Only one edit window! How do you create "Read more..."?
From Joomla! Documentation
This is now implemented by inserting a tag (button is in the editor area) a dotted line appears in the edited text showing the split location for the "Read more...". A new bot takes care of the rest.
It is worth mentioning that this does not have a negative effect on migrated data from older sites. The new implementation is fully backward compatible.