@taavi - thanks for the report! We're trying to reproduce from our side. If you can, would it be possible to check if you are opted into the "Accessiblity for reading" beta feature (available in the beta tab of your preferences)? Also, does it still appear if you're using the old parser?

This is with that beta feature enabled, without it the entire menu is gone. Somehow seems like switching to the old parser and then back new made the problem go away. So unless you can reproduce somehow I am tempted to blame this on some cache issue between wmf.2 and wmf.3 functionality. (The page in the screenshots is https://en.wikipedia.org/wiki/Long-term_nuclear_waste_warning_messages, in case that's relevant somehow.)

Per @Jdlrobson "My guess is that html was loaded from wmf.2 during the deployment alongside js from wmf.3. That would manifest just like that. I wouldnt worry about it. This scenario occurs every deploy when HTML changes for a period of 5 minutes."