@NotNull List<VirtualFile> getFilesUnderDirectory(@NotNull VirtualFile dir) {
    assertThreadIfNeeded();

    List<VirtualFile> files = myStrictDirectory2Children.get(dir);
    return files != null ? List.copyOf(files) : List.of();
  }
@NotNull List<VirtualFile> getFilesUnderDirectory(@NotNull VirtualFile dir)