so instead of space width: 4px; word-spacing: 5px; -> measured width (and then the run width): 9px; run offset is 5px; and run width is 4px;
Created attachment 415799 [details] Patch
Created attachment 416065 [details] Patch
Created attachment 416116 [details] Patch
Committed r270749: <https://trac.webkit.org/changeset/270749> All reviewed patches have been landed. Closing bug and clearing flags on attachment 416116 [details].
<rdar://problem/72270772>