QStandardItem is using un-documented value of Qt::ItemDataRole

QStandardItem is using data()/setData() with un-documented value `255` of Qt::ItemDataRole for its methods such as flags()/setFlags() and isEditable()/setEditable().

When users subclass from QStardardItem and override data() and/or setData(), those methods become unavailable if they don't call QStandardItem::data()/setData() for these un-documented values, as most the examples on Qt's docs website do, e.g. https://doc.qt.io/qt-6/model-view-programming.html .

PLEASE metion this behavior on QStandardItem's docs page, especially for data()/setData()/flags()/setFlags() and getter/setter for individual flag such as isEditable()/setEditable().

This is infact an old issue reported in 2012: https://bugreports.qt.io/browse/QTBUG-25224 , which was closed as unresolved.