While investigating Issue 769311 I noticed that bookmark bar is laid out from threads other than the main thread as the result of various notification and favicon observer callbacks. 

This isn't actively causing any issues AFAIK but is generally recommended against as NSView has restrictions when used off the main thread, among other things (https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/Multithreading/ThreadSafetySummary/ThreadSafetySummary.html#//apple_ref/doc/uid/10000057i-CH12-123427) ;