Slots that contain only text nodes render drop-zone when they should not

Thanks for the detailed bug report, with precise steps to reproduce 🤩👏🙏

@luke.leber Would you be able to try testing on the MR branch on 📌 Investigate drag-and-drop solution that removes the need to drop items into the preview iFrame Active ? I that while this MR is not intentionally trying to fix the issue you've reported, it is such a big refactor that it will just make the problem go away. Specifically, in that MR, I now determine if a slot is empty by looking at the JSON data model (slot.components.length) whereas before I think the empty placeholder was injected based on if there were any child elements in the HTML (and probably failing to spot textnodes)

Oo, shiny.

Yeah, this particular issue seems to go away with that MR applied.