[xenadu02]

This is not strictly correct. CoreAnimation does it's work in a background thread, so the mechanical work of scrolling pixels is not done on the UI thread unless you are doing custom drawing. Under the covers its a texture so the scrolling is really just translation.

[panic]

If you want to be pedantic, this isn't quite right either. CoreAnimation does some work on the main thread to encode layer properties and send them to the render server, which is in another process, not another thread.

The important part isn't the amount of work it takes to render, it's that event handling is serialized with all the other stuff happening on the main thread.