This technique might be required for applications or libraries that do their own form of thread pooling and thread dispatching.

执行自身形式的线程入池和线程调度的应用程序或库可能需要这种技术。

youdao

Incoming messages pop up on the top right in the form of what Facebook calls chat heads, which are bubbles that each represent a particular conversation thread and is displayed via a pop-over dialog box.

ENGADGET: Facebook Home hands-on (video)