Confluence 5.9.7, using Firefox/IE/Chrome
Default Theme with Sidebar, using a header of any type, even just simply 'x'
Open a Confluence page, edit it, press preview. Some crazy page is displayed where the Confluence menus are on the left half of the screen at the top with blank underneath, and the preview page appears crammed on the right side of the page.
This happens with any type of custom header applied. Clearing the header customization makes the problem go away.
I have no customization on the site in terms of CSS/HTML/Stylesheets/Layouts/Themes/Color, etc. I went through all of the theming options to make sure they were all set to defaults. I have tried this on several different machines so I don't think it is a problem with my version of browser or any of my browser extensions or caching, etc.
This seems pretty repeatable with any page on our site that I try to edit. The only thing notable about my site is that I recently upgraded from 3.5 to 5.0, then to 5.4, then 5.9.7.
Some parts of my site we are still using the Documentation theme. There are no problem with Preview mode there.