[css-scroll-anchoring-1] Unclear whether a 0-size element at edge of region is eligible as anchor node or not

https://drafts.csswg.org/css-scroll-anchoring/#anchor-node-selection defines "fully visible" and "fully clipped" as follows:

A DOM node N is fully visible in a scrolling box S if N’s scroll anchoring bounding rect is entirely within the optimal viewing region of S.
A DOM node N is fully clipped in a scrolling box S if N’s scroll anchoring bounding rect is entirely outside the optimal viewing region of S.

This leaves an edge case undefined (depending on how you interpret "within" vs. "outside"):
What if N's rect has size (0,0) and is positioned exactly on the edge of the optimal viewing region? Is that empty rect considered "entirely within" or "entirely outside" the region?

(This choice will have a user-noticeable impact; if such a node is "entirely within", then the algorithm will terminate and chooses this 0-sized node N; but if it's "entirely outside",, then the algorithm will proceed & choose some other node.)

Start with the CSS Scroll Anchoring specification's anchor node selection section and review the definitions of “fully visible” and “fully clipped.” Determine how a zero-size rectangle exactly on the optimal viewing region's edge should be classified, then update the specification to make that behavior explicit and verify that the selection algorithm's consequence is clear.