Even going to source editor gets ride of the header and brings up a box with nearly no function.
This seems to be known bug. You can report it to staff via Fandom (zendesk.com) to increase the likelihood of it being fixed.
Having the same problem; just submitted this as a bug.
For me it only happens when i try to edit a "File:" page. I submitted a bug report about yesterday.
The first thread here on Community Central about this bug was posted on Tuesday. So it has been almost a week at this point. Some has since found that you can use WikiEditor just fine to edit the file pages.
In case someone wants to use WikiEditor when it's not their default editor, it is possible by adding ?action=submit to the URL of the page.
What do you think?