I noticed an incompatibility between Qt 6.5 and earlier version. My code does something like this:

void MyWindow::event( QEvent* event ) override
{
    if ( event->type() == QEvent::Show && size().isEmpty() )
        // setting some default size ...

}

The basic idea of this code is to set a default size when the application code did not assign something explicitly.

With Qt 6.5 the following line has been added in qxcbwindow.cpp line 269:

QPlatformWindow::setGeometry(rect);

what happens before sending the Show event.

In earlier Qt versions a window gets also initialized to same default size, but later.