Hiding the dock icon after the viewer window closes triggers an
asynchronous screen/dock reconfiguration via SkyLight. Without
draining the main run loop, the resulting NSScreen invalidation
notification is dispatched to a GCD worker thread, which hits
NSScreen's main-thread assertion and crashes the process.
Drain the main NSRunLoop briefly after setActivationPolicy: so the
reconfiguration is processed on the main thread.
Kevin Zakka
327f05a567