If someone does save the document with features that aren’t supported by Word Online, you can still co-author, just not in Word Online anymore. If everyone in the group has Word 2010, Word 2013, or Word for Mac 2011, continue working together in Word (click Open in Word and continue editing).
Hi All,
The behavior that files opened in Word Online is directly in the edit mode is a behavior in SharePoint Online by default now.
For more detailed information, please refer to Modern Web experience for Office files in the article.
As a workaround, you can set Require documents to be checked out before they can be edited to Yes in Version settings in Library Settings. Meanwhile, I suggest you submit your feedback on our SharePoint UserVoice as it is the direct way for product developers to know users’ requirements.
Thanks for your understanding.
Cliff
This is very disappointing.
In my opinion this a huge change in functionality from one day to another, and because of this we are also having massive issues with files suddenly being changed unintentionally.
The check out feature is not a solution, as it adds a big unwanted step whenever one of our many users wants to change as mush a single Word in one of the many documents we have stored.
We really need a setting where we can change this behaviour back to how it was before.