diff --git a/src/Avalonia.Controls/TextBoxTextInputMethodClient.cs b/src/Avalonia.Controls/TextBoxTextInputMethodClient.cs
index d94d36ef1d..8595175192 100644
--- a/src/Avalonia.Controls/TextBoxTextInputMethodClient.cs
+++ b/src/Avalonia.Controls/TextBoxTextInputMethodClient.cs
@@ -1,9 +1,7 @@
 using System;
 using Avalonia.Controls.Presenters;
 using Avalonia.Input.TextInput;
-using Avalonia.Media.TextFormatting;
 using Avalonia.Reactive;
-using Avalonia.Utilities;
 
 namespace Avalonia.Controls
 {
@@ -13,6 +11,7 @@ namespace Avalonia.Controls
         private TextPresenter? _presenter;
         private bool _selectionChanged;
         private bool _isInChange;
+        private EventHandler? _caretBoundsChangedHandler;
 
         public override Visual TextViewVisual => _presenter!;
 
@@ -20,28 +19,22 @@ namespace Avalonia.Controls
         {
             get
             {
-                if (_presenter is null || _parent is null)
+                if (_parent is null)
                 {
                     return "";
                 }
 
-                if (_parent.CaretIndex != _presenter.CaretIndex)
+                if (_presenter is not null && _parent.CaretIndex != _presenter.CaretIndex)
                 {
                     _presenter.SetCurrentValue(TextPresenter.CaretIndexProperty, _parent.CaretIndex);
                 }
 
-                if (_parent.Text != _presenter.Text)
+                if (_presenter is not null && _parent.Text != _presenter.Text)
                 {
                     _presenter.SetCurrentValue(TextPresenter.TextProperty, _parent.Text);
                 }
 
-                var lineIndex = _presenter.TextLayout.GetLineIndexFromCharacterIndex(_presenter.CaretIndex, false);
-
-                var textLine = _presenter.TextLayout.TextLines[lineIndex];
-
-                var lineText = GetTextLineText(textLine);
-
-                return lineText;
+                return _parent.Text ?? string.Empty;
             }
         }
 
@@ -69,41 +62,22 @@ namespace Avalonia.Controls
         {
             get
             {
-                if (_presenter is null || _parent is null)
+                if (_parent is null)
                 {
                     return default;
                 }
 
-                var lineIndex = _presenter.TextLayout.GetLineIndexFromCharacterIndex(_parent.CaretIndex, false);
-
-                var textLine = _presenter.TextLayout.TextLines[lineIndex];
-
-                var lineStart = textLine.FirstTextSourceIndex;
-
-                var selectionStart = Math.Max(0, _parent.SelectionStart - lineStart);
-
-                var selectionEnd = Math.Max(0, _parent.SelectionEnd - lineStart);
-
-                return new TextSelection(selectionStart, selectionEnd);
+                return new TextSelection(_parent.SelectionStart, _parent.SelectionEnd);
             }
             set
             {
-                if (_parent is null || _presenter is null)
+                if (_parent is null)
                 {
                     return;
                 }
 
-                var lineIndex = _presenter.TextLayout.GetLineIndexFromCharacterIndex(_parent.CaretIndex, false);
-
-                var textLine = _presenter.TextLayout.TextLines[lineIndex];
-
-                var lineStart = textLine.FirstTextSourceIndex;
-
-                var selectionStart = lineStart + value.Start;
-                var selectionEnd = lineStart + value.End;
-
-                _parent.SelectionStart = selectionStart;
-                _parent.SelectionEnd = selectionEnd;
+                _parent.SelectionStart = value.Start;
+                _parent.SelectionEnd = value.End;
 
                 RaiseSelectionChanged();
             }
@@ -136,7 +110,10 @@ namespace Avalonia.Controls
                 oldPresenter.CurrentImClient = null;
                 oldPresenter.ClearValue(TextPresenter.PreeditTextProperty);
 
-                oldPresenter.CaretBoundsChanged -= (s, e) => RaiseCursorRectangleChanged();
+                if (_caretBoundsChangedHandler is not null)
+                {
+                    oldPresenter.CaretBoundsChanged -= _caretBoundsChangedHandler;
+                }
             }
 
             _presenter = presenter;
@@ -145,7 +122,8 @@ namespace Avalonia.Controls
             {
 
                 _presenter.CurrentImClient = this;
-                _presenter.CaretBoundsChanged += (s, e) => RaiseCursorRectangleChanged();
+                _caretBoundsChangedHandler ??= OnPresenterCaretBoundsChanged;
+                _presenter.CaretBoundsChanged += _caretBoundsChangedHandler;
             }
 
             RaiseTextViewVisualChanged();
@@ -153,6 +131,11 @@ namespace Avalonia.Controls
             RaiseCursorRectangleChanged();
         }
 
+        private void OnPresenterCaretBoundsChanged(object? sender, EventArgs e)
+        {
+            RaiseCursorRectangleChanged();
+        }
+
         private void OnParentTapped(object? sender, Input.TappedEventArgs e)
         {
             RaiseInputPaneActivationRequested();
@@ -171,30 +154,6 @@ namespace Avalonia.Controls
             _presenter.SetCurrentValue(TextPresenter.PreeditTextCursorPositionProperty, cursorPos);
         }
 
-        private static string GetTextLineText(TextLine textLine)
-        {
-            if (textLine.Length == 0)
-            {
-                return string.Empty;
-            }
-
-            var builder = StringBuilderCache.Acquire(textLine.Length);
-
-            foreach (var run in textLine.TextRuns)
-            {
-                if (run.Length > 0)
-                {
-                    builder.Append(run.Text.Span);
-                }
-            }
-
-            var lineText = builder.ToString();
-
-            StringBuilderCache.Release(builder);
-
-            return lineText;
-        }
-
         public override void ExecuteContextMenuAction(ContextMenuAction action)
         {
             base.ExecuteContextMenuAction(action);
diff --git a/tests/Avalonia.Controls.UnitTests/TextBoxTests.cs b/tests/Avalonia.Controls.UnitTests/TextBoxTests.cs
index 46af86b98d..1d7570c2a1 100644
--- a/tests/Avalonia.Controls.UnitTests/TextBoxTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/TextBoxTests.cs
@@ -2196,6 +2196,46 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(string.Empty, client.SurroundingText);
         }
 
+        [Fact]
+        public void InputMethodClient_SurroundingText_Uses_Full_Document_For_Multiline_Text()
+        {
+            using var _ = UnitTestApplication.Start(Services);
+
+            var textBox = new TextBox
+            {
+                Template = CreateTemplate(),
+                Text = "one\ntwo",
+                CaretIndex = 5
+            };
+            textBox.ApplyTemplate();
+
+            var client = GetInputMethodClient(textBox);
+
+            Assert.Equal("one\ntwo", client.SurroundingText);
+            Assert.Equal(new TextSelection(5, 5), client.Selection);
+        }
+
+        [Fact]
+        public void InputMethodClient_Selection_Setter_Uses_Document_Offsets_For_Multiline_Text()
+        {
+            using var _ = UnitTestApplication.Start(Services);
+
+            var textBox = new TextBox
+            {
+                Template = CreateTemplate(),
+                Text = "one\ntwo",
+                CaretIndex = 5
+            };
+            textBox.ApplyTemplate();
+
+            var client = GetInputMethodClient(textBox);
+            client.Selection = new TextSelection(0, 3);
+
+            Assert.Equal(0, textBox.SelectionStart);
+            Assert.Equal(3, textBox.SelectionEnd);
+            Assert.Equal("one", textBox.SelectedText);
+        }
+
         [Fact]
         public void Backspace_Should_Delete_Last_Character_In_Line_And_Keep_Caret_On_Same_Line()
         {
@@ -2364,6 +2404,18 @@ namespace Avalonia.Controls.UnitTests
             fontManagerImpl: new TestFontManager(),
             assetLoader: new StandardAssetLoader());
 
+        private static TextInputMethodClient GetInputMethodClient(TextBox textBox)
+        {
+            var eventArgs = new TextInputMethodClientRequestedEventArgs
+            {
+                RoutedEvent = InputElement.TextInputMethodClientRequestedEvent
+            };
+            textBox.RaiseEvent(eventArgs);
+
+            Assert.NotNull(eventArgs.Client);
+            return eventArgs.Client;
+        }
+
         internal static IControlTemplate CreateTemplate()
         {
             return new FuncControlTemplate<TextBox>((control, scope) =>