Go to: https://en.m.wikipedia.org/wiki/Undo. Click/tap on 'Last modified...' link to go to the history page. Take a random edit from the list and click/tap the 'Undo' link.
Expected behavior: You are shown a diff of what the undo would look like, as well as the editor displaying the undone edit.
Actual behavior: you are taken to the mobile editor, to edit current section 0
Version: unspecified
Severity: normal