I’ve made a plugin for this: GitHub - scossar/discourse-sticky-footer: make a custom footer sticky in a Discourse forum · GitHub
Pull requests and suggestions for improvements are welcome.
The plugin takes care of most of the flashing by keeping the footer from jumping around the window as the view is rendered. If you reload the topic-list while you are on the topic list, the footer flashes in the middle of the screen. I think that can be fixed.