Caused by Webpack clobbering the `Danbooru` variable for unknown reasons when the app/javascript/packs/flash.js pack is loaded. Disabling the output.library option seems to fix it.
Config
This directory contains configuration files for Danbooru.
To configure your Danbooru instance, copy danbooru_default_config.rb to danbooru_local_config.rb and edit it. See danbooru_default_config.rb for details.
The only file here that end users need to be concerned about is danbooru_default_config.rb. The rest of the files here are internal Rails-related configuration files that end users shouldn't need to edit.