WinAPI timer vs thread?

[Arjay]

The conclusion that I have reached is that there is no harm in manipulating the UI controls from a worker thread, but from a design perspective (and to prevent deadlocks) it is a better idea to only make the UI thread responsible for manipulating the UI controls.

Kind of. You need to be aware that any writeable shared data between the UI thread and worker thread(s) needs to be synchronize. And don't forget the negative impact of using SendMessage to communicate between worker threads and the UI threads.