private Collection<VirtualFile> collectReadOnlyAffectedFiles() {
    Collection<VirtualFile> readOnlyFiles = new ArrayList<>();
    for (UndoableAction action : myUndoableGroup.getActions()) {
      if (action instanceof MentionOnlyUndoableAction) continue;

      DocumentReference[] refs = action.getAffectedDocuments();
      if (refs == null) continue;

      for (DocumentReference ref : refs) {
        VirtualFile file = ref.getFile();
        if ((file != null) && file.isValid() && !file.isWritable()) {
          readOnlyFiles.add(file);
        }
      }
    }
    return readOnlyFiles;
  }
private Collection<VirtualFile> collectReadOnlyAffectedFiles()