Within computerPerCharacterLayoutInformation, first layoutChildBoxes is called. This function calls calculateBoundaries on all child boxes. Directly afterwards, layoutRootBox determines the rect for itself based on its children, but instead of using the values just calculated in the previous step, they're re-calculated. There is no early return in calculateBoundaries.
Created attachment 94181 [details]
Comment on attachment 94181 [details]
Good catch, r=me, assuming no regressions.
Committed r86927: <http://trac.webkit.org/changeset/86927>