Qt's tlwRect.size() reports ~16 GByte

While trying to run the app Stellarium on the L5, this fails with a message

qt.qpa.xcb: xcb_shm_create_segment() can't be called for size 17179344900

The maintainer of stellarium reports that this comes out of Qt from tlwRect.size() in qwidgetrepaintmanager.cpp reporting this size of "top-level window". See also the thread https://github.com/Stellarium/stellarium/discussions/2831#

It seems not to be exactly 16 GByte;

$ echo '16*1024*1024*1024' | bc
17179869184