(note: ticket description modified to put backticks around the //, which was preventing comments from being added to the ticket, apparently)
I like the concept of a better core solution for multi-part pages. But I'm a little unclear on how this proposal would work in terms of content structure. Is "Panel Content" a post object, of a particular post type? What would the API look like for themes displaying the information? What would the UI look like for filling in the actual content here?
I wonder whether we might be able to accomplish something similar with a more straightforward UI if it were based on widgets, and new core widgets that supported this type of functionality were introduced. That way we'd be able to leverage the existing core UI for adding/reordering/number of items/etc, and on a technical level things like selective refresh and the work on storing widgets as post objects. This idea is generally related to widgets as "content blocks".
Looking at https://piquedemo.wordpress.com/, being able to make a "container widget" that sets a number of columns, title, and maybe image, and then lets users add content via additional widget that, for example, have fields for image, title, and description, could be worth exploring. Or that part could be based on theme support with content being organized with widgets. I'm definitely concerned about how this will work across theme switches, though, especially for themes that don't support this.