This is mostly about fingering out when to stretch the inline box with glyphs coming from fallback fonts.
Created attachment 434712 [details] Patch
<rdar://problem/81661514>
Created attachment 438908 [details] Patch
Committed r282872 (242001@main): <https://commits.webkit.org/242001@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 438908 [details].