It gives the following runtime warning:
Warning: QT_AUTO_SCREEN_SCALE_FACTOR is deprecated. Instead use: QT_ENABLE_HIGHDPI_SCALING to enable platform plugin controlled per-screen factors
But we don't need to set QT_ENABLE_HIGHDPI_SCALING to 0.
Just setting QT_SCREEN_SCALE_FACTORS is enough to control scaling, after
my commit e018d11600bffc6 to qtbase.
Kudos for porting startkde to be C++ code, it makes this commit much
easier...