sticky_header [nfc]: Add asserts from studying possible child geometry; note a bug

The logic below -- particularly in the allowOverflow true case,
where it constructs a new SliverGeometry from scratch -- has been
implicitly relying on several of these facts already.  These
wouldn't be true of an arbitrary sliver child; but this sliver knows
what type of child it actually has, and they are true of that one.
So write down the specific assumptions we can take from that.

Reading through [RenderSliverList.performLayout] to see what it can
produce as the child geometry also made clear there's another case
that this method isn't currently correctly handling at all: the case
where scrollOffsetCorrection is non-null.  Filed an issue for that;
add a todo-comment for it.

Author: gnprice

lib/widgets/sticky_header.dart

@@ -556,6 +556,19 @@ class _RenderSliverStickyHeaderList extends RenderSliver with RenderSliverHelper
     child!.layout(constraints, parentUsesSize: true);
     SliverGeometry geometry = child!.geometry!;
 
+    // TODO(#1309) handle the scrollOffsetCorrection case, passing it through
+
+    // We assume [child]'s geometry is free of certain complications.
+    // Probably most or all of these *could* be handled if necessary, just at
+    // the cost of further complicating this code.  Fortunately they aren't,
+    // because [RenderSliverList.performLayout] never has these complications.
+    assert(geometry.paintOrigin == 0);
+    assert(geometry.layoutExtent == geometry.paintExtent);
+    assert(geometry.hitTestExtent == geometry.paintExtent);
+    assert(geometry.visible == (geometry.paintExtent > 0));
+    assert(geometry.maxScrollObstructionExtent == 0);
+    assert(geometry.crossAxisExtent == null);
+
     if (header != null) {
       header!.layout(constraints.asBoxConstraints(), parentUsesSize: true);
       final headerExtent = header!.size.onAxis(constraints.axis);