After upgrade to Geneva or later versions, the update set picker styling is messed up in the header.
This happens in UI15 and later, and is reproducible in all browsers.
The reason this happens if there is a "MoveUpdateSetPicker" UI Script added in the instance. This UI Script is provided by some third party websites, which allows styling the update set picker.
This UI Script moved the Update set picker from the settings menu to the header. This script is only compatible with UI14 and not newer UIs, therefore it is breaking on the current UI.
To resolve this issue, de-activate this UI script so that the Update set picker moves back to the Settings menu, and is removed from the header.
Geneva and later