How can open another document in a separate window and not in a tab?

Also, how can i stop the tab bar covering the top of the document when help opens in a tab?

Could you please show a screenshot?

LibreOffice does not support tabbed interface as far as I know; many people would love to have it… but maybe it’s an OS-specific situation (about which you said nothing, as well as about everything else, like LibreOffice version).

I’ve discovered the answer to my own question! Mike Kaganski is thinking right. I’m using an iMac running High Sierra. ‘System Preferences> Dock> Prefer tabs when opening documents’ was set to ‘Always’. (I don’t know how this happened!) This has had the effect of forcing the use of tabs in ALL applications including Mail and Pages and LibreOffice, and probably others too. I have set it to ‘Manually’ and now I can open new files in new windows. So, Mike, LibreOffice does support tabs after all !! Thank you for responding.