A few lists and settings screens of the CMS plugin work as described below. You need to be an administrator or a user in the CMS edit group; Redmine hooks and Menus are for administrators.
Sort the Pages list
The Pages list shows the tree of Pages. Click the header of Slug, Name, Layout, Version or Updated to sort it. A sort (like a filter) switches the list from the tree to a flat list with page numbers.
Menus as a tree
The Menus list is a tree with expanders, so nested items are easy to see and manage. Items you add always follow the stock items of Redmine, ordered by position. On Redmine 7 and later the Account Menu has one level only, so items there have no parent.
Redmine hooks
Open Redmine hooks in the CMS settings menu. Three code editors (HTML and Liquid) let you add HTML and Liquid to every Redmine page: Layouts base html head, Layouts base sidebar and Layouts base body bottom. Liquid tags, filters, Variables and Snippets work there; an empty editor prints nothing. There is no current Page, Layout or Part in these texts.
Parts as tabs
The edit screens of a Page and of a Part show every Part of the Page as a tab, locked Parts included, so you can edit a locked Part without activating it.