title: How to know if a widget is visible within a viewport?
body: <p>I have a view that consists of a <code>Scaffold</code> and a single <code>ListView</code> in its body, each children of the list is a different widget that represents various "sections" of the view (sections range from simple TextViews to arrangements of <code>Column</code>s and <code>Row</code>s), I want to show a <code>FloatingActionButon</code> only when the user scrolls over certain <code>Widgets</code> (which aren't initially visible due to being far down the list).</p>