Use the Visual editor source editing mode and edit some footnote within <ref>...</ref>. If you then preview your changes with Publish Changes/Show preview and click on the footnote, you will be sent to the original article's (unchanged) footnote text. This is unexpected, unhelpful, and differs from the behavior of the old-fashioned source code editor's preview mode.
So what's the expected behavior now? I see that right now, it does not navigate to any page but stays on the Preview section. Is that the change was about?
Yes, previously the reference links in the preview would open a new window.
My patch stops the reference footnote links from opening in a new window, but instead they go nowhere, because we don't set the needed ID attributes in the visual diff. I think this is still an improvement (and it is a complete fix for any other links to fragments). We should file a separate task about the remaining issue.