Normally when editing a page in the Page: namespace, the header, body and footer sections are visually separated by a blue border, like in this image.
However, recently, on editing a Page: these borders momentarily appear but then vanish, so that the left of the window is entirely white save for the text being edited and the labels for the header, body and footer.
This is happening to me on Safari. Other users have reported the problem at the English Scriptorium.