Put UI.start_timer mock callback into its own thread
Put UI.start_timer mock callback into its own thread

The block of UI.start_timer may call UI.stop_timer which was run in the
waiting thread and attempted to terminate the waiting thread causing a
freeze. When the callback is in its own thread, UI.stop_timer can
terminate the waiting thread.

Aerilius/sketchup-console-plus,Aerilius/sketchup-console-plus,Aerilius/sketchup-console-plus