maintainState property
If false, the child will be replaced with a SizedBox.shrink()
when the Collapsible is in its collapsed state, if true, the
child will remain built when collapsed.
maintainState will be set to true by default if minWidthFactor
or minHeightFactor is > 0.0 and axis affects the horizontal or
vertical axes respectively, as the child will still occupy space
when in the collapsed state in those cases.
Implementation
final bool maintainState;