I am a Mac user. I'm sure that in the past I could close even the last tab in a window, using the cmd + W (Close Tab) shortcut, effectively closing that window. Recently I have noticed this no longer works. Nor does the shortcut given in the File dropdown menu to Close Window (cmd + shift + W) nor clicking Close Window in the dropdown itself (nor the shortcut or dropdown listings to Close Tab, though these continue to work on tabs other than the final tab in a window). The only way that now remains to close a single-tab window is to click the red close button in the corner of that window.