If i check single document interface and choose 'don't show tabs" in preferences, there isn't any close buton to close each window.
One solution is to right click in windows list, and choose Close_windows.

Thanks

Perhaps you can add the File > Close function to the Quick Access Toolbar?

Ctrl-F4 will also close it, or File > Close on the ribbon/menu, or a function key assigned to File > Close.

We'll consider an additional button next to the Help/Info/Manual buttons at the upper right when using SDI mode without tabs.