I have been whining off and on for some weeks now about the sporadic freezes which have been disrupting this iMac 17,1 (iMac Retina 5K, 27″, Late 2015). It would appear that I am not alone: MacRumours is reporting this as a bug affecting MacBook Pro Retina, 13″, Early 2015 models (MacBookPro12,1) running OS X 10.11.4.
This happened again yesterday morning, most peculiarly. I had not long got up and started my rounds of the day – Twitter, email, websites in Safari – and my Mac was hardly loaded or fraught. When visiting some fairly ordinary websites, it froze again: the Trackpad became completely unresponsive, the clock stopped, and all input and pointer movement was lost. I left it for a couple of minutes, and it spontaneously restarted, as it usually does.
The logs for the minute or so before the freeze showed a crescendo of WebKit errors. There was no complaint from the kernel, then all went silent until the restart occurred.
From here it looks as if this is new with OS X 10.11.4 (it did not happen before updating to it), and probably rests somewhere between WebKit and the kernel, which could of course include quite a lot, including the graphics drivers.
Among many other solutions which I have tried are installing the 10.11.4 Combo update, stripping old extensions, updating every app that I can think of, including Parallels Desktop, OS X Server, and Xcode, and more besides. These freezes are not frequent – every 5-7 days, I suppose – and are completely unpredictable. If they have one common circumstance, it is probably browsing in Safari.
Let’s hope that Apple has a fix in hand, and that it will be removed in the forthcoming release of OS X 10.11.5.