Dennis Johnson fell victim last week to a new form of harassment known as "Zoombombing," in which intruders hijack video calls and post hate speech and offensive images such as pornography. It's a phenomenon so alarming that the FBI has issued a warning about using Zoom. Like many people these days, Johnson is doing a lot of things over the Internet that he would normally do in person. Last week, he defended his doctoral dissertation in a Zoom videoconference. He had a big audience — he estimated it was about 40 people, including "my closest friends, family and my classmates...