In QBidiAlgorithm::resolveN0() there is an assumption that only BMP characters can have categories open-punctuation or close-punctuation. This is currently the case, but if supplementary characters are added with these properties, then they will be unsupported by Qt.

For robustness, we should resolve surrogates in this code like we do elsewhere.
In the mean time, it may be a good idea to add an assertion in the qtbase/utils/unicode/ program about the current status quo remaining true, so we know if this becomes urgent.