Added support for platform text scaling, disabled by default

TomEdwardsEnscape · TomEdwardsEnscape · commit 7cc5c1019ae9 · 2025-12-16T11:50:17.000+01:00
Only TextBlock, Inline, and TextPresenter scale text by themselves
No change to low-level text rendering or FontSize values
Implemented for iOS and Windows
Windows scaling is currently uniform regardless of base font size, which is incorrect
diff --git a/samples/ControlCatalog/MainView.xaml b/samples/ControlCatalog/MainView.xaml
@@ -6,7 +6,8 @@
              xmlns:models="using:ControlCatalog.Models"
              xmlns:pages="using:ControlCatalog.Pages"
              xmlns:viewModels="using:ControlCatalog.ViewModels"
-             x:DataType="viewModels:MainWindowViewModel">
+             x:DataType="viewModels:MainWindowViewModel"
+             TextElement.IsPlatformTextScalingEnabled="True">
   <Grid>
     <Grid.Styles>
       <Style Selector="TextBlock.h2">
diff --git a/src/Avalonia.Base/Platform/DefaultPlatformSettings.cs b/src/Avalonia.Base/Platform/DefaultPlatformSettings.cs
@@ -1,10 +1,8 @@
 ﻿using System;
 using Avalonia.Input;
 using Avalonia.Input.Platform;
-using Avalonia.Media;
 using Avalonia.Metadata;
 using Avalonia.Threading;
-using Avalonia.VisualTree;
 
 namespace Avalonia.Platform
 {
@@ -47,12 +45,18 @@ public virtual PlatformColorValues GetColorValues()
             };
         }
 
-        public virtual event EventHandler<PlatformColorValues>? ColorValuesChanged;
+        public event EventHandler<PlatformColorValues>? ColorValuesChanged;
 
-        protected void OnColorValuesChanged(PlatformColorValues colorValues)
+        protected virtual void OnColorValuesChanged(PlatformColorValues colorValues)
         {
             Dispatcher.UIThread.Send(
                 _ => ColorValuesChanged?.Invoke(this, colorValues));
         }
+
+        public event EventHandler<EventArgs>? TextScalingChanged;
+
+        protected virtual void OnTextScaleChanged() => Dispatcher.UIThread.Send(_ => TextScalingChanged?.Invoke(this, EventArgs.Empty));
+
+        public virtual double GetScaledFontSize(double baseFontSize) => baseFontSize;
     }
 }
diff --git a/src/Avalonia.Base/Platform/IPlatformSettings.cs b/src/Avalonia.Base/Platform/IPlatformSettings.cs
@@ -42,7 +42,7 @@ public interface IPlatformSettings
         /// Get a configuration for platform-specific hotkeys in an Avalonia application.
         /// </summary>
         PlatformHotkeyConfiguration HotkeyConfiguration { get; }
-        
+
         /// <summary>
         /// Gets current system color values including dark mode and accent colors.
         /// </summary>
@@ -52,5 +52,15 @@ public interface IPlatformSettings
         /// 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
@@ -3,7 +3,7 @@
 
 namespace Avalonia.Controls.Documents
 {
-    internal interface IInlineHost : ILogical
+    internal interface IInlineHost : ILogical, IPlatformTextScaleable
     {
         void Invalidate();
 
diff --git a/src/Avalonia.Controls/Documents/Inline.cs b/src/Avalonia.Controls/Documents/Inline.cs
@@ -83,7 +83,7 @@ protected TextRunProperties CreateTextRunProperties()
             return new GenericTextRunProperties(
                 typeface,
                 FontFeatures, 
-                FontSize,
+                InlineHost?.GetScaledFontSize(FontSize) ?? FontSize,
                 TextDecorations, 
                 Foreground,
                 parentOrSelfBackground,
diff --git a/src/Avalonia.Controls/Documents/TextElement.cs b/src/Avalonia.Controls/Documents/TextElement.cs
@@ -14,6 +14,15 @@ public abstract class TextElement : StyledElement
         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>
@@ -102,6 +111,16 @@ public IBrush? Background
             set => SetValue(BackgroundProperty, value);
         }
 
+        /// <summary>
+        /// Gets or sets whether the <see cref="FontSize"/> should be scaled according to platform text scaling
+        /// rules when measuring and rendering this control.
+        /// </summary>
+        public bool IsPlatformTextScalingEnabled
+        {
+            get => GetValue(IsPlatformTextScalingEnabledProperty);
+            set => SetValue(IsPlatformTextScalingEnabledProperty, value);
+        }
+
         /// <summary>
         /// Gets or sets the font family.
         /// </summary>
@@ -174,6 +193,9 @@ public double LetterSpacing
             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>
@@ -359,6 +381,7 @@ protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs chang
                 case nameof(Background):
                 case nameof(FontFamily):
                 case nameof(FontSize):
+                case nameof(IsPlatformTextScalingEnabled):
                 case nameof(FontStyle):
                 case nameof(FontWeight):
                 case nameof(FontStretch):
diff --git a/src/Avalonia.Controls/IPlatformTextScaleable.cs b/src/Avalonia.Controls/IPlatformTextScaleable.cs
@@ -0,0 +1,17 @@
+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/Presenters/TextPresenter.cs b/src/Avalonia.Controls/Presenters/TextPresenter.cs
@@ -15,7 +15,7 @@
 
 namespace Avalonia.Controls.Presenters
 {
-    public class TextPresenter : Control
+    public class TextPresenter : Control, IPlatformTextScaleable
     {
         public static readonly StyledProperty<bool> ShowSelectionHighlightProperty =
             AvaloniaProperty.Register<TextPresenter, bool>(nameof(ShowSelectionHighlight), defaultValue: true);
@@ -47,6 +47,12 @@ public class TextPresenter : Control
         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>
@@ -124,6 +130,13 @@ public IBrush? Background
             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>
@@ -181,6 +194,13 @@ public double FontSize
             get => TextElement.GetFontSize(this);
             set => TextElement.SetFontSize(this, value);
         }
+        
+        double IPlatformTextScaleable.GetScaledFontSize(double baseFontSize) => IsPlatformTextScalingEnabled && TopLevel.GetTopLevel(this) is { PlatformSettings: { } platformSettings } ? platformSettings.GetScaledFontSize(baseFontSize) : baseFontSize;
+        
+        /// <summary>
+        /// Gets <see cref="FontSize"/> scaled according to the platform's current text scaling rules.
+        /// </summary>
+        protected double EffectiveFontSize => ((IPlatformTextScaleable)this).GetScaledFontSize(FontSize);
 
         /// <summary>
         /// Gets or sets the font style.
@@ -335,6 +355,14 @@ public int SelectionEnd
 
         internal TextSelectionHandleCanvas? TextSelectionHandleCanvas { get; set; }
 
+        void IPlatformTextScaleable.OnPlatformTextScalingChanged()
+        {
+            if (IsPlatformTextScalingEnabled)
+            {
+                InvalidateMeasure();
+            }
+        }
+
         /// <summary>
         /// Creates the <see cref="TextLayout"/> used to render the text.
         /// </summary>
@@ -350,7 +378,7 @@ private TextLayout CreateTextLayoutInternal(Size constraint, string? text, Typef
             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, FontSize, foreground, TextAlignment,
+            var textLayout = new TextLayout(text, typeface, FontFeatures, EffectiveFontSize, foreground, TextAlignment,
                 TextWrapping, maxWidth: maxWidth, maxHeight: maxHeight, textStyleOverrides: textStyleOverrides,
                 flowDirection: FlowDirection, lineHeight: LineHeight, letterSpacing: LetterSpacing);
 
@@ -553,7 +581,7 @@ protected virtual TextLayout CreateTextLayout()
             if (!string.IsNullOrEmpty(preeditText))
             {
                 var preeditHighlight = new ValueSpan<TextRunProperties>(caretIndex, preeditText.Length,
-                        new GenericTextRunProperties(typeface, FontFeatures, FontSize,
+                        new GenericTextRunProperties(typeface, FontFeatures, EffectiveFontSize,
                         foregroundBrush: foreground,
                         textDecorations: TextDecorations.Underline));
 
@@ -569,7 +597,7 @@ protected virtual TextLayout CreateTextLayout()
                     textStyleOverrides = new[]
                     {
                         new ValueSpan<TextRunProperties>(start, length,
-                        new GenericTextRunProperties(typeface, FontFeatures, FontSize,
+                        new GenericTextRunProperties(typeface, FontFeatures, EffectiveFontSize,
                             foregroundBrush: SelectionForegroundBrush))
                     };
                 }
diff --git a/src/Avalonia.Controls/Primitives/TextSelectionCanvas.cs b/src/Avalonia.Controls/Primitives/TextSelectionCanvas.cs
@@ -154,7 +154,7 @@ private void DragSelectionHandle(TextSelectionHandle handle)
                 }
 
                 var point = ToPresenter(handle.IndicatorPosition);
-                point = point.WithY(point.Y - _presenter.FontSize / 2);
+                point = point.WithY(point.Y - ((IPlatformTextScaleable)_presenter).GetScaledFontSize(_presenter.FontSize) / 2);
                 var hit = _presenter.TextLayout.HitTestPoint(point);
                 var position = hit.CharacterHit.FirstCharacterIndex + hit.CharacterHit.TrailingLength;
 
diff --git a/src/Avalonia.Controls/SelectableTextBlock.cs b/src/Avalonia.Controls/SelectableTextBlock.cs
@@ -187,7 +187,7 @@ protected override TextLayout CreateTextLayout(string? text)
             var defaultProperties = new GenericTextRunProperties(
                 typeface,
                 FontFeatures,
-                FontSize,
+                EffectiveFontSize,
                 TextDecorations,
                 Foreground);
 
@@ -236,7 +236,7 @@ protected override TextLayout CreateTextLayout(string? text)
                                 new GenericTextRunProperties(
                                     textRun.Properties?.Typeface ?? typeface,
                                     textRun.Properties?.FontFeatures ?? FontFeatures,
-                                    FontSize,
+                                    EffectiveFontSize,
                                     foregroundBrush: SelectionForegroundBrush)));
 
                         accumulatedLength += runLength;
@@ -247,7 +247,7 @@ protected override TextLayout CreateTextLayout(string? text)
                     textStyleOverrides =
                     [
                         new ValueSpan<TextRunProperties>(start, length,
-                            new GenericTextRunProperties(typeface, FontFeatures, FontSize,
+                            new GenericTextRunProperties(typeface, FontFeatures, EffectiveFontSize,
                                 foregroundBrush: SelectionForegroundBrush))
                     ];
                 }
diff --git a/src/Avalonia.Controls/TextBlock.cs b/src/Avalonia.Controls/TextBlock.cs
@@ -15,14 +15,20 @@ namespace Avalonia.Controls
     /// <summary>
     /// A control that displays a block of text.
     /// </summary>
-    public class TextBlock : Control, IInlineHost
+    public class TextBlock : Control, IInlineHost, IPlatformTextScaleable
     {
         /// <summary>
         /// Defines the <see cref="Background"/> property.
         /// </summary>
         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>
@@ -98,7 +104,7 @@ public class TextBlock : Control, IInlineHost
         /// Defines the <see cref="LetterSpacing"/> property.
         /// </summary>
         /// <remarks>
-        /// This property uses <see cref="AvaloniaProperty.AddOwner{TOwner}(AvaloniaProperty)"/> to share the same
+        /// This property uses <see cref="StyledProperty{TValue}.AddOwner{TOwner}(StyledPropertyMetadata{TValue}?)"/> to share the same
         /// definition as <see cref="TextElement.LetterSpacingProperty"/>, ensuring consistent behavior across text
         /// elements and templated controls.
         /// </remarks>
@@ -192,6 +198,13 @@ public TextBlock()
         /// </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>
@@ -237,6 +250,13 @@ public double FontSize
             set => SetValue(FontSizeProperty, value);
         }
 
+        double IPlatformTextScaleable.GetScaledFontSize(double baseFontSize) => IsPlatformTextScalingEnabled && TopLevel.GetTopLevel(this) is { PlatformSettings: { } platformSettings } ? platformSettings.GetScaledFontSize(baseFontSize) : baseFontSize;
+
+        /// <summary>
+        /// Gets <see cref="FontSize"/> scaled according to the platform's current text scaling rules.
+        /// </summary>
+        protected double EffectiveFontSize => ((IPlatformTextScaleable)this).GetScaledFontSize(FontSize);
+
         /// <summary>
         /// Gets or sets the font style used to draw the control's text.
         /// </summary>
@@ -652,6 +672,14 @@ internal void ClearTextInternal()
             }
         }
 
+        void IPlatformTextScaleable.OnPlatformTextScalingChanged()
+        {
+            if (IsPlatformTextScalingEnabled)
+            {
+                InvalidateMeasure();
+            }
+        }
+
         /// <summary>
         /// Creates the <see cref="TextLayout"/> used to render the text.
         /// </summary>
@@ -663,7 +691,7 @@ protected virtual TextLayout CreateTextLayout(string? text)
             var defaultProperties = new GenericTextRunProperties(
                 typeface,
                 FontFeatures,
-                FontSize,
+                EffectiveFontSize,
                 TextDecorations,
                 Foreground);
 
@@ -838,6 +866,7 @@ protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs chang
             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/TopLevel.cs b/src/Avalonia.Controls/TopLevel.cs
diff --git a/src/Windows/Avalonia.Win32/Win32PlatformSettings.cs b/src/Windows/Avalonia.Win32/Win32PlatformSettings.cs
diff --git a/src/Windows/Avalonia.Win32/WinRT/winrt.idl b/src/Windows/Avalonia.Win32/WinRT/winrt.idl
diff --git a/src/iOS/Avalonia.iOS/PlatformSettings.cs b/src/iOS/Avalonia.iOS/PlatformSettings.cs

Original file line number	Diff line number	Diff line change
`@@ -1,10 +1,8 @@`
`1`	`1`	`using System;`
`2`	`2`	`using Avalonia.Input;`
`3`	`3`	`using Avalonia.Input.Platform;`
`4`		`-using Avalonia.Media;`
`5`	`4`	`using Avalonia.Metadata;`
`6`	`5`	`using Avalonia.Threading;`
`7`		`-using Avalonia.VisualTree;`
`8`	`6`
`9`	`7`	`namespace Avalonia.Platform`
`10`	`8`	`{`
`@@ -47,12 +45,18 @@ public virtual PlatformColorValues GetColorValues()`
`47`	`45`	`};`
`48`	`46`	`}`
`49`	`47`
`50`		`- public virtual event EventHandler<PlatformColorValues>? ColorValuesChanged;`
	`48`	`+ public event EventHandler<PlatformColorValues>? ColorValuesChanged;`
`51`	`49`
`52`		`- protected void OnColorValuesChanged(PlatformColorValues colorValues)`
	`50`	`+ protected virtual void OnColorValuesChanged(PlatformColorValues colorValues)`
`53`	`51`	`{`
`54`	`52`	`Dispatcher.UIThread.Send(`
`55`	`53`	`_ => ColorValuesChanged?.Invoke(this, colorValues));`
`56`	`54`	`}`
	`55`	`+`
	`56`	`+ public event EventHandler<EventArgs>? TextScalingChanged;`
	`57`	`+`
	`58`	`+ protected virtual void OnTextScaleChanged() => Dispatcher.UIThread.Send(_ => TextScalingChanged?.Invoke(this, EventArgs.Empty));`
	`59`	`+`
	`60`	`+ public virtual double GetScaledFontSize(double baseFontSize) => baseFontSize;`
`57`	`61`	`}`
`58`	`62`	`}`
Original file line number	Diff line number	Diff line change
`@@ -3,7 +3,7 @@`
`3`	`3`
`4`	`4`	`namespace Avalonia.Controls.Documents`
`5`	`5`	`{`
`6`		`- internal interface IInlineHost : ILogical`
	`6`	`+ internal interface IInlineHost : ILogical, IPlatformTextScaleable`
`7`	`7`	`{`
`8`	`8`	`void Invalidate();`
`9`	`9`
Original file line number	Diff line number	Diff line change
`@@ -154,7 +154,7 @@ private void DragSelectionHandle(TextSelectionHandle handle)`
`154`	`154`	`}`
`155`	`155`
`156`	`156`	`var point = ToPresenter(handle.IndicatorPosition);`
`157`		`- point = point.WithY(point.Y - _presenter.FontSize / 2);`
	`157`	`+ point = point.WithY(point.Y - ((IPlatformTextScaleable)_presenter).GetScaledFontSize(_presenter.FontSize) / 2);`
`158`	`158`	`var hit = _presenter.TextLayout.HitTestPoint(point);`
`159`	`159`	`var position = hit.CharacterHit.FirstCharacterIndex + hit.CharacterHit.TrailingLength;`
`160`	`160`