Tight constraints are those that can be satisfied by exactly one
valid geometry. For example, if the min and max widths are equal to
each other and the min and max heights are equal to each other,
the only size that satisfies those constraints is one with that
width and height. If the parent provides tight constraints,
then the parent need not recompute its layout whenever the child
recomputes its layout, even if the parent uses the child’s size
in its layout, because the child cannot change size without new
constraints from its parent.

A render object can declare that it uses the constraints provided
by the parent only to determine its geometry. Such a declaration
informs the framework that the parent of that render object does
not need to recompute its layout when the child recomputes its layout
even if the constraints are not tight and even if the parent’s
layout depends on the child’s size, because the child cannot change
size without new constraints from its parent.