 #nullable enable
@@ -795,6 +796,12 @@ namespace Avalonia.Layout
             base.OnVisualParentChanged(oldParent, newParent);
         }
 
+        private protected override void OnControlThemeChanged()
+        {
+            base.OnControlThemeChanged();
+            InvalidateMeasure();
+        }
+
         /// <summary>
         /// Called when the layout manager raises a LayoutUpdated event.
         /// </summary>

src/Avalonia.Base/PropertyStore/FramePriority.cs

@@ -0,0 +1,36 @@
+using System.Diagnostics;
+using Avalonia.Data;
+
+namespace Avalonia.PropertyStore
+{
+    internal enum FramePriority : sbyte
+    {
+        Animation,
+        AnimationTemplatedParentTheme,
+        AnimationTheme,
+        StyleTrigger,
+        StyleTriggerTemplatedParentTheme,
+        StyleTriggerTheme,
+        Template,
+        TemplateTemplatedParentTheme,
+        TemplateTheme,
+        Style,
+        StyleTemplatedParentTheme,
+        StyleTheme,
+    }
+
+    internal static class FramePriorityExtensions
+    {
+        public static FramePriority ToFramePriority(this BindingPriority priority, FrameType type = FrameType.Style)
+        {
+            Debug.Assert(priority != BindingPriority.LocalValue);
+            var p = (int)(priority > 0 ? priority : priority + 1);
+            return (FramePriority)(p * 3 + (int)type);
+        }
+
+        public static bool IsType(this FramePriority priority, FrameType type)
+        {
+            return (FrameType)((int)priority % 3) == type;
+        }
+    }
+}

src/Avalonia.Base/PropertyStore/ImmediateValueFrame.cs

@@ -10,8 +10,8 @@ namespace Avalonia.PropertyStore
     internal class ImmediateValueFrame : ValueFrame
     {
         public ImmediateValueFrame(BindingPriority priority)
+            : base(priority, FrameType.Style)
         {
-            Priority = priority;
         }
 
         public TypedBindingEntry<T> AddBinding<T>(

src/Avalonia.Base/PropertyStore/ValueFrame.cs

@@ -1,13 +1,18 @@
-using System;
-using System.Collections.Generic;
+using System.Collections.Generic;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using Avalonia.Data;
 using Avalonia.Utilities;
-using static Avalonia.Rendering.Composition.Animations.PropertySetSnapshot;
 
 namespace Avalonia.PropertyStore
 {
+    internal enum FrameType
+    {
+        Style,
+        TemplatedParentTheme,
+        Theme,
+    }
+
     internal abstract class ValueFrame
     {
         private List<IValueEntry>? _entries;
@@ -15,11 +20,18 @@ namespace Avalonia.PropertyStore
         private ValueStore? _owner;
         private bool _isShared;
 
+        protected ValueFrame(BindingPriority priority, FrameType type)
+        {
+            Priority = priority;
+            FramePriority = priority.ToFramePriority(type);
+        }
+
         public int EntryCount => _index.Count;
         public bool IsActive => GetIsActive(out _);
         public ValueStore? Owner => !_isShared ? _owner : 
             throw new AvaloniaInternalException("Cannot get owner for shared ValueFrame");
-        public BindingPriority Priority { get; protected set; }
+        public BindingPriority Priority { get; }
+        public FramePriority FramePriority { get; }
 
         public bool Contains(AvaloniaProperty property) => _index.ContainsKey(property);
 

src/Avalonia.Base/PropertyStore/ValueStore.cs

@@ -4,8 +4,8 @@ using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using Avalonia.Data;
 using Avalonia.Diagnostics;
-using Avalonia.Logging;
 using Avalonia.Utilities;
+using static Avalonia.Rendering.Composition.Animations.PropertySetSnapshot;
 
 namespace Avalonia.PropertyStore
 {
@@ -580,6 +580,29 @@ namespace Avalonia.PropertyStore
             return false;
         }
 
+        public void RemoveFrames(FrameType type)
+        {
+            var removed = false;
+
+            for (var i = _frames.Count - 1; i >= 0; --i)
+            {
+                var frame = _frames[i];
+
+                if (frame.FramePriority.IsType(type))
+                {
+                    _frames.RemoveAt(i);
+                    frame.Dispose();
+                    removed = true;
+                }
+            }
+
+            if (removed)
+            {
+                ++_frameGeneration;
+                ReevaluateEffectiveValues();
+            }
+        }
+
         public AvaloniaPropertyValue GetDiagnostic(AvaloniaProperty property)
         {
             object? value;
@@ -612,7 +635,7 @@ namespace Avalonia.PropertyStore
         {
             Debug.Assert(!_frames.Contains(frame));
 
-            var index = BinarySearchFrame(frame.Priority);
+            var index = BinarySearchFrame(frame.FramePriority);
             _frames.Insert(index, frame);
             ++_frameGeneration;
             frame.SetOwner(this);
@@ -626,7 +649,7 @@ namespace Avalonia.PropertyStore
         {
             Debug.Assert(priority != BindingPriority.LocalValue);
 
-            var index = BinarySearchFrame(priority);
+            var index = BinarySearchFrame(priority.ToFramePriority());
 
             if (index > 0 && _frames[index - 1] is ImmediateValueFrame f &&
                 f.Priority == priority &&
@@ -914,7 +937,7 @@ namespace Avalonia.PropertyStore
             }
         }
 
-        private int BinarySearchFrame(BindingPriority priority)
+        private int BinarySearchFrame(FramePriority priority)
         {
             var lo = 0;
             var hi = _frames.Count - 1;
@@ -923,7 +946,7 @@ namespace Avalonia.PropertyStore
             while (lo <= hi)
             {
                 var i = lo + ((hi - lo) >> 1);
-                var order = priority - _frames[i].Priority;
+                var order = priority - _frames[i].FramePriority;
 
                 if (order <= 0)
                 {

src/Avalonia.Base/StyledElement.cs

@@ -3,6 +3,7 @@ using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.ComponentModel;
+using System.Diagnostics;
 using System.Linq;
 using Avalonia.Animation;
 using Avalonia.Collections;
@@ -11,6 +12,7 @@ using Avalonia.Data;
 using Avalonia.Diagnostics;
 using Avalonia.Logging;
 using Avalonia.LogicalTree;
+using Avalonia.PropertyStore;
 using Avalonia.Styling;
 
 namespace Avalonia
@@ -69,10 +71,10 @@ namespace Avalonia
         private IAvaloniaList<ILogical>? _logicalChildren;
         private IResourceDictionary? _resources;
         private Styles? _styles;
-        private bool _styled;
+        private bool _stylesApplied;
+        private bool _themeApplied;
         private ITemplatedControl? _templatedParent;
         private bool _dataContextUpdating;
-        private bool _hasPromotedTheme;
         private ControlTheme? _implicitTheme;
 
         /// <summary>
@@ -141,7 +143,7 @@ namespace Avalonia
 
             set
             {
-                if (_styled)
+                if (_stylesApplied)
                 {
                     throw new InvalidOperationException("Cannot set Name : styled element already styled.");
                 }
@@ -353,31 +355,31 @@ namespace Avalonia
         /// </returns>
         public bool ApplyStyling()
         {
-            if (_initCount == 0 && !_styled)
+            if (_initCount == 0 && (!_stylesApplied || !_themeApplied))
             {
-                var hasPromotedTheme = _hasPromotedTheme;
-
                 GetValueStore().BeginStyling();
 
                 try
                 {
-                    ApplyControlTheme();
-                    ApplyStyles(this);
+                    if (!_themeApplied)
+                    {
+                        ApplyControlTheme();
+                        _themeApplied = true;
+                    }
+
+                    if (!_stylesApplied)
+                    {
+                        ApplyStyles(this);
+                        _stylesApplied = true;
+                    }
                 }
                 finally
                 {
-                    _styled = true;
                     GetValueStore().EndStyling();
                 }
-
-                if (hasPromotedTheme)
-                {
-                    _hasPromotedTheme = false;
-                    ClearValue(ThemeProperty);
-                }
             }
 
-            return _styled;
+            return _stylesApplied;
         }
 
         /// <summary>
@@ -615,31 +617,25 @@ namespace Avalonia
 
             if (change.Property == ThemeProperty)
             {
-                var (oldValue, newValue) = change.GetOldAndNewValue<ControlTheme?>();
-
-                // Changing the theme detaches all styles, meaning that if the theme property was
-                // set via a style, it will get cleared! To work around this, if the value was
-                // applied at less than local value priority then promote the value to local value
-                // priority until styling is re-applied.
-                if (change.Priority > BindingPriority.LocalValue)
-                {
-                    Theme = newValue;
-                    _hasPromotedTheme = true;
-                }
-                else if (_hasPromotedTheme && change.Priority == BindingPriority.LocalValue)
-                {
-                    _hasPromotedTheme = false;
-                }
-
-                InvalidateStyles();
-
-                if (oldValue is not null)
-                    DetachControlThemeFromTemplateChildren(oldValue);
+                OnControlThemeChanged();
+                _themeApplied = false;
             }
         }
 
-        internal virtual void DetachControlThemeFromTemplateChildren(ControlTheme theme)
+        private protected virtual void OnControlThemeChanged()
         {
+            var values = GetValueStore();
+            values.BeginStyling();
+            try { values.RemoveFrames(FrameType.Theme); }
+            finally { values.EndStyling(); }
+        }
+
+        internal virtual void OnTemplatedParentControlThemeChanged()
+        {
+            var values = GetValueStore();
+            values.BeginStyling();
+            try { values.RemoveFrames(FrameType.TemplatedParentTheme); }
+            finally { values.EndStyling(); }
         }
 
         internal ControlTheme? GetEffectiveTheme()
@@ -736,26 +732,28 @@ namespace Avalonia
             var theme = GetEffectiveTheme();
 
             if (theme is not null)
-                ApplyControlTheme(theme);
+                ApplyControlTheme(theme, FrameType.Theme);
 
             if (TemplatedParent is StyledElement styleableParent &&
                 styleableParent.GetEffectiveTheme() is { } parentTheme)
             {
-                ApplyControlTheme(parentTheme);
+                ApplyControlTheme(parentTheme, FrameType.TemplatedParentTheme);
             }
         }
 
-        private void ApplyControlTheme(ControlTheme theme)
+        private void ApplyControlTheme(ControlTheme theme, FrameType type)
         {
+            Debug.Assert(type is FrameType.Theme or FrameType.TemplatedParentTheme);
+
             if (theme.BasedOn is ControlTheme basedOn)
-                ApplyControlTheme(basedOn);
+                ApplyControlTheme(basedOn, type);
 
-            theme.TryAttach(this, null);
+            theme.TryAttach(this, type);
 
             if (theme.HasChildren)
             {
                 foreach (var child in theme.Children)
-                    ApplyStyle(child, null);
+                    ApplyStyle(child, null, type);
             }
         }
 
@@ -768,17 +766,17 @@ namespace Avalonia
             if (host.IsStylesInitialized)
             {
                 foreach (var style in host.Styles)
-                    ApplyStyle(style, host);
+                    ApplyStyle(style, host, FrameType.Style);
             }
         }
 
-        private void ApplyStyle(IStyle style, IStyleHost? host)
+        private void ApplyStyle(IStyle style, IStyleHost? host, FrameType type)
         {
             if (style is Style s)
-                s.TryAttach(this, host);
+                s.TryAttach(this, host, type);
 
             foreach (var child in style.Children)
-                ApplyStyle(child, host);
+                ApplyStyle(child, host, type);
         }
 
         private void OnAttachedToLogicalTreeCore(LogicalTreeAttachmentEventArgs e)
@@ -895,6 +893,7 @@ namespace Avalonia
             for (var i = valueStore.Frames.Count - 1; i >= 0; --i)
             {
                 if (valueStore.Frames[i] is StyleInstance si &&
+                    si.Source is not ControlTheme &&
                     (styles is null || styles.Contains(si.Source)))
                 {
                     valueStore.RemoveFrame(si);
@@ -902,7 +901,7 @@ namespace Avalonia
             }
 
             valueStore.EndStyling();
-            _styled = false;
+            _stylesApplied = false;
         }
 
         private void InvalidateStylesOnThisAndDescendents()

src/Avalonia.Base/Styling/ControlTheme.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Diagnostics;
 using Avalonia.PropertyStore;
 
 namespace Avalonia.Styling
@@ -36,8 +37,10 @@ namespace Avalonia.Styling
             throw new InvalidOperationException("ControlThemes cannot be added as a nested style.");
         }
 
-        internal override SelectorMatchResult TryAttach(IStyleable target, object? host)
+        internal SelectorMatchResult TryAttach(IStyleable target, FrameType type)
         {
+            Debug.Assert(type is FrameType.Theme or FrameType.TemplatedParentTheme);
+
             _ = target ?? throw new ArgumentNullException(nameof(target));
 
             if (TargetType is null)
@@ -45,7 +48,7 @@ namespace Avalonia.Styling
 
             if (HasSettersOrAnimations && TargetType.IsAssignableFrom(target.StyleKey))
             {
-                Attach(target, null);
+                Attach(target, null, type);
                 return SelectorMatchResult.AlwaysThisType;
             }
 

src/Avalonia.Base/Styling/Style.cs

@@ -1,4 +1,5 @@
 using System;
+using Avalonia.PropertyStore;
 
 namespace Avalonia.Styling
 {
@@ -58,7 +59,7 @@ namespace Avalonia.Styling
             base.SetParent(parent);
         }
 
-        internal override SelectorMatchResult TryAttach(IStyleable target, object? host)
+        internal SelectorMatchResult TryAttach(IStyleable target, object? host, FrameType type)
         {
             _ = target ?? throw new ArgumentNullException(nameof(target));
 
@@ -73,7 +74,7 @@ namespace Avalonia.Styling
 
                 if (match.IsMatch)
                 {
-                    Attach(target, match.Activator);
+                    Attach(target, match.Activator, type);
                 }
 
                 result = match.Result;

src/Avalonia.Base/Styling/StyleBase.cs

@@ -92,9 +92,7 @@ namespace Avalonia.Styling
             return false;
         }
 
-        internal abstract SelectorMatchResult TryAttach(IStyleable target, object? host);
-
-        internal ValueFrame Attach(IStyleable target, IStyleActivator? activator)
+        internal ValueFrame Attach(IStyleable target, IStyleActivator? activator, FrameType type)
         {
             if (target is not AvaloniaObject ao)
                 throw new InvalidOperationException("Styles can only be applied to AvaloniaObjects.");
@@ -109,7 +107,7 @@ namespace Avalonia.Styling
             {
                 var canShareInstance = activator is null;
 
-                instance = new StyleInstance(this, activator);
+                instance = new StyleInstance(this, activator, type);
 
                 if (_setters is not null)
                 {

src/Avalonia.Base/Styling/StyleInstance.cs

@@ -1,6 +1,5 @@
 using System;
 using System.Collections.Generic;
-using System.Diagnostics;
 using System.Reactive.Subjects;
 using Avalonia.Animation;
 using Avalonia.Data;
@@ -27,10 +26,13 @@ namespace Avalonia.Styling
         private List<IAnimation>? _animations;
         private Subject<bool>? _animationTrigger;
 
-        public StyleInstance(IStyle style, IStyleActivator? activator)
+        public StyleInstance(
+            IStyle style,
+            IStyleActivator? activator,
+            FrameType type)
+            : base(GetPriority(activator), type)
         {
             _activator = activator;
-            Priority = activator is object ? BindingPriority.StyleTrigger : BindingPriority.Style;
             Source = style;
         }
 
@@ -99,5 +101,10 @@ namespace Avalonia.Styling
             hasChanged = _isActive != previous;
             return _isActive;
         }
+
+        private static BindingPriority GetPriority(IStyleActivator? activator)
+        {
+            return activator is not null ? BindingPriority.StyleTrigger : BindingPriority.Style;
+        }
     }
 }

src/Avalonia.Base/Styling/Styles.cs

@@ -4,6 +4,7 @@ using System.Collections.Generic;
 using System.Collections.Specialized;
 using Avalonia.Collections;
 using Avalonia.Controls;
+using Avalonia.PropertyStore;
 
 namespace Avalonia.Styling
 {
@@ -233,7 +234,7 @@ namespace Avalonia.Styling
             {
                 if (s is not Style style)
                     continue;
-                var r = style.TryAttach(target, host);
+                var r = style.TryAttach(target, host, FrameType.Style);
                 if (r > result)
                     result = r;
             }

src/Avalonia.Controls/Primitives/TemplatedControl.cs

@@ -5,6 +5,7 @@ using Avalonia.Interactivity;
 using Avalonia.Logging;
 using Avalonia.LogicalTree;
 using Avalonia.Media;
+using Avalonia.PropertyStore;
 using Avalonia.Styling;
 using Avalonia.VisualTree;
 
@@ -395,56 +396,36 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
-        internal override void DetachControlThemeFromTemplateChildren(ControlTheme theme)
+        private protected override void OnControlThemeChanged()
         {
-            static ControlTheme? GetControlTheme(StyleBase style)
-            {
-                var s = style;
+            base.OnControlThemeChanged();
 
-                while (s is not null)
+            var count = VisualChildren.Count;
+            for (var i = 0; i < count; ++i)
+            {
+                if (VisualChildren[i] is StyledElement child &&
+                    child.TemplatedParent == this)
                 {
-                    if (s is ControlTheme c)
-                        return c;
-                    s = s.Parent as StyleBase;
+                    child.OnTemplatedParentControlThemeChanged();
                 }
-
-                return null;
             }
+        }
 
-            static void Detach(Visual control, ITemplatedControl templatedParent, ControlTheme theme)
-            {
-                var valueStore = control.GetValueStore();
-                var count = valueStore.Frames.Count;
-
-                if (control != templatedParent)
-                {
-                    valueStore.BeginStyling();
-
-                    for (var i = count - 1; i >= 0; --i)
-                    {
-                        if (valueStore.Frames[i] is StyleInstance si &&
-                            si.Source is StyleBase style &&
-                            GetControlTheme(style) == theme)
-                        {
-                            valueStore.RemoveFrame(si);
-                        }
-                    }
-
-                    valueStore.EndStyling();
-                }
+        internal override void OnTemplatedParentControlThemeChanged()
+        {
+            base.OnTemplatedParentControlThemeChanged();
 
-                var children = ((IVisual)control).VisualChildren;
-                count = children.Count;
+            var count = VisualChildren.Count;
+            var templatedParent = TemplatedParent;
 
-                for (var i = 0; i < count; i++)
+            for (var i = 0; i < count; ++i)
+            {
+                if (VisualChildren[i] is TemplatedControl child &&
+                    child.TemplatedParent == templatedParent)
                 {
-                    if (children[i] is Visual v &&
-                        v.TemplatedParent == templatedParent)
-                        Detach(v, templatedParent, theme);
+                    child.OnTemplatedParentControlThemeChanged();
                 }
             }
-
-            Detach(this, this, theme);
         }
     }
 }

tests/Avalonia.Base.UnitTests/Animation/AnimatableTests.cs

@@ -5,6 +5,7 @@ using Avalonia.Controls.Shapes;
 using Avalonia.Data;
 using Avalonia.Layout;
 using Avalonia.Media;
+using Avalonia.PropertyStore;
 using Avalonia.Styling;
 using Avalonia.UnitTests;
 using Moq;
@@ -435,7 +436,7 @@ namespace Avalonia.Base.UnitTests.Animation
                 }
             };
 
-            style.TryAttach(control, control);
+            style.TryAttach(control, control, FrameType.Style);
 
             // Which means that the transition state hasn't been initialized with the new
             // Transitions when the Opacity change notification gets raised here.

tests/Avalonia.Base.UnitTests/PropertyStore/ValueStoreTests_Frames.cs

@@ -117,7 +117,7 @@ namespace Avalonia.Base.UnitTests.PropertyStore
 
         private static StyleInstance InstanceStyle(Style style, StyledElement target)
         {
-            var result = new StyleInstance(style, null);
+            var result = new StyleInstance(style, null, FrameType.Style);
 
             foreach (var setter in style.Setters)
                 result.Add(setter.Instance(result, target));

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

@@ -6,6 +6,7 @@ using Avalonia.Controls.Templates;
 using Avalonia.Data;
 using Avalonia.Data.Converters;
 using Avalonia.Media;
+using Avalonia.PropertyStore;
 using Avalonia.Styling;
 using Avalonia.UnitTests;
 using Moq;
@@ -503,7 +504,7 @@ namespace Avalonia.Base.UnitTests.Styling
 
         private void Apply(Style style, Control control)
         {
-            style.TryAttach(control, null);
+            style.TryAttach(control, null, FrameType.Style);
         }
 
         private void Apply(Setter setter, Control control)

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

@@ -5,6 +5,7 @@ using Avalonia.Base.UnitTests.Animation;
 using Avalonia.Controls;
 using Avalonia.Controls.Templates;
 using Avalonia.Data;
+using Avalonia.PropertyStore;
 using Avalonia.Styling;
 using Avalonia.UnitTests;
 using Moq;
@@ -27,7 +28,7 @@ namespace Avalonia.Base.UnitTests.Styling
 
             var target = new Class1();
 
-            style.TryAttach(target, null);
+            style.TryAttach(target, null, FrameType.Style);
 
             Assert.Equal("Foo", target.Foo);
         }
@@ -45,7 +46,7 @@ namespace Avalonia.Base.UnitTests.Styling
 
             var target = new Class1();
 
-            style.TryAttach(target, null);
+            style.TryAttach(target, null, FrameType.Style);
             Assert.Equal("foodefault", target.Foo);
             target.Classes.Add("foo");
             Assert.Equal("Foo", target.Foo);
@@ -66,7 +67,7 @@ namespace Avalonia.Base.UnitTests.Styling
 
             var target = new Class1();
 
-            style.TryAttach(target, target);
+            style.TryAttach(target, target, FrameType.Style);
 
             Assert.Equal("Foo", target.Foo);
         }
@@ -92,7 +93,7 @@ namespace Avalonia.Base.UnitTests.Styling
             var target = new Class1();
             var other = new Class1();
 
-            style.TryAttach(target, other);
+            style.TryAttach(target, other, FrameType.Style);
 
             Assert.Equal("foodefault", target.Foo);
         }
@@ -113,7 +114,7 @@ namespace Avalonia.Base.UnitTests.Styling
                 Foo = "Original",
             };
 
-            style.TryAttach(target, null);
+            style.TryAttach(target, null, FrameType.Style);
             Assert.Equal("Original", target.Foo);
         }
 
@@ -577,7 +578,7 @@ namespace Avalonia.Base.UnitTests.Styling
                 Child = border = new Border(),
             };
 
-            style.TryAttach(border, null);
+            style.TryAttach(border, null, FrameType.Style);
 
             Assert.Equal(new Thickness(4), border.BorderThickness);
             root.Child = null;
@@ -761,7 +762,7 @@ namespace Avalonia.Base.UnitTests.Styling
 
             var target = new Class1();
 
-            style.TryAttach(target, null);
+            style.TryAttach(target, null, FrameType.Style);
 
             Assert.Equal(1, target.Classes.ListenerCount);
 
@@ -874,7 +875,7 @@ namespace Avalonia.Base.UnitTests.Styling
             var clock = new TestClock();
             var target = new Class1 { Clock = clock };
 
-            style.TryAttach(target, null);
+            style.TryAttach(target, null, FrameType.Style);
 
             Assert.Equal(0.0, target.Double);
 

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

@@ -23,7 +23,7 @@ public class StyledElementTests_Theming
 
             Assert.Null(target.Template);
 
-            var root = CreateRoot(target);
+            CreateRoot(target);
             Assert.NotNull(target.Template);
 
             var border = Assert.IsType<Border>(target.VisualChild);
@@ -43,7 +43,7 @@ public class StyledElementTests_Theming
 
             Assert.Null(target.Template);
 
-            var root = CreateRoot(target);
+            CreateRoot(target);
             Assert.NotNull(target.Template);
 
             var border = Assert.IsType<Border>(target.VisualChild);
@@ -57,7 +57,7 @@ public class StyledElementTests_Theming
         public void Theme_Is_Detached_When_Theme_Property_Cleared()
         {
             var target = CreateTarget();
-            var root = CreateRoot(target);
+            CreateRoot(target);
 
             Assert.NotNull(target.Template);
 
@@ -66,7 +66,47 @@ public class StyledElementTests_Theming
         }
 
         [Fact]
-        public void Theme_Is_Detached_From_Template_Controls_When_Theme_Property_Cleared()
+        public void Setting_Explicit_Theme_Detaches_Default_Theme()
+        {
+            var target = new ThemedControl();
+            var root = new TestRoot
+            {
+                Resources = { { typeof(ThemedControl), CreateTheme() } },
+                Child = target,
+            };
+
+            root.LayoutManager.ExecuteInitialLayoutPass();
+
+            Assert.Equal("theme", target.Tag);
+
+            target.Theme = new ControlTheme(typeof(ThemedControl))
+            {
+                Setters =
+                {
+                    new Setter(ThemedControl.BackgroundProperty, Brushes.Yellow),
+                }
+            };
+
+            root.LayoutManager.ExecuteLayoutPass();
+
+            Assert.Null(target.Tag);
+            Assert.Equal(Brushes.Yellow, target.Background);
+        }
+
+        [Fact]
+        public void Unrelated_Styles_Are_Not_Detached_When_Theme_Property_Cleared()
+        {
+            var target = CreateTarget();
+            CreateRoot(target, createAdditionalStyles: true);
+
+            Assert.Equal("style", target.Tag);
+
+            target.Theme = null;
+            Assert.Equal("style", target.Tag);
+        }
+        
+        [Fact]
+        public void TemplatedParent_Theme_Is_Detached_From_Template_Controls_When_Theme_Property_Cleared()
         {
             var theme = new ControlTheme
             {
@@ -93,10 +133,115 @@ public class StyledElementTests_Theming
 
             target.Theme = null;
 
-            Assert.IsType<Canvas>(target.VisualChild);
+            Assert.Same(canvas, target.VisualChild);
             Assert.Null(canvas.Background);
         }
 
+        [Fact]
+        public void Primary_Theme_Is_Not_Detached_From_Template_Controls_When_Theme_Property_Cleared()
+        {
+            var templatedParentTheme = new ControlTheme
+            {
+                TargetType = typeof(ThemedControl),
+                Children =
+                {
+                    new Style(x => x.Nesting().Template().OfType<Button>())
+                    {
+                        Setters =
+                        {
+                            new Setter(Panel.BackgroundProperty, Brushes.Red),
+                        }
+                    },
+                }
+            };
+
+            var childTheme = new ControlTheme
+            {
+                TargetType = typeof(Button),
+                Setters =
+                {
+                    new Setter(TemplatedControl.ForegroundProperty, Brushes.Green),
+                }
+            };
+
+            var target = CreateTarget(templatedParentTheme);
+            target.Template = new FuncControlTemplate<ThemedControl>((o, n) => new Button
+            {
+                Theme = childTheme,
+            });
+
+            var root = CreateRoot(target, createAdditionalStyles: true);
+
+            var templateChild = Assert.IsType<Button>(target.VisualChild);
+            Assert.Equal(Brushes.Red, templateChild.Background);
+            Assert.Equal(Brushes.Green, templateChild.Foreground);
+
+            target.Theme = null;
+
+            Assert.Null(templateChild.Background);
+            Assert.Equal(Brushes.Green, templateChild.Foreground);
+        }
+
+        [Fact]
+        public void TemplatedParent_Theme_Is_Not_Detached_From_Template_Controls_When_Primary_Theme_Property_Cleared()
+        {
+            var templatedParentTheme = new ControlTheme
+            {
+                TargetType = typeof(ThemedControl),
+                Children =
+                {
+                    new Style(x => x.Nesting().Template().OfType<Button>())
+                    {
+                        Setters =
+                        {
+                            new Setter(Panel.BackgroundProperty, Brushes.Red),
+                        }
+                    },
+                }
+            };
+
+            var childTheme = new ControlTheme
+            {
+                TargetType = typeof(Button),
+                Setters =
+                {
+                    new Setter(Button.TagProperty, "childTheme"),
+                }
+            };
+
+            var target = CreateTarget(templatedParentTheme);
+            target.Template = new FuncControlTemplate<ThemedControl>((o, n) => new Button
+            {
+                Theme = childTheme,
+            });
+
+            var root = CreateRoot(target, createAdditionalStyles: true);
+
+            var templateChild = Assert.IsType<Button>(target.VisualChild);
+            Assert.Equal(Brushes.Red, templateChild.Background);
+            Assert.Equal("childTheme", templateChild.Tag);
+
+            templateChild.Theme = null;
+
+            Assert.Equal(Brushes.Red, templateChild.Background);
+            Assert.Null(templateChild.Tag);
+        }
+
+        [Fact]
+        public void Unrelated_Styles_Are_Not_Detached_From_Template_Controls_When_Theme_Property_Cleared()
+        {
+            var target = CreateTarget();
+            var root = CreateRoot(target, createAdditionalStyles: true);
+
+            var canvas = Assert.IsType<Border>(target.VisualChild);
+            Assert.Equal("style", canvas.Tag);
+
+            target.Theme = null;
+
+            Assert.Same(canvas, target.VisualChild);
+            Assert.Equal("style", canvas.Tag);
+        }
+
         [Fact]
         public void Theme_Is_Applied_On_Layout_After_Theme_Property_Changes()
         {
@@ -122,7 +267,7 @@ public class StyledElementTests_Theming
 
             Assert.Null(target.Template);
 
-            var root = CreateRoot(target);
+            CreateRoot(target);
             Assert.NotNull(target.Template);
             Assert.Equal(Brushes.Blue, target.BorderBrush);
 
@@ -135,6 +280,29 @@ public class StyledElementTests_Theming
             Assert.Equal(Brushes.Cyan, border.BorderBrush);
         }
 
+        [Fact]
+        public void Theme_Has_Lower_Priority_Than_Style()
+        {
+            var target = CreateTarget();
+            CreateRoot(target, createAdditionalStyles: true);
+
+            Assert.Equal("style", target.Tag);
+        }
+
+        [Fact]
+        public void Theme_Has_Lower_Priority_Than_Style_After_Change()
+        {
+            var target = CreateTarget();
+            var theme = target.Theme;
+            CreateRoot(target, createAdditionalStyles: true);
+
+            target.Theme = null;
+            target.Theme = theme;
+            target.ApplyStyling();
+
+            Assert.Equal("style", target.Tag);
+        }
+
         private static ThemedControl CreateTarget(ControlTheme? theme = null)
         {
             return new ThemedControl
@@ -143,9 +311,32 @@ public class StyledElementTests_Theming
             };
         }
 
-        private static TestRoot CreateRoot(IControl child)
+        private static TestRoot CreateRoot(
+            IControl child,
+            bool createAdditionalStyles = false)
         {
-            var result = new TestRoot(child);
+            var result = new TestRoot();
+
+            if (createAdditionalStyles)
+            {
+                result.Styles.Add(new Style(x => x.OfType<ThemedControl>())
+                {
+                    Setters =
+                    {
+                        new Setter(Control.TagProperty, "style"),
+                    }
+                });
+
+                result.Styles.Add(new Style(x => x.OfType<Border>())
+                {
+                    Setters =
+                    {
+                        new Setter(Control.TagProperty, "style"),
+                    }
+                });
+            }
+
+            result.Child = child;
             result.LayoutManager.ExecuteInitialLayoutPass();
             return result;
         }
@@ -157,7 +348,7 @@ public class StyledElementTests_Theming
         public void Implicit_Theme_Is_Applied_When_Attached_To_Logical_Tree()
         {
             var target = CreateTarget();
-            var root = CreateRoot(target);
+            CreateRoot(target);
             Assert.NotNull(target.Template);
 
             var border = Assert.IsType<Border>(target.VisualChild);
@@ -231,7 +422,7 @@ public class StyledElementTests_Theming
             Assert.Null(target.Theme);
             Assert.Null(target.Template);
 
-            var root = CreateRoot(target);
+            CreateRoot(target);
 
             Assert.NotNull(target.Theme);
             Assert.NotNull(target.Template);
@@ -348,6 +539,7 @@ public class StyledElementTests_Theming
             TargetType = typeof(ThemedControl),
             Setters =
             {
+                new Setter(Control.TagProperty, "theme"),
                 new Setter(TemplatedControl.TemplateProperty, template),
                 new Setter(TemplatedControl.CornerRadiusProperty, new CornerRadius(5)),
             },
@@ -355,7 +547,11 @@ public class StyledElementTests_Theming
             {
                 new Style(x => x.Nesting().Template().OfType<Border>())
                 {
-                    Setters = { new Setter(Border.BackgroundProperty, Brushes.Red) }
+                    Setters = 
+                    { 
+                        new Setter(Border.BackgroundProperty, Brushes.Red),
+                        new Setter(Control.TagProperty, "theme"),
+                    }
                 },
                 new Style(x => x.Nesting().Class("foo").Template().OfType<Border>())
                 {

tests/Avalonia.Benchmarks/Styling/Style_Activation.cs

@@ -1,5 +1,6 @@
 using System.Runtime.CompilerServices;
 using Avalonia.Controls;
+using Avalonia.PropertyStore;
 using Avalonia.Styling;
 using BenchmarkDotNet.Attributes;
 
@@ -27,7 +28,7 @@ namespace Avalonia.Benchmarks.Styling
                 Setters = { new Setter(TestClass.StringProperty, "foo") }
             };
 
-            style.TryAttach(_target, null);
+            style.TryAttach(_target, null, FrameType.Style);
         }
 
         [Benchmark]

tests/Avalonia.Benchmarks/Styling/Style_Apply.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 using Avalonia.Controls;
+using Avalonia.PropertyStore;
 using Avalonia.Styling;
 using BenchmarkDotNet.Attributes;
 
@@ -56,7 +57,7 @@ namespace Avalonia.Benchmarks.Styling
             target.GetValueStore().BeginStyling();
 
             foreach (var style in _styles)
-                style.TryAttach(target, null);
+                style.TryAttach(target, null, FrameType.Style);
 
             target.GetValueStore().EndStyling();
         }

tests/Avalonia.Benchmarks/Styling/Style_ClassSelector.cs

@@ -1,6 +1,7 @@
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 using Avalonia.Controls;
+using Avalonia.PropertyStore;
 using Avalonia.Styling;
 using BenchmarkDotNet.Attributes;
 
@@ -35,7 +36,7 @@ namespace Avalonia.Benchmarks.Styling
             target.GetValueStore().BeginStyling();
 
             for (var i = 0; i < 50; ++i)
-                _style.TryAttach(target, null);
+                _style.TryAttach(target, null, FrameType.Style);
 
             target.GetValueStore().EndStyling();
         }
@@ -48,7 +49,7 @@ namespace Avalonia.Benchmarks.Styling
             target.GetValueStore().BeginStyling();
 
             for (var i = 0; i < 50; ++i)
-                _style.TryAttach(target, null);
+                _style.TryAttach(target, null, FrameType.Style);
 
             target.GetValueStore().EndStyling();
 
@@ -64,7 +65,7 @@ namespace Avalonia.Benchmarks.Styling
             target.GetValueStore().BeginStyling();
 
             for (var i = 0; i < 50; ++i)
-                _style.TryAttach(target, null);
+                _style.TryAttach(target, null, FrameType.Style);
 
             target.GetValueStore().EndStyling();
 

tests/Avalonia.Benchmarks/Styling/Style_NonActive.cs

@@ -1,5 +1,6 @@
 using System.Runtime.CompilerServices;
 using Avalonia.Controls;
+using Avalonia.PropertyStore;
 using Avalonia.Styling;
 using BenchmarkDotNet.Attributes;
 
@@ -27,7 +28,7 @@ namespace Avalonia.Benchmarks.Styling
                 Setters = { new Setter(TestClass.StringProperty, "foo") }
             };
 
-            style.TryAttach(_target, null);
+            style.TryAttach(_target, null, FrameType.Style);
             _target.SetValue(TestClass.StringProperty, "foo");
             _target.SetValue(TestClass.Struct1Property, new(1));
             _target.SetValue(TestClass.Struct2Property, new(1));

tests/Avalonia.Markup.Xaml.UnitTests/StyleTests.cs

@@ -2,6 +2,7 @@ using System.Linq;
 using System.Reactive.Linq;
 using Avalonia.Controls;
 using Avalonia.Data;
+using Avalonia.PropertyStore;
 using Avalonia.Styling;
 using Avalonia.UnitTests;
 using Xunit;
@@ -54,7 +55,7 @@ namespace Avalonia.Markup.Xaml.UnitTests
                     }
                 };
 
-                style.TryAttach(control, control);
+                style.TryAttach(control, control, FrameType.Style);
                 Assert.Equal("foo", control.Text);
 
                 control.Text = "bar";