Inspector's timeline agent used to send only first layout invalidation to front-end, so we displayed first JS stack as the reason for re-layout. It's probably more useful to display the last one, as the scope of layout is expanded, hence the last one has a good chances to be the one with larger impact. Ultimately, we should probably aggregate these and find a way to visualize many stacks.
Created attachment 165793 [details] Patch
Committed r129645: <http://trac.webkit.org/changeset/129645>