While adding a test for toLower() handling of Greek Final Sigma (see https://en.wikipedia.org/wiki/Sigma), I noticed that QString::toLower() doesn't handle it (very long-standing bug QTBUG-2163), but the Windows backend of QLocale::toLower() also doesn't. See https://codereview.qt-project.org/c/qt/qtbase/+/663457 for the failing test (earlier versions; I'll update the test with a QEXPECT_FAIL pointing to this bug report after filing this).

It's hard to believe that the world's most-used OS gets this wrong in a non-ignorable locale like Greek, so we should rule out that it's us doing something wrong (locale not installed on the CI, wrong function used (cf. QTBUG-138629), ...).

The litmus test is: Lower-casing "ΛΟΓΟΣ" should produce "λογος", not "λογοσ". If a non-Qt application gets this correct, then that's a strong indication that it's Qt's fault it doesn't work in QLocale.