Let's start with moving RenderSVGRoot -> LegacyRenderSVGRoot. A follow-up patch will introduce the new RenderSVGRoot stub implementation. After that switching from LBSE to legacy engine and back has a visual effect (LBSE enabled == nothing renderered). After some experimentation on how to upstream, I found that starting from a clean RenderSVGRoot is the easiest approach.
Created attachment 445481 [details] Patch, v1
Committed r286392 (244748@main): <https://commits.webkit.org/244748@main>
<rdar://problem/85943424>