Fix #3996: edit the display font size in points, not pixels

The options dialog bound DisplaySettings.SelectedFontSize (device-independent
pixels) straight into a NumericUpDown, so a fresh profile showed 13.33 and the
6-72 bounds were pixels. The WPF host presented points via FontSizeConverter;
this restores that behavior on Avalonia with an editable size ComboBox (like
the Windows font dialogs) backed by a pt/px proxy on the viewmodel. The stored
value stays pixels so settings files keep round-tripping with ILSpy 9.x.

Assisted-by: Claude:claude-fable-5:Claude Code

ILSpy.Tests/Options/DisplayFontSizeTests.cs

@@ -0,0 +1,216 @@
+// Copyright (c) 2026 Christoph Wille
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+using Avalonia.Controls;
+using Avalonia.Headless.NUnit;
+using Avalonia.Threading;
+
+using AwesomeAssertions;
+
+using ICSharpCode.ILSpy.AppEnv;
+using ICSharpCode.ILSpy.Commands;
+using ICSharpCode.ILSpy.Docking;
+using ICSharpCode.ILSpy.Options;
+using ICSharpCode.ILSpy.Options.Panels;
+using ICSharpCode.ILSpy.Properties;
+using ICSharpCode.ILSpy.ViewModels;
+using ICSharpCode.ILSpy.Views;
+
+using NUnit.Framework;
+
+namespace ICSharpCode.ILSpy.Tests;
+
+/// <summary>
+/// The options dialog edits the font size in points (like the Windows font dialogs and the
+/// WPF host's FontSizeConverter), while <see cref="DisplaySettings.SelectedFontSize"/> keeps
+/// storing device-independent pixels so persisted settings round-trip with ILSpy 9.x.
+/// The pt/px conversion lives in <see cref="DisplaySettingsViewModel.SelectedFontSizePoints"/>.
+/// </summary>
+[TestFixture]
+public class DisplayFontSizeTests
+{
+	static (DisplaySettingsViewModel page, DisplaySettings settings) CreatePage()
+	{
+		var service = AppComposition.Current.GetExport<SettingsService>();
+		var page = new DisplaySettingsViewModel();
+		page.Load(service);
+		return (page, service.DisplaySettings);
+	}
+
+	[AvaloniaTest]
+	public void Default_Pixel_Size_Displays_As_10_Points()
+	{
+		var (page, settings) = CreatePage();
+		var original = settings.SelectedFontSize;
+		try
+		{
+			settings.SelectedFontSize = 10.0 * 4 / 3;
+			page.SelectedFontSizePoints.Should().Be("10",
+				"the default 13.33 px must be presented as the 10 pt it actually is");
+		}
+		finally
+		{
+			settings.SelectedFontSize = original;
+		}
+	}
+
+	[AvaloniaTest]
+	public void Typed_Point_Size_Is_Stored_As_Pixels()
+	{
+		var (page, settings) = CreatePage();
+		var original = settings.SelectedFontSize;
+		try
+		{
+			page.SelectedFontSizePoints = "12";
+			settings.SelectedFontSize.Should().BeApproximately(12.0 * 4 / 3, 1e-9,
+				"the stored value stays device-independent pixels for 9.x round-tripping");
+			page.SelectedFontSizePoints.Should().Be("12");
+		}
+		finally
+		{
+			settings.SelectedFontSize = original;
+		}
+	}
+
+	[AvaloniaTest]
+	public void External_Pixel_Change_Refreshes_The_Points_Text()
+	{
+		// Reset-to-defaults and LoadFromXml write SelectedFontSize directly; the dialog text
+		// must follow via PropertyChanged on SelectedFontSizePoints.
+		var (page, settings) = CreatePage();
+		var original = settings.SelectedFontSize;
+		try
+		{
+			var notified = new List<string?>();
+			page.PropertyChanged += (_, e) => notified.Add(e.PropertyName);
+
+			settings.SelectedFontSize = 20;
+
+			notified.Should().Contain(nameof(DisplaySettingsViewModel.SelectedFontSizePoints));
+			page.SelectedFontSizePoints.Should().Be("15");
+		}
+		finally
+		{
+			settings.SelectedFontSize = original;
+		}
+	}
+
+	[AvaloniaTest]
+	public void Setting_Points_Through_The_Dialog_Does_Not_Echo_A_Text_Notification()
+	{
+		// While the user is typing in the size box, the setter must not raise PropertyChanged
+		// for SelectedFontSizePoints - the binding would rewrite the box mid-keystroke
+		// (typing "10." would snap back to "10" before the fraction can be completed).
+		var (page, settings) = CreatePage();
+		var original = settings.SelectedFontSize;
+		try
+		{
+			var notified = new List<string?>();
+			page.PropertyChanged += (_, e) => notified.Add(e.PropertyName);
+
+			page.SelectedFontSizePoints = "14";
+
+			notified.Should().NotContain(nameof(DisplaySettingsViewModel.SelectedFontSizePoints));
+		}
+		finally
+		{
+			settings.SelectedFontSize = original;
+		}
+	}
+
+	[AvaloniaTest]
+	public void NonNumeric_Input_Is_Ignored()
+	{
+		var (page, settings) = CreatePage();
+		var original = settings.SelectedFontSize;
+		try
+		{
+			settings.SelectedFontSize = 16;
+			page.SelectedFontSizePoints = "abc";
+			settings.SelectedFontSize.Should().Be(16,
+				"transient garbage while typing must not move the stored size");
+		}
+		finally
+		{
+			settings.SelectedFontSize = original;
+		}
+	}
+
+	[AvaloniaTest]
+	public void Typed_Sizes_Are_Clamped_To_The_6_To_72_Point_Range()
+	{
+		var (page, settings) = CreatePage();
+		var original = settings.SelectedFontSize;
+		try
+		{
+			page.SelectedFontSizePoints = "1";
+			settings.SelectedFontSize.Should().BeApproximately(6.0 * 4 / 3, 1e-9);
+
+			page.SelectedFontSizePoints = "500";
+			settings.SelectedFontSize.Should().BeApproximately(72.0 * 4 / 3, 1e-9);
+		}
+		finally
+		{
+			settings.SelectedFontSize = original;
+		}
+	}
+
+	[AvaloniaTest]
+	public void Size_List_Offers_6_Through_24_Points_Like_The_WPF_Host()
+	{
+		var (page, _) = CreatePage();
+		page.FontSizes.Should().Equal(Enumerable.Range(6, 24 - 6 + 1));
+	}
+
+	[AvaloniaTest]
+	public async Task Display_Panel_Size_Box_Is_An_Editable_ComboBox_Showing_Points()
+	{
+		var window = AppComposition.Current.GetExport<MainWindow>();
+		window.Show();
+		var settings = AppComposition.Current.GetExport<SettingsService>();
+		var original = settings.DisplaySettings.SelectedFontSize;
+		try
+		{
+			settings.DisplaySettings.SelectedFontSize = 10.0 * 4 / 3;
+
+			AppComposition.Current.GetExport<MainMenuCommandRegistry>()
+				.GetCommand(nameof(Resources._Options)).Execute(null);
+			var vm = (MainWindowViewModel)window.DataContext!;
+			var model = (OptionsPageModel)vm.DockWorkspace.Documents!.VisibleDockables!
+				.OfType<ContentTabPage>().First(t => t.Content is OptionsPageModel).Content!;
+			model.SelectedPage = model.Pages.OfType<DisplaySettingsViewModel>().Single();
+			TestCapture.Step("display-page-selected");
+
+			var panel = await window.WaitForComponent<DisplaySettingsPanel>();
+			var box = panel.FindControl<ComboBox>("fontSizeComboBox");
+			((object?)box).Should().NotBeNull("the size box must be the named editable ComboBox");
+			Dispatcher.UIThread.RunJobs();
+
+			box!.IsEditable.Should().BeTrue("custom sizes must be typeable, like Notepad's font page");
+			box.Text.Should().Be("10", "the box shows points, not device-independent pixels");
+		}
+		finally
+		{
+			settings.DisplaySettings.SelectedFontSize = original;
+		}
+	}
+}

ILSpy/Options/DisplaySettingsPanel.axaml

@@ -20,16 +20,21 @@
 				<ComboBox MinWidth="120" ItemsSource="{Binding AllThemes}"
 				          SelectedItem="{Binding SessionSettings.Theme, Mode=TwoWay}" />
 			</StackPanel>
-			<Grid ColumnDefinitions="Auto,*,Auto" RowDefinitions="Auto,Auto" ColumnSpacing="8" RowSpacing="4">
+			<Grid ColumnDefinitions="Auto,*,Auto,Auto" RowDefinitions="Auto,Auto" ColumnSpacing="8" RowSpacing="4">
 				<TextBlock Grid.Row="0" Grid.Column="0" VerticalAlignment="Center"
 				           Text="{x:Static res:Resources.Font}" />
 				<ComboBox Grid.Row="0" Grid.Column="1" MinWidth="200"
 				          ItemsSource="{Binding AvailableFonts}"
 				          SelectedItem="{Binding Settings.SelectedFont, Mode=TwoWay}" />
-				<NumericUpDown Grid.Row="0" Grid.Column="2" Width="100"
-				               Minimum="6" Maximum="72" Increment="1"
-				               Value="{Binding Settings.SelectedFontSize, Mode=TwoWay}" />
-				<Border Grid.Row="1" Grid.Column="1" Grid.ColumnSpan="2"
+				<TextBlock Grid.Row="0" Grid.Column="2" VerticalAlignment="Center"
+				           Text="{x:Static res:Resources.Size}" />
+				<!-- Edits in points (the unit every Windows font dialog uses); the viewmodel
+				     converts to the device-independent pixels DisplaySettings stores. -->
+				<ComboBox Grid.Row="0" Grid.Column="3" Width="100" x:Name="fontSizeComboBox"
+				          IsEditable="True"
+				          ItemsSource="{Binding FontSizes}"
+				          Text="{Binding SelectedFontSizePoints, Mode=TwoWay}" />
+				<Border Grid.Row="1" Grid.Column="1" Grid.ColumnSpan="3"
 				        BorderBrush="{DynamicResource ILSpy.ChromeBorder}" BorderThickness="1" Padding="6" Margin="0,2,0,0">
 					<!-- FontFamily binds to a derived FontFamily property because Avalonia's
 					     runtime binding pipeline doesn't coerce string → FontFamily (the

ILSpy/Options/DisplaySettingsViewModel.cs

@@ -16,8 +16,10 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+using System;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Globalization;
 using System.Linq;
 using System.Xml.Linq;
 
@@ -55,6 +57,40 @@ namespace ICSharpCode.ILSpy.Options
 			.OrderBy(n => n, System.StringComparer.OrdinalIgnoreCase)
 			.ToArray();
 
+		/// <summary>Point sizes offered in the size dropdown; same list as the WPF host.</summary>
+		public IReadOnlyList<int> FontSizes { get; } = Enumerable.Range(6, 24 - 6 + 1).ToArray();
+
+		/// <summary>
+		/// The font size as the user sees and edits it: points, like the Windows font dialogs.
+		/// <see cref="DisplaySettings.SelectedFontSize"/> itself stays in device-independent
+		/// pixels (1 pt = 4/3 px) so persisted settings round-trip with the WPF host; the
+		/// conversion happens only at this dialog boundary. Non-numeric input is ignored
+		/// (it is usually a transient typing state), numeric input is clamped to 6-72 pt.
+		/// </summary>
+		public string SelectedFontSizePoints {
+			get => Settings == null
+				? string.Empty
+				: Math.Round(Settings.SelectedFontSize * 3 / 4).ToString(CultureInfo.CurrentCulture);
+			set {
+				if (Settings == null || !double.TryParse(value, NumberStyles.Float, CultureInfo.CurrentCulture, out double points))
+					return;
+				points = Math.Clamp(points, 6, 72);
+				// Suppress the PropertyChanged echo for this property: the binding would
+				// immediately rewrite the size box with the rounded value mid-keystroke.
+				updatingFontSizeFromText = true;
+				try
+				{
+					Settings.SelectedFontSize = points * 4 / 3;
+				}
+				finally
+				{
+					updatingFontSizeFromText = false;
+				}
+			}
+		}
+
+		bool updatingFontSizeFromText;
+
 		/// <summary>
 		/// Derived FontFamily for the preview TextBlock. Avalonia's runtime binding pipeline
 		/// doesn't auto-coerce a <c>string</c> source to <see cref="FontFamily"/> (the implicit
@@ -75,12 +111,15 @@ namespace ICSharpCode.ILSpy.Options
 			Settings.PropertyChanged += OnSettingsPropertyChanged;
 			SessionSettings = service.SessionSettings;
 			OnPropertyChanged(nameof(CurrentFontFamily));
+			OnPropertyChanged(nameof(SelectedFontSizePoints));
 		}
 
 		void OnSettingsPropertyChanged(object? sender, PropertyChangedEventArgs e)
 		{
 			if (e.PropertyName == nameof(DisplaySettings.SelectedFont))
 				OnPropertyChanged(nameof(CurrentFontFamily));
+			if (e.PropertyName == nameof(DisplaySettings.SelectedFontSize) && !updatingFontSizeFromText)
+				OnPropertyChanged(nameof(SelectedFontSizePoints));
 		}
 
 		public void LoadDefaults()