Why should I avoid using the CloseButtonClosesWindow property in Office Online?

Browsers prevent JavaScript from closing windows that aren’t owned by the script that calls window.close, so in many cases CloseButtonClosesWindow will not behave properly. In such cases an error may be logged on the browser developer console:

Scripts may close only the windows that were opened by it.

The calling script is not reliably given any information that the close() call failed, so Office Online can’t detect when this happens and change behavior. Thus, hosts who wish to close the current window when the Close UI is activated should prefer to use ClosePostMessage and handle the UI_Close message for reliable close behavior.