ps you have an example, or what you think is an example? That I can try?
Take a 3.6 alpha open some windows, browser, then invoke a deprecated method such as Smalltalk beep. When you proceed the notifier, you have a big chance to see all the panes getting refresh extremely slowly one after the one ones even those that should not get redrawn.
You can almost certainly make it happen by adding "Processor yield" as the first line in Debugger>>resumeProcess: - it all looks as if there's a confusion between which of the processes (the debugged or the active one) is the "UI process" here. I would suspect that the Mac VM (being multi-threaded) issues a signal on the input semaphore which invokes a process switch very similar to adding Processor yield in #resumeProcess:.
Cheers, - Andreas