After bug 1708735, this popup has a shadow root with a slot for its contents, but this popup wants attr inheritance to work on the light tree, which used to work due to the fallback here to `this`: https://searchfox.org/mozilla-central/rev/294f10eff7398d6b05beac6aa256d86ac3cb7113/toolkit/content/customElements.js#431 So override getElementForAttrInheritance to ignore the shadow tree in order to preserve the behavior. Differential Revision: https://phabricator.services.mozilla.com/D117982