Faster on interactive pages that measure and rebuild themselves as they load

A focused speed pass on the built-in engine for pages that read their own layout and then change themselves while loading - web apps, feeds, dashboards and editors. When such a page measures itself and then tweaks a part, the engine now redoes the layout for just the part that actually moved instead of the whole page, and skips layout entirely when a change only affects colour or other appearance. Pages that build their content in stages settle faster, and repeated measurements return consistent results so elements positioned relative to each other line up reliably.