56d4033423
This changes how SnackBar works so you can use it anywhere, not just on the bottom edge of the screen (it used to rely on overflowing its bounds and having negative offsets... I'm not really sure how hit testing worked on it before!). To do this I introduced a new RenderBox, RenderOverflowBox, that lets you set your child's size independent of your own. I needed this so that the snack bar could use a SquashTransition to change its size, while not affecting the layout of its child. This is exposed as OverflowBox in fn3. I'm not sure if it's the best API. It doesn't let you position the child (which is an issue if the size you give is smaller), it doesn't let you give a loose constraint (which maybe you might want?). But it handles this use case, so for now it's probably ok. Making the FAB get repositioned out of the way of the Snack Bar is now done in the Scaffold, which is in charge of positioning both of those and is the place that knows that both exist.