This makes this call work as expected: https://searchfox.org/mozilla-central/rev/4c065f1df299065c305fb48b36cdae571a43d97c/dom/view-transitions/ViewTransition.cpp#569 Right now the Gecko and Servo values don't really match. Nobody uses the sideways flag on this value. Differential Revision: https://phabricator.services.mozilla.com/D248229