Try fix Korean IME on iOS (#17680)

* Do note propagate SurroundingTextChanged back to iOS

* Fix

* Optimize

src/iOS/Avalonia.iOS/TextInputResponder.cs

@@ -90,6 +90,7 @@ partial class AvaloniaView
         private int _inSurroundingTextUpdateEvent;
         private readonly UITextPosition _beginningOfDocument = new AvaloniaTextPosition(0);
         private readonly UITextInputStringTokenizer _tokenizer;
+        private bool _isInUpdate;
 
         public TextInputMethodClient? Client => _client;
 
@@ -142,15 +143,15 @@ partial class AvaloniaView
         private void SurroundingTextChanged(object? sender, EventArgs e)
         {
             Logger.TryGet(LogEventLevel.Debug, ImeLog)?.Log(null, "SurroundingTextChanged");
-            if (WeakInputDelegate == null)
+            if (WeakInputDelegate == null || _isInUpdate)
                 return;
             _inSurroundingTextUpdateEvent++;
             try
             {
                 objc_msgSend(WeakInputDelegate.Handle.Handle, TextWillChange, Handle.Handle);
                 objc_msgSend(WeakInputDelegate.Handle.Handle, TextDidChange, Handle.Handle);
-                objc_msgSend(WeakInputDelegate.Handle.Handle, SelectionWillChange, this.Handle.Handle);
+                objc_msgSend(WeakInputDelegate.Handle.Handle, SelectionWillChange, Handle.Handle);
-                objc_msgSend(WeakInputDelegate.Handle.Handle, SelectionDidChange, this.Handle.Handle);
+                objc_msgSend(WeakInputDelegate.Handle.Handle, SelectionDidChange, Handle.Handle);
             }
             finally
             {
@@ -160,6 +161,7 @@ partial class AvaloniaView
 
         private void KeyPress(Key key, PhysicalKey physicalKey, string? keySymbol)
         {
+            _isInUpdate = true;
             Logger.TryGet(LogEventLevel.Debug, ImeLog)?.Log(null, "Triggering key press {key}", key);
 
             if (_view._topLevelImpl.Input is { } input)
@@ -170,12 +172,15 @@ partial class AvaloniaView
                 input.Invoke(new RawKeyEventArgs(KeyboardDevice.Instance!, 0, _view.InputRoot,
                     RawKeyEventType.KeyUp, key, RawInputModifiers.None, physicalKey, keySymbol));
             }
+            _isInUpdate = false;
         }
 
         private void TextInput(string text)
         {
+            _isInUpdate = true;
             Logger.TryGet(LogEventLevel.Debug, ImeLog)?.Log(null, "Triggering text input {text}", text);
             _view._topLevelImpl.Input?.Invoke(new RawTextInputEventArgs(KeyboardDevice.Instance!, 0, _view.InputRoot, text));
+            _isInUpdate = false;
         }
 
         void IUIKeyInput.InsertText(string text)
@@ -214,23 +219,22 @@ partial class AvaloniaView
             var r = (AvaloniaTextRange)range;
             var surroundingText = _client.SurroundingText;
 
-            var currentSelection = _client.Selection;
-
             Logger.TryGet(LogEventLevel.Debug, ImeLog)?.Log(null, "IUIKeyInput.TextInRange {start} {end}", r.StartIndex, r.EndIndex);
 
             string result = "";
             if (string.IsNullOrEmpty(_markedText))
             {
-                if(surroundingText != null && r.EndIndex < surroundingText.Length)
+                if (surroundingText != null && r.EndIndex <= surroundingText.Length)
                 {
                     result = surroundingText[r.StartIndex..r.EndIndex];
                 }
             }
             else
             {
-                var span = new CombinedSpan3<char>(surroundingText.AsSpan().Slice(0, currentSelection.Start),
+                var currentSelection = _client.Selection;
+                var span = new CombinedSpan3<char>(surroundingText.AsSpan(0, currentSelection.Start),
                     _markedText,
-                    surroundingText.AsSpan().Slice(currentSelection.Start, currentSelection.End - currentSelection.Start));
+                    surroundingText.AsSpan(currentSelection.Start, currentSelection.End - currentSelection.Start));
                 var buf = new char[r.EndIndex - r.StartIndex];
                 span.CopyTo(buf, r.StartIndex);
                 result = new string(buf);