SlidingUpPanel animates its own height between minHeight and maxHeight
(full screen) while opening/closing. Since it sits as a plain child in
the Scaffold's footers column, that transient height briefly exceeds
the fixed collapsed slot it was given, overflowing the footers column
by ~18-24px during the animation.
Wrap it in a fixed-size SizedBox + OverflowBox so the panel can still
paint at its actual (larger) size while reporting only the collapsed
height to its parent, avoiding the layout overflow.
