- Open any Wikisource page in the Page namespace and edit. E.g. https://en.wikisource.org/wiki/Page%3AOrph%C3%A9e_aux_Enfers_(Chicago_1868).djvu/9
- Observe the page is in a left/right default split: edit box on left, scan on right
- Click the "Change layout button":
- The page changes to a top/bottom split (correct)
- "Show Preview" for the page
- The page has reverted back to the left/right split (incorrect)
- The page remains in the top/bottom split mode until manually set back.
- Presumably the editor set this based on their window layout, and "Show Preview" or changing page won't change that. So, this setting should stick until an editor manually changes it back to left/right split mode.