Interleaving build and layout is possible only because of the strict
controls on information propagation in the build and layout algorithms.
Specifically, during the build phase, information can propagate only
down the tree. When a render object is performing layout, the layout
walk has not visited the subtree below that render object, which means
writes generated by building in that subtree cannot invalidate any
information that has entered the layout calculation thus far. Similarly,
once layout has returned from a render object, that render object will
never be visited again during this layout, which means any writes
generated by subsequent layout calculations cannot invalidate the
information used to build the render object’s subtree.

Additionally, linear reconciliation and tree surgery are essential
for efficiently updating elements during scrolling and for modifying
the render tree when elements are scrolled into and out of view at
the edge of the viewport.

API Ergonomics