- Set display: none on the close link using a generic CSS class from core (rather than from the extension). The extension's stylesheet would override this. I don't know whether a suitable class already exists in core CSS.
- Variation on #2: use client-nojs, similarly to how it is used for the enhanced changes list. This would require a core change (unless, of course, the extension were to use addModuleStyles() as in #1).
- Fix T50815: reserve a fixed amount of space for the close link (possibly changing it to an icon) and add it to the document from JS.