edit pom.xml, add some dependencies for example. don't save editor contents or switch to diff. application, but press Synchronize:
the new changes are not synchronized.
If you press SaveAll before synchronizing - ok.
(you can check this also if you create some invalid code, like adding dependency with empty groupid/version, then save file, then correct mistake and synchronize: the error is detected)
This is confusing behavior since generally in IDEA all unsaved files are saved without any user input on actions performing on them .