Corrected the title and this description. The proofread page in edit mode is affected when the syntax highlighter is enabled. When not logged in, or when in read mode, the proofread page displays correctly.
I am using Firefox 71.0 and Vivaldi 2.9.1705.41 (Stable channel) (64-bit). When I log in, the header is on the top of the footer at the bottom of the textbox in both browsers in both Linux and Windows. Please see this image link. https://en.wikisource.org/wiki/File:Header_footer_layout_error.jpg