I accidentally clicked on a “Mark this as spam” link at a web page where the comments are managed by Facebook. Brave tried to pop up a Facebook extension, which was a mostly blank window. Clicking the X to get rid of it did not work, multiple times.
Eventually, I clicked the X on the main Brave window, and that window disappeared, but the annoying Facebook extension window remained. After waiting a minute or two, I went to Windows Task Manager (on Windows 10) and clicked on the Brave process and clicked End Task.
Now I have a second problem: When I start Brave, instead of loading all the tabs it used to load, it only loads the annoying blank Facebook extension tab. And I see no way to display “Recently closed tabs” as in other browsers. So all the tabs I was saving are lost, apparently.