This is preparation for incremental inline layout. We should be able to vertically align the content without rebuilding the line.
Created attachment 435630 [details] Patch
<rdar://problem/82260272>
Created attachment 436249 [details] Patch
Committed r281479 (240857@main): <https://commits.webkit.org/240857@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 436249 [details].