@Override
  public @NotNull LineFragment subFragment(int startOffset, int endOffset) {
    assert startOffset >= 0;
    assert endOffset <= myCharPositions.length;
    assert startOffset < endOffset;
    if (startOffset == 0 && endOffset == myCharPositions.length) return this;
    return new TextFragmentWindow(startOffset, endOffset);
  }
@Override
  public @NotNull LineFragment subFragment(int startOffset, int endOffset)