If the defaulthandler is set with the latter pair of methods, any thread that doesn't have its own exception handler will use the default.
如果使用第二对方法设置默认处理程序,那么没有自己的异常处理程序的所有线程都将使用默认处理程序。
3
If no event handlers processed the click, the event will travel back up the same path until it comes back to the root element, where the default click handler will ignore it.