qt6 6.11.0 is broken (or qgis 4 has a latent bug, seems less likely)

[Greg Troxel <gdt%lexort.com@localhost>]

Yesterday I wrote that starting up qgis 4.0.2(+18 commits) (from
wip/qgis) resulted in a crash.   It turns out the thing that matters for
crash vs works is not qgis version, but qt6 version.

This is all about qgis 4, because qgis 3.44 uses qt5 (and both qgis 3.44
and qt5 are apparently stable).

QGIS	QT	RESULT	
4.0.0	6.10.2	works
4.0.2   6.10.2	works
4.0.0	6.11.0	crashes on opening project file
4.0.2	6.11.0	crashes on opening project file

Have other people tried to run complicated qt programs built against qt6
6.11.0?