I turns out there’s a flowchart diagram of the Gtk drag-and-drop process on the Developer documentation site. (I know there had been some recent questions here regarding DnD.)
Anyone who wasn’t aware would be forgiven for having missed it, as it’s filed in the How Do I…? section of the Guides page (that part’s fine) as a page titled “Diagram Overview”. (That part, not so much.)
Honestly I think it was intended to be labeled “Drag and Drop Overview”, it’s just that whoever copied it from the wiki accidentally renamed it to the most generic thing possible… making it basically invisible in the process, because not many people are as bored as I am. Fortunately I’ll click on something labeled “diagram overview” out of sheer curiosity.
(Also, at the bottom of the page it claims that the SVG source for the diagram is available as an attachment on the wiki page, which is no longer true — or at least, no longer accessible — now that it’s not a wiki page. So, it’d be great if that could be removed, or even better if the source could be retrieved and added to the Developer site page as a link.)