What has changed in the macOS 10.15.6 Supplemental Update?

macOS Catalina 10.15.6 Supplemental Update is promised to address two problems: some Macs running Virtual Machines under VMWare and VirtualBox have been suffering kernel panics, and the 27-inch displays of some new 2020 iMacs have appeared washed out when they wake from sleep. Although these should be relatively limited bugs to be fixed, the installer is substantial, at around 2.8-3.2 GB, and brings some surprising changes.

Among the bundled apps, there are only two changes:

• Books, which has a small build increment,
• VoiceOver Utility, which has a minor build increment.

There are more substantial changes in the /System/Library folder, which includes:

• VoiceOver app, with a minor build increment,
• AMDRadeon kernel extensions, with minor version increments,
• PDFKit framework, which has a major build increment,
• ScreenReader private framework, with a minor version increment,
• BrailleDrivers, with build increments.

All Macs with a T2 chip also have new firmware, with both the EFI and Bridge components undergoing minor version increments. Although the installers include a full set of firmware updates for other models, none of those has changed. One side-effect of this is that Macs with T2 chips which are running Catalina now have different firmware version numbers from those which are still running Mojave or High Sierra.

Apple hasn’t posted any security release notes for this update either.

Early reports are that users who had been suffering kernel panics and other problems as a result of the 10.15.6 update have recovered stability in their Macs. It thus appears most likely that the memory leak was in the AMDRadeon drivers.