FramelessWindowHint + showMaximized bug in qt6

I have an application (FreeCAD) with a FramelessWindowHint:
FramelessWindow::FramelessWindow(QWidget * parent, Qt::WindowFlags f) : QMainWindow( parent, f )

{      setWindowFlags(windowFlags() | Qt::FramelessWindowHint | Qt::Window); }

Trying to maximize the window gives weird behavior:

Start with a non-maximized window.

Try to maximize it (with python or cpp is the same). The window visually maximize. However the QEvent::WindowStateChange is duplicated. So the window is kind-of-restored (but not in size) :

>>> mainWindow.showMaximized()
>>> mainWindow.isMaximized()
False
>>> mainWindow.showMaximized()
>>> mainWindow.isMaximized()
True

 

The problem is that if you have a button to maximize/restore like so : 

void CustomTitleBar::toggleMaximizeRestore() {
    if (m_mainWindow->isMaximized())

{         m_mainWindow->showNormal();     }

    else

{         m_mainWindow->showMaximized();     }

}

Then here what happens if you click it : 

• Start normal
• Click 1 : Becomes visually maximized, but is in fact 'restored'
• Click 2 : stays visually maximize, become 'maximized' for real.
• Click 3 : restores correctly.

This was behaving perfectly with Qt 5.15 and fails after updating to Qt 6.8.1