Fix SelectableTextBlock selection when text content changes (#21894)

* Fix SelectableTextBlock selection when text content changes

* Clean up

src/Avalonia.Controls/SelectableTextBlock.cs

@@ -1,7 +1,7 @@
 using System;
 using System.Collections.Generic;
-using System.Data;
 using System.Linq;
+using Avalonia.Collections;
 using Avalonia.Controls.Documents;
 using Avalonia.Controls.Utils;
 using Avalonia.Input;
@@ -9,7 +9,6 @@ using Avalonia.Input.Platform;
 using Avalonia.Interactivity;
 using Avalonia.Media;
 using Avalonia.Media.TextFormatting;
-using Avalonia.Platform;
 using Avalonia.Utilities;
 
 namespace Avalonia.Controls
@@ -20,10 +19,10 @@ namespace Avalonia.Controls
     public class SelectableTextBlock : TextBlock, IInlineHost
     {
         public static readonly StyledProperty<int> SelectionStartProperty =
-            TextBox.SelectionStartProperty.AddOwner<SelectableTextBlock>();
+            TextBox.SelectionStartProperty.AddOwner<SelectableTextBlock>(new(coerce: TextBox.CoerceCaretIndex));
 
         public static readonly StyledProperty<int> SelectionEndProperty =
-            TextBox.SelectionEndProperty.AddOwner<SelectableTextBlock>();
+            TextBox.SelectionEndProperty.AddOwner<SelectableTextBlock>(new(coerce: TextBox.CoerceCaretIndex));
 
         public static readonly DirectProperty<SelectableTextBlock, string> SelectedTextProperty =
             AvaloniaProperty.RegisterDirect<SelectableTextBlock, string>(
@@ -48,7 +47,7 @@ namespace Avalonia.Controls
 
         static SelectableTextBlock()
         {
-            FocusableProperty.OverrideDefaultValue(typeof(SelectableTextBlock), true);
+            FocusableProperty.OverrideDefaultValue<SelectableTextBlock>(true);
             AffectsRender<SelectableTextBlock>(SelectionStartProperty, SelectionEndProperty, SelectionBrushProperty);
         }
 
@@ -331,15 +330,31 @@ namespace Avalonia.Controls
         {
             base.OnPropertyChanged(change);
 
-            if (change.Property == SelectionStartProperty || 
-                change.Property == SelectionEndProperty)
+            if (change.Property == InlinesProperty)
+            {
+                if (change.OldValue is InlineCollection oldInlines)
+                {
+                    oldInlines.Invalidated -= OnInlinesInvalidated;
+                }
+
+                if (change.NewValue is InlineCollection newInlines)
+                {
+                    newInlines.Invalidated += OnInlinesInvalidated;
+                }
+
+                OnTextOrInlinesChanged();
+            }
+            else if (change.Property == TextProperty)
+            {
+                OnTextOrInlinesChanged();
+            }
+            else if (change.Property == SelectionStartProperty || change.Property == SelectionEndProperty)
             {
                 RaisePropertyChanged(SelectedTextProperty, "", "");
                 UpdateCommandStates();
                 InvalidateTextLayout();
             }
-
-            if(change.Property == SelectionForegroundBrushProperty)
+            else if (change.Property == SelectionForegroundBrushProperty)
             {
                 InvalidateTextLayout();
             }
@@ -505,6 +520,16 @@ namespace Avalonia.Controls
             e.Pointer.Capture(null);
         }
 
+        private void OnInlinesInvalidated(object? sender, EventArgs e) => OnTextOrInlinesChanged();
+
+        private void OnTextOrInlinesChanged()
+        {
+            CoerceValue(SelectionStartProperty);
+            CoerceValue(SelectionEndProperty);
+            RaisePropertyChanged(SelectedTextProperty, "", "");
+            UpdateCommandStates();
+        }
+
         private void UpdateCommandStates()
         {
             var text = GetSelection();

src/Avalonia.Controls/TextBox.cs

@@ -2136,7 +2136,10 @@ namespace Avalonia.Controls
 
         internal static int CoerceCaretIndex(AvaloniaObject sender, int value)
         {
-            var text = sender.GetValue(TextProperty); // method also used by TextPresenter and SelectableTextBlock
+            // method also used by TextPresenter and SelectableTextBlock
+            var text = sender is SelectableTextBlock { HasComplexContent: true } textBlock
+                ? textBlock.Inlines?.Text
+                : sender.GetValue(TextProperty);
 
             if (text == null)
             {

tests/Avalonia.Controls.UnitTests/SelectableTextBlockTests.cs

@@ -95,5 +95,91 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
+        [Fact]
+        public void Inlines_Changes_Should_Update_Selection()
+        {
+            using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface))
+            {
+                var target = new SelectableTextBlock();
+                target.Inlines!.Add(new Run("foo"));
+                target.SelectionEnd = 3;
+
+                var selectedTextChanged = false;
+                target.PropertyChanged += (_, e) =>
+                {
+                    if (e.Property == SelectableTextBlock.SelectedTextProperty)
+                    {
+                        selectedTextChanged = true;
+                    }
+                };
+
+                target.Inlines.Add(new Run("bar"));
+
+                Assert.True(selectedTextChanged);
+
+                target.SelectionStart = 6;
+                target.SelectionEnd = 0;
+                target.Inlines.RemoveAt(1);
+
+                Assert.Equal(3, target.SelectionStart);
+                Assert.Equal(0, target.SelectionEnd);
+
+                target.SelectionStart = 0;
+                target.SelectionEnd = 3;
+
+                target.Inlines[0] = new Run("a");
+
+                Assert.Equal(0, target.SelectionStart);
+                Assert.Equal(1, target.SelectionEnd);
+                Assert.Equal("a", target.SelectedText);
+            }
+        }
+
+        [Fact]
+        public void Text_Changes_Should_Update_Selection()
+        {
+            using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface))
+            {
+                var target = new SelectableTextBlock
+                {
+                    Text = "foo",
+                    SelectionEnd = 3
+                };
+
+                var selectedTextChanged = false;
+                target.PropertyChanged += (_, e) =>
+                {
+                    if (e.Property == SelectableTextBlock.SelectedTextProperty)
+                    {
+                        selectedTextChanged = true;
+                    }
+                };
+
+                target.Text = "a";
+
+                Assert.Equal(0, target.SelectionStart);
+                Assert.Equal(1, target.SelectionEnd);
+                Assert.True(selectedTextChanged);
+            }
+        }
+
+        [Fact]
+        public void CoerceCaretIndex_OnTextChanged()
+        {
+            using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface))
+            {
+                var target = new SelectableTextBlock
+                {
+                    Text = "foo",
+                    SelectionStart = 3,
+                    SelectionEnd = 3
+                };
+
+                target.Text = "a";
+
+                Assert.Equal(1, target.SelectionStart);
+                Assert.Equal(1, target.SelectionEnd);
+            }
+        }
     }
 }