Fix IME position bug for Mac and Windows (#1154)

lib/src/widgets/raw_editor/raw_editor_state_text_input_client_mixin.dart

@@ -63,12 +63,44 @@ mixin RawEditorStateTextInputClientMixin on EditorState
       );
 
       _updateSizeAndTransform();
+      //update IME position for Windows
+      _updateComposingRectIfNeeded();
+      //update IME position for Macos
+      _updateCaretRectIfNeeded();
       _textInputConnection!.setEditingState(_lastKnownRemoteTextEditingValue!);
     }
-
     _textInputConnection!.show();
   }
 
+  void _updateComposingRectIfNeeded() {
+    final composingRange = _lastKnownRemoteTextEditingValue?.composing ??
+        textEditingValue.composing;
+    if (hasConnection) {
+      assert(mounted);
+      final offset = composingRange.isValid ? composingRange.start : 0;
+      final composingRect =
+          renderEditor.getLocalRectForCaret(TextPosition(offset: offset));
+      _textInputConnection!.setComposingRect(composingRect);
+      SchedulerBinding.instance
+          .addPostFrameCallback((_) => _updateComposingRectIfNeeded());
+    }
+  }
+
+  void _updateCaretRectIfNeeded() {
+    if (hasConnection) {
+      if (renderEditor.selection.isValid &&
+          renderEditor.selection.isCollapsed) {
+        final currentTextPosition =
+            TextPosition(offset: renderEditor.selection.baseOffset);
+        final caretRect =
+            renderEditor.getLocalRectForCaret(currentTextPosition);
+        _textInputConnection!.setCaretRect(caretRect);
+      }
+      SchedulerBinding.instance
+          .addPostFrameCallback((_) => _updateCaretRectIfNeeded());
+    }
+  }
+
   /// Closes input connection if it's currently open. Otherwise does nothing.
   void closeConnectionIfNeeded() {
     if (!hasConnection) {