append as if it were a regular text run and use the new Line::addTrailingHyphen to mark the line with trailing hyphen.
Created attachment 410449 [details] Patch
Created attachment 410454 [details] Patch
Created attachment 410461 [details] Patch
Committed r267946: <https://trac.webkit.org/changeset/267946> All reviewed patches have been landed. Closing bug and clearing flags on attachment 410461 [details].
<rdar://problem/69928193>