It outputs calc(var(--toolbar-height)+var(--tab-bar-height)), which is not valid. This is breaking the UI after http://trac.webkit.org/changeset/198065.
<rdar://problem/25152196>
Created attachment 274029 [details] Patch
Comment on attachment 274029 [details] Patch r=me!
Comment on attachment 274029 [details] Patch Clearing flags on attachment: 274029 Committed r198164: <http://trac.webkit.org/changeset/198164>
All reviewed patches have been landed. Closing bug.