3f57fdd8e4
XUL is special and doesn't follow the usual native-appearance-prevention rules from HTML. That means that if you don't specify a border we'll now happily paint it on the content box. Also specify a background-color while at it. Honestly there's not too much the appearance property does for us here, so we could probably remove it, but this is the lower risk fix... Differential Revision: https://phabricator.services.mozilla.com/D250916