Crash in QSortFilterProxyModelPrivate::create_mapping_recursive

With Qt 6.3.0 and Qt 6.3.1 (possibly earlier; not tested) our application crashes in QSortFilterProxyModelPrivate::create_mapping_recursive. Full backtrace attached.

After checking with a debug compile of qtbase, it hits an assertion

ASSERT failure in QList::at: "index out of range", file /home/arehn/src/qt6/qtbase/src/corelib/tools/qlist.h, line 459

Arising from

        if (gm->proxy_rows.at(source_parent.row()) == -1 ||
                gm->proxy_columns.at(source_parent.column()) == -1) {

in QSortFilterProxyModelPrivate::create_mapping_recursive(). The code assumes that proxy_rows for sure is at least as long as source_parent.row() + 1. I'm not sure whether this assumption is warranted; I haven't grokked it completley yet.

We do have some other proxy models in front of the QSortFilterProxyModel, so I cannot completely rule out that we're exposing an inconsistent model to QSFPM. However, the code is well tested and it works flawlessly with Qt 5.15.