3 Technically, the child’s position is not
  part of its RenderBox geometry and therefore need not actually be
  calculated during layout. Many render objects implicitly position
  their single child at 0,0 relative to their own origin, which
  requires no computation or storage at all. Some render objects
  avoid computing the position of their children until the last
  possible moment (for example, during the paint phase), to avoid
  the computation entirely if they are not subsequently painted.

4  There exists one exception to this rule.
  As discussed in the Building widgets on demand
  section, some widgets can be rebuilt as a result of a change in layout
  constraints. If a widget marked itself dirty for unrelated reasons in
  the same frame that it also is affected by a change in layout constraints,
  it will be updated twice. This redundant build is limited to the
  widget itself and does not impact its descendants.

5 A key is an opaque object optionally
  associated with a widget whose equality operator is used to influence
  the reconciliation algorithm.