Overhaul.

.gitignore

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+.idea/
+.vscode/
+.vs/
+
+bin/
+obj/
+
+*.user

App.xaml

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+<Application x:Class="HocrEditor.App"
+             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:local="clr-namespace:HocrEditor"
+             StartupUri="MainWindow.xaml">
+    <Application.Resources>
+
+    </Application.Resources>
+</Application>

App.xaml.cs

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+using System;
+using System.Collections.Generic;
+using System.Configuration;
+using System.Data;
+using System.Linq;
+using System.Threading.Tasks;
+using System.Windows;
+
+namespace HocrEditor
+{
+    /// <summary>
+    /// Interaction logic for App.xaml
+    /// </summary>
+    public partial class App : Application
+    {
+    }
+}

AssemblyInfo.cs

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+using System.Windows;
+
+[assembly: ThemeInfo(
+    ResourceDictionaryLocation.None, //where theme specific resource dictionaries are located
+    //(used if a resource is not found in the page,
+    // or application resource dictionaries)
+    ResourceDictionaryLocation.SourceAssembly //where the generic resource dictionary is located
+    //(used if a resource is not found in the page,
+    // app, or any theme specific resource dictionaries)
+)]

BindingHelpers.cs

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+using System.Collections.ObjectModel;
+using System.Collections.Specialized;
+using System.ComponentModel;
+
+namespace HocrEditor;
+
+public static class BindingHelpers
+{
+    public static void SubscribeItemPropertyChanged<T>(
+        this ObservableCollection<T> collection,
+        PropertyChangedEventHandler handler
+    )
+        where T : INotifyPropertyChanged
+    {
+        void CollectionOnCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
+        {
+            if (e.OldItems != null)
+            {
+                foreach (var oldItem in e.OldItems)
+                {
+                    ((INotifyPropertyChanged)oldItem).PropertyChanged -= ItemOnPropertyChanged;
+                }
+            }
+
+            if (e.NewItems != null)
+            {
+                foreach (var newItem in e.NewItems)
+                {
+                    ((INotifyPropertyChanged)newItem).PropertyChanged += ItemOnPropertyChanged;
+                }
+            }
+        }
+
+        void ItemOnPropertyChanged(object? sender, PropertyChangedEventArgs e)
+        {
+            handler.Invoke(sender, e);
+        }
+
+        collection.CollectionChanged += CollectionOnCollectionChanged;
+
+        foreach (var item in collection)
+        {
+            item.PropertyChanged += ItemOnPropertyChanged;
+        }
+    }
+}

Controls/DocumentCanvas.xaml

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+<av:UserControl xmlns="https://github.com/avaloniaui"
+                xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+                xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+                xmlns:av="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+                mc:Ignorable="d"
+                d:DesignWidth="800"
+                d:DesignHeight="450"
+                xmlns:skia="clr-namespace:SkiaSharp.Views.WPF;assembly=SkiaSharp.Views.WPF"
+                x:Class="HocrEditor.Controls.DocumentCanvas">
+    <av:Grid>
+        <skia:SKElement Name="Canvas"
+                        PaintSurface="Canvas_OnPaintSurface" />
+    </av:Grid>
+</av:UserControl>