Fix TextBox sync issues (#14960)

* Sync change text and changed caret index with the TextPresenter when text input is handled

* Do not add length again

src/Avalonia.Controls/TextBox.cs

@@ -1063,7 +1063,15 @@ namespace Avalonia.Controls
                     _undoRedoHelper.DiscardRedo();
                 }
 
-                SetCurrentValue(CaretIndexProperty, caretIndex + input.Length);
+                //Make sure updated text is in sync
+                _presenter?.SetCurrentValue(TextPresenter.TextProperty, text);
+
+                caretIndex += input.Length;
+
+                //Make sure caret is in sync
+                _presenter?.MoveCaretToTextPosition(caretIndex);
+
+                SetCurrentValue(CaretIndexProperty, caretIndex);
             }
         }