From e5babc8a6f4c6522f52d22cb9b1cf6c673ea0ab6 Mon Sep 17 00:00:00 2001
From: Tom Edwards <tom.edwards@chaos.com>
Date: Sun, 21 Dec 2025 13:10:07 +0100
Subject: [PATCH] Added TextScaling static class with configuration properties
 Added ITextScaler to support custom text scaling Calculate a font scale
 factor and use this to adjust height and spacing values Fixed TextBox not
 updating when LineHeight changes and MaxLines or MinLines are active

---
 samples/ControlCatalog/MainView.xaml          |   2 +-
 src/Avalonia.Base/ITextScaler.cs              |  16 ++
 .../Platform/DefaultPlatformSettings.cs       |   2 +-
 .../Platform/IPlatformSettings.cs             |  12 +-
 .../Documents/IInlineHost.cs                  |   2 +-
 src/Avalonia.Controls/Documents/Inline.cs     |   2 +-
 .../Documents/TextElement.cs                  |  32 +---
 .../IPlatformTextScaleable.cs                 |  17 ---
 src/Avalonia.Controls/ITextScaleable.cs       |  13 ++
 .../Presenters/TextPresenter.cs               |  57 +++----
 .../Primitives/TextSelectionCanvas.cs         |   6 +-
 src/Avalonia.Controls/SelectableTextBlock.cs  |   9 +-
 src/Avalonia.Controls/TextBlock.cs            |  46 ++----
 src/Avalonia.Controls/TextBox.cs              |  29 +++-
 src/Avalonia.Controls/TextScaling.cs          | 139 ++++++++++++++++++
 src/Avalonia.Controls/TopLevel.cs             |   9 +-
 .../Avalonia.Win32/Win32PlatformSettings.cs   |   4 +-
 src/iOS/Avalonia.iOS/PlatformSettings.cs      |   2 +-
 18 files changed, 256 insertions(+), 143 deletions(-)
 create mode 100644 src/Avalonia.Base/ITextScaler.cs
 delete mode 100644 src/Avalonia.Controls/IPlatformTextScaleable.cs
 create mode 100644 src/Avalonia.Controls/ITextScaleable.cs
 create mode 100644 src/Avalonia.Controls/TextScaling.cs

diff --git a/samples/ControlCatalog/MainView.xaml b/samples/ControlCatalog/MainView.xaml
index 401d96fad2..5d6b5c6c86 100644
--- a/samples/ControlCatalog/MainView.xaml
+++ b/samples/ControlCatalog/MainView.xaml
@@ -7,7 +7,7 @@
              xmlns:pages="using:ControlCatalog.Pages"
              xmlns:viewModels="using:ControlCatalog.ViewModels"
              x:DataType="viewModels:MainWindowViewModel"
-             TextElement.IsPlatformTextScalingEnabled="True">
+             TextScaling.IsEnabled="True">
   <Grid>
     <Grid.Styles>
       <Style Selector="TextBlock.h2">
diff --git a/src/Avalonia.Base/ITextScaler.cs b/src/Avalonia.Base/ITextScaler.cs
new file mode 100644
index 0000000000..624a47d0e2
--- /dev/null
+++ b/src/Avalonia.Base/ITextScaler.cs
@@ -0,0 +1,16 @@
+using System;
+
+namespace Avalonia;
+
+/// <summary>
+/// Represents an object which can algoritmically scale text.
+/// </summary>
+public interface ITextScaler
+{
+    double GetScaledFontSize(Visual target, double baseFontSize);
+
+    /// <summary>
+    /// Raised when the text scaling algorithm has changed. Indicates that all text should be rescaled.
+    /// </summary>
+    event EventHandler<EventArgs>? TextScalingChanged;
+}
diff --git a/src/Avalonia.Base/Platform/DefaultPlatformSettings.cs b/src/Avalonia.Base/Platform/DefaultPlatformSettings.cs
index 6c07484aa0..7888b3c534 100644
--- a/src/Avalonia.Base/Platform/DefaultPlatformSettings.cs
+++ b/src/Avalonia.Base/Platform/DefaultPlatformSettings.cs
@@ -57,6 +57,6 @@ namespace Avalonia.Platform
 
         protected virtual void OnTextScaleChanged() => Dispatcher.UIThread.Send(_ => TextScalingChanged?.Invoke(this, EventArgs.Empty));
 
-        public virtual double GetScaledFontSize(double baseFontSize) => baseFontSize;
+        public virtual double GetScaledFontSize(Visual target, double baseFontSize) => baseFontSize;
     }
 }
diff --git a/src/Avalonia.Base/Platform/IPlatformSettings.cs b/src/Avalonia.Base/Platform/IPlatformSettings.cs
index e94c765593..077db569dd 100644
--- a/src/Avalonia.Base/Platform/IPlatformSettings.cs
+++ b/src/Avalonia.Base/Platform/IPlatformSettings.cs
@@ -10,7 +10,7 @@ namespace Avalonia.Platform
     /// Some of these settings might be changed by used globally in the OS in runtime.
     /// </summary>
     [NotClientImplementable]
-    public interface IPlatformSettings
+    public interface IPlatformSettings : ITextScaler
     {
         /// <summary>
         /// The size of the rectangle around the location of a pointer down that a pointer up
@@ -52,15 +52,5 @@ namespace Avalonia.Platform
         /// Raises when current system color values are changed. Including changing of a dark mode and accent colors.
         /// </summary>
         event EventHandler<PlatformColorValues>? ColorValuesChanged;
-
-        /// <summary>
-        /// Raised when the system text scaling changes.
-        /// </summary>
-        event EventHandler<EventArgs>? TextScalingChanged;
-
-        /// <summary>
-        /// Scales a font size according to the current system text scaling rules.
-        /// </summary>
-        double GetScaledFontSize(double baseFontSize);
     }
 }
diff --git a/src/Avalonia.Controls/Documents/IInlineHost.cs b/src/Avalonia.Controls/Documents/IInlineHost.cs
index 3fa6140bbe..167e186721 100644
--- a/src/Avalonia.Controls/Documents/IInlineHost.cs
+++ b/src/Avalonia.Controls/Documents/IInlineHost.cs
@@ -3,7 +3,7 @@ using Avalonia.LogicalTree;
 
 namespace Avalonia.Controls.Documents
 {
-    internal interface IInlineHost : ILogical, IPlatformTextScaleable
+    internal interface IInlineHost : ILogical, ITextScaleable
     {
         void Invalidate();
 
diff --git a/src/Avalonia.Controls/Documents/Inline.cs b/src/Avalonia.Controls/Documents/Inline.cs
index d89cdc9cf9..62bc429352 100644
--- a/src/Avalonia.Controls/Documents/Inline.cs
+++ b/src/Avalonia.Controls/Documents/Inline.cs
@@ -83,7 +83,7 @@ namespace Avalonia.Controls.Documents
             return new GenericTextRunProperties(
                 typeface,
                 FontFeatures, 
-                InlineHost?.GetScaledFontSize(FontSize) ?? FontSize,
+                InlineHost is Visual hostVisual ? TextScaling.GetScaledFontSize(hostVisual, FontSize) : FontSize,
                 TextDecorations, 
                 Foreground,
                 parentOrSelfBackground,
diff --git a/src/Avalonia.Controls/Documents/TextElement.cs b/src/Avalonia.Controls/Documents/TextElement.cs
index 245d4fd7bf..46fc3f1518 100644
--- a/src/Avalonia.Controls/Documents/TextElement.cs
+++ b/src/Avalonia.Controls/Documents/TextElement.cs
@@ -14,15 +14,6 @@ namespace Avalonia.Controls.Documents
         public static readonly StyledProperty<IBrush?> BackgroundProperty =
             Border.BackgroundProperty.AddOwner<TextElement>();
 
-        /// <summary>
-        /// Defines the <see cref="IsPlatformTextScalingEnabled"/> property.
-        /// </summary>
-        public static readonly AttachedProperty<bool> IsPlatformTextScalingEnabledProperty =
-            AvaloniaProperty.RegisterAttached<TextElement, Control, bool>(
-                nameof(IsPlatformTextScalingEnabled),
-                defaultValue: false,
-                inherits: true);
-
         /// <summary>
         /// Defines the <see cref="FontFamily"/> property.
         /// </summary>
@@ -111,17 +102,6 @@ namespace Avalonia.Controls.Documents
             set => SetValue(BackgroundProperty, value);
         }
 
-        /// <summary>
-        /// Gets or sets whether <see cref="FontSize"/>, <see cref="LetterSpacing"/>, <see cref="TextBlock.LineHeight"/>, and 
-        /// <see cref="TextBlock.LineSpacing"/> should be scaled according to platform text scaling rules when measuring and rendering this control.
-        /// </summary>
-        /// <remarks>Text scaling is typically not uniform. Smaller text scales up faster than larger text.</remarks>
-        public bool IsPlatformTextScalingEnabled
-        {
-            get => GetValue(IsPlatformTextScalingEnabledProperty);
-            set => SetValue(IsPlatformTextScalingEnabledProperty, value);
-        }
-
         /// <summary>
         /// Gets or sets the font family.
         /// </summary>
@@ -194,9 +174,6 @@ namespace Avalonia.Controls.Documents
             set => SetValue(LetterSpacingProperty, value);
         }
 
-        public static bool GetIsPlatformTextScalingEnabled(Control control) => control.GetValue(IsPlatformTextScalingEnabledProperty);
-        public static void SetIsPlatformTextScalingEnabled(Control control, bool value) => control.SetValue(IsPlatformTextScalingEnabledProperty, value);
-
         /// <summary>
         /// Gets the value of the attached <see cref="FontFamilyProperty"/> on a control.
         /// </summary>
@@ -377,17 +354,22 @@ namespace Avalonia.Controls.Documents
         {
             base.OnPropertyChanged(change);
 
+            if (change.Property.OwnerType == typeof(TextScaling))
+            {
+                InlineHost?.Invalidate();
+                return;
+            }
+
             switch (change.Property.Name)
             {
                 case nameof(Background):
                 case nameof(FontFamily):
                 case nameof(FontSize):
-                case nameof(IsPlatformTextScalingEnabled):
                 case nameof(FontStyle):
                 case nameof(FontWeight):
                 case nameof(FontStretch):
                 case nameof(Foreground):
-                    InlineHost?.Invalidate();
+                InlineHost?.Invalidate();
                     break;
             }
         }
diff --git a/src/Avalonia.Controls/IPlatformTextScaleable.cs b/src/Avalonia.Controls/IPlatformTextScaleable.cs
deleted file mode 100644
index 6cf6c1b742..0000000000
--- a/src/Avalonia.Controls/IPlatformTextScaleable.cs
+++ /dev/null
@@ -1,17 +0,0 @@
-namespace Avalonia.Controls;
-
-/// <summary>
-/// Represents an object which particpates in platform text scaling. This is an accessibility feature
-/// which allows the user to request that text be drawn larger (or on some platforms smaller) than normal,
-/// without altering other UI elements.
-/// </summary>
-public interface IPlatformTextScaleable
-{
-    bool IsPlatformTextScalingEnabled { get; }
-    void OnPlatformTextScalingChanged();
-
-    /// <summary>
-    /// Scales a font size according to the current system text scaling rules and the value of <see cref="IsPlatformTextScalingEnabled"/>.
-    /// </summary>
-    double GetScaledFontSize(double baseFontSize);
-}
diff --git a/src/Avalonia.Controls/ITextScaleable.cs b/src/Avalonia.Controls/ITextScaleable.cs
new file mode 100644
index 0000000000..7dd5512ae4
--- /dev/null
+++ b/src/Avalonia.Controls/ITextScaleable.cs
@@ -0,0 +1,13 @@
+namespace Avalonia.Controls;
+
+/// <summary>
+/// Represents an object which particpates in <see cref="TextScaling"/>.
+/// </summary>
+/// <seealso cref="ITextScaler"/>
+public interface ITextScaleable
+{
+    /// <summary>
+    /// Called when the active text scaling algorithm for this object changes.
+    /// </summary>
+    void OnTextScalingChanged();
+}
diff --git a/src/Avalonia.Controls/Presenters/TextPresenter.cs b/src/Avalonia.Controls/Presenters/TextPresenter.cs
index 847da4f390..948228f1a6 100644
--- a/src/Avalonia.Controls/Presenters/TextPresenter.cs
+++ b/src/Avalonia.Controls/Presenters/TextPresenter.cs
@@ -1,9 +1,7 @@
 using System;
 using System.Collections.Generic;
-using System.Data;
 using Avalonia.Controls.Documents;
 using Avalonia.Controls.Primitives;
-using Avalonia.Interactivity;
 using Avalonia.Layout;
 using Avalonia.Media;
 using Avalonia.Media.Immutable;
@@ -15,7 +13,7 @@ using Avalonia.VisualTree;
 
 namespace Avalonia.Controls.Presenters
 {
-    public class TextPresenter : Control, IPlatformTextScaleable
+    public class TextPresenter : Control, ITextScaleable
     {
         public static readonly StyledProperty<bool> ShowSelectionHighlightProperty =
             AvaloniaProperty.Register<TextPresenter, bool>(nameof(ShowSelectionHighlight), defaultValue: true);
@@ -47,12 +45,6 @@ namespace Avalonia.Controls.Presenters
         public static readonly StyledProperty<int> SelectionEndProperty =
             TextBox.SelectionEndProperty.AddOwner<TextPresenter>(new(coerce: TextBox.CoerceCaretIndex));
 
-        /// <summary>
-        /// Defines the <see cref="IsPlatformTextScalingEnabled"/> property.
-        /// </summary>
-        public static readonly StyledProperty<bool> IsPlatformTextScalingEnabledProperty =
-            TextElement.IsPlatformTextScalingEnabledProperty.AddOwner<TextPresenter>();
-
         /// <summary>
         /// Defines the <see cref="Text"/> property.
         /// </summary>
@@ -64,7 +56,7 @@ namespace Avalonia.Controls.Presenters
         /// </summary>
         public static readonly StyledProperty<string?> PreeditTextProperty =
             AvaloniaProperty.Register<TextPresenter, string?>(nameof(PreeditText));
-        
+
         /// <summary>
         /// Defines the <see cref="PreeditText"/> property.
         /// </summary>
@@ -130,13 +122,6 @@ namespace Avalonia.Controls.Presenters
             set => SetValue(BackgroundProperty, value);
         }
 
-        /// <inheritdoc cref="TextElement.IsPlatformTextScalingEnabled"/>
-        public bool IsPlatformTextScalingEnabled
-        {
-            get => GetValue(IsPlatformTextScalingEnabledProperty);
-            set => SetValue(IsPlatformTextScalingEnabledProperty, value);
-        }
-
         /// <summary>
         /// Gets or sets a value that determines whether the TextPresenter shows a selection highlight.
         /// </summary>
@@ -161,7 +146,7 @@ namespace Avalonia.Controls.Presenters
             get => GetValue(PreeditTextProperty);
             set => SetValue(PreeditTextProperty, value);
         }
-        
+
         public int? PreeditTextCursorPosition
         {
             get => GetValue(PreeditTextCursorPositionProperty);
@@ -194,12 +179,6 @@ namespace Avalonia.Controls.Presenters
             get => TextElement.GetFontSize(this);
             set => TextElement.SetFontSize(this, value);
         }
-        
-        /// <inheritdoc cref="IPlatformTextScaleable.GetScaledFontSize(double)"/>
-        protected double GetScaledFontSize(double baseFontSize) => !double.IsNaN(baseFontSize) && IsPlatformTextScalingEnabled && 
-            TopLevel.GetTopLevel(this) is { PlatformSettings: { } platformSettings } ? platformSettings.GetScaledFontSize(baseFontSize) : baseFontSize;
-
-        double IPlatformTextScaleable.GetScaledFontSize(double baseFontSize) => GetScaledFontSize(baseFontSize);
 
         /// <summary>
         /// Gets or sets the font style.
@@ -354,13 +333,7 @@ namespace Avalonia.Controls.Presenters
 
         internal TextSelectionHandleCanvas? TextSelectionHandleCanvas { get; set; }
 
-        void IPlatformTextScaleable.OnPlatformTextScalingChanged()
-        {
-            if (IsPlatformTextScalingEnabled)
-            {
-                InvalidateMeasure();
-            }
-        }
+        void ITextScaleable.OnTextScalingChanged() => InvalidateMeasure();
 
         /// <summary>
         /// Creates the <see cref="TextLayout"/> used to render the text.
@@ -377,9 +350,12 @@ namespace Avalonia.Controls.Presenters
             var maxWidth = MathUtilities.IsZero(constraint.Width) ? double.PositiveInfinity : constraint.Width;
             var maxHeight = MathUtilities.IsZero(constraint.Height) ? double.PositiveInfinity : constraint.Height;
 
-            var textLayout = new TextLayout(text, typeface, FontFeatures, GetScaledFontSize(FontSize), foreground, TextAlignment,
+            var effectiveFontSize = TextScaling.GetScaledFontSize(this, FontSize);
+            var fontScaleFactor = effectiveFontSize / FontSize;
+
+            var textLayout = new TextLayout(text, typeface, FontFeatures, effectiveFontSize, foreground, TextAlignment,
                 TextWrapping, maxWidth: maxWidth, maxHeight: maxHeight, textStyleOverrides: textStyleOverrides,
-                flowDirection: FlowDirection, lineHeight: GetScaledFontSize(LineHeight), letterSpacing: GetScaledFontSize(LetterSpacing));
+                flowDirection: FlowDirection, lineHeight: LineHeight * fontScaleFactor, letterSpacing: LetterSpacing * fontScaleFactor);
 
             return textLayout;
         }
@@ -568,7 +544,7 @@ namespace Avalonia.Controls.Presenters
             var preeditText = PreeditText;
             var text = GetCombinedText(Text, caretIndex, preeditText);
             var typeface = new Typeface(FontFamily, FontStyle, FontWeight, FontStretch);
-            var effectiveFontSize = GetScaledFontSize(FontSize);
+            var effectiveFontSize = TextScaling.GetScaledFontSize(this, FontSize);
             var selectionStart = SelectionStart;
             var selectionEnd = SelectionEnd;
             var start = Math.Min(selectionStart, selectionEnd);
@@ -775,7 +751,7 @@ namespace Avalonia.Controls.Presenters
 
             CaretChanged();
         }
-        
+
         private void EnsureCaretTimer()
         {
             if (_caretTimer == null)
@@ -801,7 +777,7 @@ namespace Avalonia.Controls.Presenters
                 _caretTimer = null;
             }
 
-            if (CaretBlinkInterval.TotalMilliseconds > 0) 
+            if (CaretBlinkInterval.TotalMilliseconds > 0)
             {
                 _caretTimer = new DispatcherTimer { Interval = CaretBlinkInterval };
                 _caretTimer.Tick += CaretTimerTick;
@@ -998,7 +974,7 @@ namespace Avalonia.Controls.Presenters
                 _caretTimer.Tick -= CaretTimerTick;
             }
         }
-        
+
         private void OnPreeditChanged(string? preeditText, int? cursorPosition)
         {
             if (string.IsNullOrEmpty(preeditText))
@@ -1029,7 +1005,7 @@ namespace Avalonia.Controls.Presenters
             {
                 OnPreeditChanged(change.NewValue as string, PreeditTextCursorPosition);
             }
-            
+
             if(change.Property == PreeditTextCursorPositionProperty)
             {
                 OnPreeditChanged(PreeditText, PreeditTextCursorPosition);
@@ -1056,6 +1032,11 @@ namespace Avalonia.Controls.Presenters
                 ResetCaretTimer();
             }
 
+            if (change.Property.OwnerType == typeof(TextScaling))
+            {
+                InvalidateTextLayout();
+            }
+
             switch (change.Property.Name)
             {
                 case nameof(PreeditText):
diff --git a/src/Avalonia.Controls/Primitives/TextSelectionCanvas.cs b/src/Avalonia.Controls/Primitives/TextSelectionCanvas.cs
index 1911c34926..eaac5db7d8 100644
--- a/src/Avalonia.Controls/Primitives/TextSelectionCanvas.cs
+++ b/src/Avalonia.Controls/Primitives/TextSelectionCanvas.cs
@@ -154,7 +154,7 @@ namespace Avalonia.Controls.Primitives
                 }
 
                 var point = ToPresenter(handle.IndicatorPosition);
-                point = point.WithY(point.Y - ((IPlatformTextScaleable)_presenter).GetScaledFontSize(_presenter.FontSize) / 2);
+                point = point.WithY(point.Y - TextScaling.GetScaledFontSize(_presenter, _presenter.FontSize) / 2);
                 var hit = _presenter.TextLayout.HitTestPoint(point);
                 var position = hit.CharacterHit.FirstCharacterIndex + hit.CharacterHit.TrailingLength;
 
@@ -327,9 +327,6 @@ namespace Avalonia.Controls.Primitives
             {
                 if (_textBox.ContextFlyout is PopupFlyoutBase flyout)
                 {
-                    var verticalOffset = ((IPlatformTextScaleable)_textBox).GetScaledFontSize(double.IsNaN(_textBox.LineHeight) ? _textBox.FontSize : _textBox.LineHeight) +
-                                         ContextMenuPadding;
-
                     TextSelectionHandle? handle = null;
 
                     if (_textBox.SelectionStart != _textBox.SelectionEnd)
@@ -349,6 +346,7 @@ namespace Avalonia.Controls.Primitives
 
                     if (handle != null)
                     {
+                        var verticalOffset = (_textBox.GetLineHeight() ?? 0) + ContextMenuPadding;
                         var topLeft = ToTextBox(handle.GetTopLeft());
                         flyout.VerticalOffset = topLeft.Y - verticalOffset;
                         flyout.HorizontalOffset = topLeft.X;
diff --git a/src/Avalonia.Controls/SelectableTextBlock.cs b/src/Avalonia.Controls/SelectableTextBlock.cs
index 78c533f0e5..b27486b853 100644
--- a/src/Avalonia.Controls/SelectableTextBlock.cs
+++ b/src/Avalonia.Controls/SelectableTextBlock.cs
@@ -183,8 +183,9 @@ namespace Avalonia.Controls
         protected override TextLayout CreateTextLayout(string? text)
         {
             var typeface = new Typeface(FontFamily, FontStyle, FontWeight, FontStretch);
-            var effectiveFontSize = GetScaledFontSize(FontSize);
-            
+            var effectiveFontSize = TextScaling.GetScaledFontSize(this, FontSize);
+            var fontScaleFactor = effectiveFontSize / FontSize;
+
             var defaultProperties = new GenericTextRunProperties(
                 typeface,
                 FontFeatures,
@@ -193,9 +194,9 @@ namespace Avalonia.Controls
                 Foreground);
 
             var paragraphProperties = new GenericTextParagraphProperties(FlowDirection, TextAlignment, true, false,
-                defaultProperties, TextWrapping, GetScaledFontSize(LineHeight), 0, GetScaledFontSize(LetterSpacing))
+                defaultProperties, TextWrapping, LineHeight * fontScaleFactor, 0, LetterSpacing * fontScaleFactor)
             {
-                LineSpacing = GetScaledFontSize(LineSpacing),
+                LineSpacing = LineSpacing * fontScaleFactor,
             };
 
             List<ValueSpan<TextRunProperties>>? textStyleOverrides = null;
diff --git a/src/Avalonia.Controls/TextBlock.cs b/src/Avalonia.Controls/TextBlock.cs
index 1be9b43e00..253beef00b 100644
--- a/src/Avalonia.Controls/TextBlock.cs
+++ b/src/Avalonia.Controls/TextBlock.cs
@@ -15,7 +15,7 @@ namespace Avalonia.Controls
     /// <summary>
     /// A control that displays a block of text.
     /// </summary>
-    public class TextBlock : Control, IInlineHost, IPlatformTextScaleable
+    public class TextBlock : Control, IInlineHost, ITextScaleable
     {
         /// <summary>
         /// Defines the <see cref="Background"/> property.
@@ -23,12 +23,6 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<IBrush?> BackgroundProperty =
             Border.BackgroundProperty.AddOwner<TextBlock>();
 
-        /// <summary>
-        /// Defines the <see cref="IsPlatformTextScalingEnabled"/> property.
-        /// </summary>
-        public static readonly StyledProperty<bool> IsPlatformTextScalingEnabledProperty =
-            TextElement.IsPlatformTextScalingEnabledProperty.AddOwner<TextBlock>();
-
         /// <summary>
         /// Defines the <see cref="Padding"/> property.
         /// </summary>
@@ -198,13 +192,6 @@ namespace Avalonia.Controls
         /// </summary>
         public TextLayout TextLayout => _textLayout ??= CreateTextLayout(Text);
 
-        /// <inheritdoc cref="TextElement.IsPlatformTextScalingEnabled"/>
-        public bool IsPlatformTextScalingEnabled
-        {
-            get => GetValue(IsPlatformTextScalingEnabledProperty);
-            set => SetValue(IsPlatformTextScalingEnabledProperty, value);
-        }
-
         /// <summary>
         /// Gets or sets the padding to place around the <see cref="Text"/>.
         /// </summary>
@@ -250,12 +237,6 @@ namespace Avalonia.Controls
             set => SetValue(FontSizeProperty, value);
         }
 
-        /// <inheritdoc cref="IPlatformTextScaleable.GetScaledFontSize(double)"/>
-        protected double GetScaledFontSize(double baseFontSize) => !double.IsNaN(baseFontSize) && IsPlatformTextScalingEnabled && 
-            TopLevel.GetTopLevel(this) is { PlatformSettings: { } platformSettings } ? platformSettings.GetScaledFontSize(baseFontSize) : baseFontSize;
-        
-        double IPlatformTextScaleable.GetScaledFontSize(double baseFontSize) => GetScaledFontSize(baseFontSize);
-
         /// <summary>
         /// Gets or sets the font style used to draw the control's text.
         /// </summary>
@@ -671,13 +652,7 @@ namespace Avalonia.Controls
             }
         }
 
-        void IPlatformTextScaleable.OnPlatformTextScalingChanged()
-        {
-            if (IsPlatformTextScalingEnabled)
-            {
-                InvalidateMeasure();
-            }
-        }
+        void ITextScaleable.OnTextScalingChanged() => InvalidateMeasure();
 
         /// <summary>
         /// Creates the <see cref="TextLayout"/> used to render the text.
@@ -690,14 +665,16 @@ namespace Avalonia.Controls
             var defaultProperties = new GenericTextRunProperties(
                 typeface,
                 FontFeatures,
-                GetScaledFontSize(FontSize),
+                TextScaling.GetScaledFontSize(this, FontSize),
                 TextDecorations,
                 Foreground);
 
+            var fontScaleFactor = defaultProperties.FontRenderingEmSize / FontSize;
+
             var paragraphProperties = new GenericTextParagraphProperties(FlowDirection, IsMeasureValid ? TextAlignment : TextAlignment.Left, true, false,
-                defaultProperties, TextWrapping, GetScaledFontSize(LineHeight), 0, GetScaledFontSize(LetterSpacing))
+                defaultProperties, TextWrapping, LineHeight * fontScaleFactor, 0, LetterSpacing * fontScaleFactor)
             {
-                LineSpacing = GetScaledFontSize(LineSpacing),
+                LineSpacing = LineSpacing * fontScaleFactor,
             };
 
             ITextSource textSource;
@@ -762,7 +739,7 @@ namespace Avalonia.Controls
                 //Force arrange so text will be properly aligned.
                 InvalidateArrange();
             }
-           
+
             var inlines = Inlines;
 
             if (HasComplexContent)
@@ -862,10 +839,15 @@ namespace Avalonia.Controls
                 }
             }
 
+            if (change.Property.OwnerType == typeof(TextScaling))
+            {
+                InvalidateTextLayout();
+                return;
+            }
+
             switch (change.Property.Name)
             {
                 case nameof(FontSize):
-                case nameof(IsPlatformTextScalingEnabled):
                 case nameof(FontWeight):
                 case nameof(FontStyle):
                 case nameof(FontFamily):
diff --git a/src/Avalonia.Controls/TextBox.cs b/src/Avalonia.Controls/TextBox.cs
index 52f2900662..ca9cf5c9db 100644
--- a/src/Avalonia.Controls/TextBox.cs
+++ b/src/Avalonia.Controls/TextBox.cs
@@ -824,6 +824,20 @@ namespace Avalonia.Controls
             return this._presenter?.TextLayout.TextLines.Count ?? -1;
         }
 
+        /// <summary>
+        /// Gets the height of each line in the <see cref="TextBox"/>, or null if no layout information is available.
+        /// </summary>
+        /// <returns></returns>
+        public double? GetLineHeight()
+        {
+            if (_presenter == null)
+                return null;
+
+            var scaledFontSize = TextScaling.GetScaledFontSize(_presenter, _presenter.FontSize);
+
+            return double.IsNaN(LineHeight) ? scaledFontSize : LineHeight * (scaledFontSize / _presenter.FontSize);
+        }
+
         /// <summary>
         /// Raised when content is being copied to the clipboard
         /// </summary>
@@ -985,6 +999,10 @@ namespace Avalonia.Controls
             {
                 InvalidateMeasure();
             }
+            else if (change.Property == LineHeightProperty)
+            {
+                InvalidateMeasure();
+            }
             else if (change.Property == UndoLimitProperty)
             {
                 OnUndoLimitChanged(change.GetNewValue<int>());
@@ -2392,9 +2410,14 @@ namespace Avalonia.Controls
         /// <returns>Height of <paramref name="numLines"/> lines of text in device-independent pixels.</returns>
         private double CalculateLineDIPHeight(int numLines)
         {
-            var textScaler = _presenter as IPlatformTextScaleable;
-            var effectiveFontSize = textScaler?.GetScaledFontSize(FontSize) ?? FontSize;
-            var effectiveLineHeight = textScaler?.GetScaledFontSize(LineHeight) ?? LineHeight;
+            var effectiveFontSize = FontSize;
+            var effectiveLineHeight = LineHeight;
+
+            if (_presenter != null)
+            { 
+                effectiveFontSize = TextScaling.GetScaledFontSize(_presenter, effectiveFontSize);
+                effectiveLineHeight = effectiveFontSize / _presenter.FontSize * LineHeight;
+            }
 
             var typeface = new Typeface(FontFamily, FontStyle, FontWeight, FontStretch);
             var paragraphProperties = TextLayout.CreateTextParagraphProperties(typeface, effectiveFontSize, null, default, default, null, default, effectiveLineHeight, default, FontFeatures);
diff --git a/src/Avalonia.Controls/TextScaling.cs b/src/Avalonia.Controls/TextScaling.cs
new file mode 100644
index 0000000000..6aa68d7f3a
--- /dev/null
+++ b/src/Avalonia.Controls/TextScaling.cs
@@ -0,0 +1,139 @@
+using System;
+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
+using Avalonia.Controls.Documents;
+using Avalonia.Threading;
+
+namespace Avalonia.Controls;
+
+/// <summary>
+/// Configures and computes text scaling. This is an accessibility feature which allows the user to request that text be 
+/// drawn at a different size than normal, without altering other UI elements. The default scaling algorithm is determined 
+/// by the platform and may make text smaller as well as larger.
+/// </summary>
+/// <remarks>
+/// Text scaling is applied when text is measured. It does not modify the value of <see cref="TextElement.FontSize"/>.
+/// </remarks>
+/// <seealso cref="ITextScaleable"/>
+/// <seealso cref="ITextScaler"/>
+public static class TextScaling
+{
+    private static readonly ConditionalWeakTable<ITextScaler, HashSet<Visual>> s_customTextScalerSubscribers = [];
+
+    /// <summary>
+    /// Determines whether <see cref="TextElement.FontSize"/> (along with <see cref="TextElement.LetterSpacing"/>, <see cref="TextBlock.LineHeight"/>, and 
+    /// <see cref="TextBlock.LineSpacing"/>) should be scalled by calling <see cref="GetScaledFontSize"/> when measuring text content. The value is inherited.
+    /// </summary>
+    /// <remarks>Text scaling is typically not uniform. Smaller text scales up faster than larger text.</remarks>
+    public static readonly AttachedProperty<bool> IsEnabledProperty =
+        AvaloniaProperty.RegisterAttached<Visual, bool>("IsEnabled", typeof(TextScaling), inherits: true);
+
+    /// <summary>
+    /// Determines the minimum size (in em units) to which text will be scaled by <see cref="GetScaledFontSize"/>. The value is inherited.
+    /// </summary>
+    /// <remarks>This value is used only when <see cref="IsEnabledProperty"/> is true.</remarks>
+    public static readonly AttachedProperty<double> MinFontSizeProperty =
+        AvaloniaProperty.RegisterAttached<Visual, double>("MinFontSize", typeof(TextScaling), inherits: true, validate: size => size >= 0);
+
+    /// <summary>
+    /// Determines the maximum size (in em units) to which text will be scaled by <see cref="GetScaledFontSize"/>. The value is inherited.
+    /// </summary>
+    /// <remarks>This value is used only when <see cref="IsEnabledProperty"/> is true.</remarks>
+    public static readonly AttachedProperty<double> MaxFontSizeProperty =
+        AvaloniaProperty.RegisterAttached<Visual, double>("MaxFontSize", typeof(TextScaling), defaultValue: double.PositiveInfinity, inherits: true,
+            validate: size => size > 0);
+
+    /// <summary>
+    /// Determines a user-defined text scaling algorithm, which overrides platform text scaling in <see cref="GetScaledFontSize"/>. The value is inherited.
+    /// </summary>
+    /// <remarks>This value is used only when <see cref="IsEnabledProperty"/> is true.</remarks>
+    public static readonly AttachedProperty<ITextScaler?> CustomTextScalerProperty =
+        AvaloniaProperty.RegisterAttached<Visual, ITextScaler?>("CustomTextScaler", typeof(TextScaling), inherits: true);
+  
+    /// <inheritdoc cref="IsEnabledProperty"/> <see cref="IsEnabledProperty"/>
+    public static bool GetIsEnabled(Visual visual) => visual.GetValue(IsEnabledProperty);
+    /// <inheritdoc cref="IsEnabledProperty"/> <see cref="IsEnabledProperty"/>
+    public static void SetIsEnabled(Visual visual, bool value) => visual.SetValue(IsEnabledProperty, value);
+    /// <inheritdoc cref="MinFontSizeProperty"/> <see cref="MinFontSizeProperty"/>
+    public static double GetMinFontSize(Visual visual) => visual.GetValue(MinFontSizeProperty);
+    /// <inheritdoc cref="MinFontSizeProperty"/> <see cref="MinFontSizeProperty"/>
+    public static void SetMinFontSize(Visual visual, double value) => visual.SetValue(MinFontSizeProperty, value);
+    /// <inheritdoc cref="MaxFontSizeProperty"/> <see cref="MaxFontSizeProperty"/>
+    public static double GetMaxFontSize(Visual visual) => visual.GetValue(MaxFontSizeProperty);
+    /// <inheritdoc cref="MaxFontSizeProperty"/> <see cref="MaxFontSizeProperty"/>
+    public static void SetMaxFontSize(Visual visual, double value) => visual.SetValue(MaxFontSizeProperty, value);
+    /// <inheritdoc cref="CustomTextScalerProperty"/> <see cref="CustomTextScalerProperty"/>
+    public static ITextScaler? GetCustomTextScaler(Visual visual) => visual.GetValue(CustomTextScalerProperty);
+    /// <inheritdoc cref="CustomTextScalerProperty"/> <see cref="CustomTextScalerProperty"/>
+    public static void SetCustomTextScaler(Visual visual, ITextScaler? value) => visual.SetValue(CustomTextScalerProperty, value);
+
+    static TextScaling()
+    {
+        CustomTextScalerProperty.Changed.AddClassHandler<Visual>(OnCustomTextScalerChanged);
+    }
+
+    private static void OnCustomTextScalerChanged(Visual visual, AvaloniaPropertyChangedEventArgs args)
+    {
+        if (visual is not ITextScaleable)
+        {
+            return;
+        }
+
+        var (oldScaler, newScaler) = args.GetOldAndNewValue<ITextScaler?>();
+
+        if (oldScaler != null && s_customTextScalerSubscribers.TryGetValue(oldScaler, out var oldSubscribers))
+        {
+            oldSubscribers.Remove(visual);
+        }
+
+        if (newScaler != null)
+        {
+            if (s_customTextScalerSubscribers.TryGetValue(newScaler, out var newSubscribers))
+            {
+                newSubscribers.Add(visual);
+            }
+            else
+            {
+                s_customTextScalerSubscribers.Add(newScaler, [visual]);
+                newScaler.TextScalingChanged += OnCustomTextScalingChanged;
+            }
+        }
+    }
+
+    private static void OnCustomTextScalingChanged(object? sender, EventArgs e)
+    {
+        Dispatcher.UIThread.VerifyAccess();
+
+        if (sender is ITextScaler scaler && s_customTextScalerSubscribers.TryGetValue(scaler, out var subscribers))
+        {
+            foreach (var visual in subscribers)
+            {
+                if (GetIsEnabled(visual))
+                {
+                    ((ITextScaleable)visual).OnTextScalingChanged();
+                }
+            }
+        }
+    }
+
+    /// <summary>
+    /// Scales <paramref name="baseFontSize"/> according to either the current platform text scaling rules, or the rules
+    /// defined by the object assigned to <see cref="CustomTextScalerProperty"/> for <paramref name="visual"/>.
+    /// </summary>
+    /// <remarks>The values of <see cref="IsEnabledProperty"/>, <see cref="MinFontSizeProperty"/>, and <see cref="MaxFontSizeProperty"/>
+    /// are enforced, even when <see cref="CustomTextScalerProperty"/> is in use.</remarks>
+    public static double GetScaledFontSize(Visual visual, double baseFontSize)
+    {
+        if (double.IsNaN(baseFontSize) || baseFontSize <= 0 || !GetIsEnabled(visual) || 
+            (GetCustomTextScaler(visual) ?? TopLevel.GetTopLevel(visual)?.PlatformSettings) is not { } scaler)
+        {
+            return baseFontSize;
+        }
+
+        // Extend min/max to encompass the base size. Conveniently, this means that min > max is not possible.
+        var min = Math.Min(GetMinFontSize(visual), baseFontSize);
+        var max = Math.Max(GetMaxFontSize(visual), baseFontSize);
+
+        return Math.Clamp(scaler.GetScaledFontSize(visual, baseFontSize), min, max);
+    }
+}
diff --git a/src/Avalonia.Controls/TopLevel.cs b/src/Avalonia.Controls/TopLevel.cs
index 5156830d4f..40569cdee5 100644
--- a/src/Avalonia.Controls/TopLevel.cs
+++ b/src/Avalonia.Controls/TopLevel.cs
@@ -347,9 +347,14 @@ namespace Avalonia.Controls
 
         private void OnPlatformTextScalingChanged(object? sender, EventArgs e)
         {
-            foreach (var scaleable in this.GetVisualDescendants().OfType<IPlatformTextScaleable>())
+            foreach (var visual in this.GetVisualDescendants())
             {
-                scaleable.OnPlatformTextScalingChanged();
+                if (visual is not ITextScaleable scaleable || !TextScaling.GetIsEnabled(visual) || TextScaling.GetCustomTextScaler(visual) != null)
+                {
+                    continue;
+                }
+
+                scaleable.OnTextScalingChanged();
             }
         }
 
diff --git a/src/Windows/Avalonia.Win32/Win32PlatformSettings.cs b/src/Windows/Avalonia.Win32/Win32PlatformSettings.cs
index cac4191ed7..35a8c41a4f 100644
--- a/src/Windows/Avalonia.Win32/Win32PlatformSettings.cs
+++ b/src/Windows/Avalonia.Win32/Win32PlatformSettings.cs
@@ -110,9 +110,9 @@ internal class Win32PlatformSettings : DefaultPlatformSettings
     /// <summary>
     /// The algorithm used is undocumented, but <see href="https://github.com/microsoft/microsoft-ui-xaml/blob/5788dee271452753d5b2c70179f976c3e96a45c7/src/dxaml/xcp/core/text/common/TextFormatting.cpp#L181-L204">defined in Microsoft's source code</see>.
     /// </summary>
-    public override double GetScaledFontSize(double baseFontSize)
+    public override double GetScaledFontSize(Visual target, double baseFontSize)
     {
-        if (baseFontSize <= 0)
+        if (baseFontSize <= 0 || _textScaleFactor == 1)
         {
             return baseFontSize;
         }
diff --git a/src/iOS/Avalonia.iOS/PlatformSettings.cs b/src/iOS/Avalonia.iOS/PlatformSettings.cs
index a6114da363..dfd57a1dd4 100644
--- a/src/iOS/Avalonia.iOS/PlatformSettings.cs
+++ b/src/iOS/Avalonia.iOS/PlatformSettings.cs
@@ -83,7 +83,7 @@ internal class PlatformSettings : DefaultPlatformSettings
         }
     }
 
-    public override double GetScaledFontSize(double baseFontSize)
+    public override double GetScaledFontSize(Visual target, double baseFontSize)
     {
         if (baseFontSize <= 0)
         {