Handle special case for Vietnamese input on iOS to prevent accidental deletion of embedded objects

lib/src/editor/raw_editor/raw_editor_state_text_input_client_mixin.dart

@@ -1,3 +1,4 @@
+import 'dart:io';
 import 'dart:ui' show lerpDouble;
 
 import 'package:flutter/cupertino.dart';
@@ -224,6 +225,50 @@ mixin RawEditorStateTextInputClientMixin on EditorState
       return;
     }
 
+    // On iOS, we need to handle a special case where user input modifies a previous word.
+    // For example, in Vietnamese keyboard with the Telex input method,
+    // the word "dinh" becomes "định" when the user types "d".
+    // iOS handles this by deleting several characters
+    // from the end of the word to the beginning (plus one extra character)
+    // and then re-inserting the updated characters.
+    //
+    // The issue arises when the word is adjacent to an embedded object (e.g., an embedded block).
+    // iOS may accidentally remove the embedded object character (\uFFFC)
+    // during this replacement process. To prevent this,
+    // we should skip updating the text value
+    // if the change affects the starting character and is next to an embedded block.
+    if (Platform.isIOS) {
+      final lastKnownValue = _lastKnownRemoteTextEditingValue;
+
+      if (
+          //verify that lastKnownValue is not null
+          lastKnownValue != null &&
+              // verify that lastKnownValue selection extentOffset is more than
+              // 2 characters away from baseOffset
+              lastKnownValue.selection.extentOffset -
+                      lastKnownValue.selection.baseOffset ==
+                  2 &&
+              // verify that the lastKnownValue text length is more than 2
+              // characters away from the value text length
+              lastKnownValue.text.length - value.text.length == 2 &&
+              value.selection.isCollapsed &&
+              value.selection.affinity == TextAffinity.downstream &&
+              value.selection.isDirectional == false) {
+        // take the last two characters from the last known value
+        final deletedText = lastKnownValue.text.substring(
+          value.selection.baseOffset,
+          value.selection.baseOffset + 2,
+        );
+
+        // if the deleted text starts with the zero-width embed character,
+        // we don't want to send an update to the native side
+        // as it will cause the embedded block to be deleted
+        if (deletedText.startsWith('\uFFFC')) {
+          return;
+        }
+      }
+    }
+
     final effectiveLastKnownValue = _lastKnownRemoteTextEditingValue!;
     _lastKnownRemoteTextEditingValue = value;
     final oldText = effectiveLastKnownValue.text;