Comment # 15 on bug 974627 from Stefan Hundhammer

IMHO this problem is on the Qt side. It might also be the window manager we are
using during installation, but if the problem appears since that new Qt 5.6, it
really looks like a Qt problem.

Back many years ago, we did not use a window manager during installation, so we
drew our own window borders and actively grabbed the keyboard focus for each
new window. But of course this is highly antisocial behaviour, and it led to
all kinds of problems. Those problems went away when we introducted a window
manager during installation and no longer actively grabbed the keyboard focus.

AFAIK that worked well throughout many years - until this Qt 5.6. So this would
be the obvious place for any fix.