Foreign XCB QWindows get destroyed

QWindows created by QWindow::fromWinId get destroyed with xcb_destroy_window, which is not expected. For instance, it breaks setting a transient parent of a window to a foreign window if the forein window gets deleted during cleanup.

See also https://bugs.kde.org/show_bug.cgi?id=380392

Minimal reproducer, launch with winId of target window as argument.

#include <QApplication>
#include <QWindow>

int main(int argc, char *argv[])
{
        QApplication app(argc, argv);
        delete QWindow::fromWinId(strtol(argv[1], nullptr, 0));
}

With Qt < 5.9.0, nothing happens. With Qt 5.9.0 however, the target window disappears.
With a breakpoint on xcb_destroy_window I can confirm that it's called with 5.9.0 but not with earlier versions.