macOS 10.13.5
The only new user feature identified by Apple is Messages in iCloud, which Apple states is a feature of High Sierra only; there is no comparable option in the Sierra version of the Messages app. The High Sierra update still seems very large: its App Store download is 2.12 GB in size.
There is no mention of whether 10.13.5 extends support for APFS to Fusion Drives.
The list of security fixes is quite long, and contains some important ones.
The combo updater is here.
Update (2018-06-02): Kirk McElhearn:
It’s surprising that Apple hasn’t rolled AirPlay 2 into macOS. You have long been able to stream to multiple speakers from the Mac, using iTunes, and that is new on iOS with AirPlay 2, but the inability for the Mac to be able to use AirPlay 2 and see a stereo pair of HomePods is surprising.
iCloud sync seems completely stuck on my desktop Mac across apps. Photos stop in December 2017; iMessage sync has been running for >12 hours without any visible progress. Ideas?
Update (2018-06-02): Since updating, I’ve had frequent problems with my Magic Mouse disconnecting. Once, Bluetooth even turned itself off, and I was not able to re-enable it until after restarting.
