Client-side section editing (T76541) works by loading the whole document data and building the whole document model (DM) tree, but only building and attaching a DOM tree for a single branch of that tree (the branch at the "attachedRoot").
This prevents the editing view from being slowed and cluttered by the presence of extraneous content (outside the attachedRoot). Building, attaching and scrolling DOM nodes is expensive, so the computational speedup is substantial.
Conceptually between the logical DM tree and the rendered DOM tree lies the CE tree. As of T76541 we build the whole CE tree, which in principle is wasteful. This task is to instead build only the CE branch at the attachedRoot.