Using Qt::ExpandedClientAreaHint and interacting with system buttons causes window to behave incorrectly:

If you click (press and release) mouse in the client area and then drag mouse over any of the system buttons (minimize, maximize, close), the hovered button is clicked without any mouse buttons being pressed.

If you click the minimize button the window is minimized, but a title bar with system buttons remains and is not clickable anymore (though has hover highlight).