In the Page namespace, even without modifying anything in the edit form, the 'are you sure?' confirmation dialog is always shown when navigating away from the page. This applies when the "Warn me when I leave an edit page with unsaved changes" preference is enabled.
Scriptorium discussion: https://en.wikisource.org/wiki/Wikisource:Scriptorium#Always_needing_to_confirm_when_leaving_Page_editing%3F