winId(), effectiveWinId(), and internalWinId() are all affected.

Steps to reproduce

1. Build and run the attached project on Linux (it is a modified version of https://doc.qt.io/qt-6/qtwidgets-widgets-windowflags-example.html where the ControllerWindow calls xcb_get_geometry_reply() to test the winId() whenever updating the PreviewWindow)

1. Select different window "Type"s, and toggle the different "Hints"

Outcomes

• On Qt 5.15.8, the winId()s are always OK
• On newer versions of Qt, selecting Qt::WindowType::Tooltip, enabling/disabling Qt::WindowType::WindowStaysOnBottomHint, or enabling/disabling Qt::WindowType::WindowStaysOnTopHint will produce an invalid winId(). This impacts the customer's ability to call the XCB API directly on their windows.

Partial workaround
Always set all flags before show()-ing a widget for the first time - this seems to avoid the problem. (However, this doesn't help with changing the window flags on an existing window)