The macOS 10.15.4 Supplemental Update released on 8 April 2020 isn’t a small update by any means, at a minimum of 1 GB. Much of that is taken up by the same set of EFI firmware updates which have already been delivered in the recent 10.15.4 update, so what else is there in this unscheduled update?

The simple answer for most users is not a lot.

Apple’s bundled apps, stored in /System/Applications, contain no changes at all, even in their build numbers.

A little more has changed, though, in /System/Library:

1. minor increment in Exchange and Exchange Web Services support, from build 806 to 806.1
2. minor increments in Setup Assistant and some of its plugins
3. minor increment in AVConference.framework (private)
4. minor increment in GameKitServices.framework (private)

Coupled with new firmware for Macs with T2 chips taking their versions up to 1037.100.362.0.0, iBridge 17.16.14281.0.0,0, these match Apple’s list of fixes to:

• problems with repeated password prompts for Office 365 accounts (1 above);
• problems with the latest MacBook Air 2020, with hanging in Setup Assistant or when connecting or disconnecting 4K or 5K external displays (2 and firmware)
• problems taking part in FaceTime calls with devices running older versions of iOS or macOS (3);
• unresponsive USB-C ports (firmware).

This is the smallest macOS update that I have ever analysed in detail, suggesting that those bugs were severe in their effects.

A standalone updater is also available from here.