Example: app has a light-dismiss window that has WindowFlags | Qt.Popup with a button that opens a regular window. Using keyboard to press the button, the new window gets shown and activated but no FocusScope receives focus. Workaround is to click away from the window, then back into the window, then TAB-ing will finally work.

This is because when the previous window was Qt.Popup, QQuickWindow sets focusReason = PopupClosed, then the code that handles focusInEvent completely no-ops on focusReason=PopupClosed.