Manual 中说的清楚:
Although QObject is reentrant, the GUI classes, notably QWidget and all its subclasses, are not reentrant. They can only be used from the main thread.
题外:你贴的这句话与本问题无关。
This class is not part of the Qt GUI Framework Edition.
它告诉你,如果你购买的是商业版的Qt GUI Framework Edition,那么将不包含该类。