wip sticky_header let paintOrigin go negative; TODO test, doc; NB req…

…uires right paint order

lib/widgets/sticky_header.dart

@@ -595,11 +595,18 @@ class _RenderSliverStickyHeaderList extends RenderSliver with RenderSliverHelper
         // even if the (visible part of the) item is smaller than the header,
         // and even if the whole child sliver is smaller than the header.
 
-        final paintedHeaderSize = calculatePaintOffset(constraints, from: 0, to: headerExtent);
+        final headerGrowthPlacement =
+          _widget.headerPlacement._byGrowth(constraints.growthDirection);
+
         geometry = SliverGeometry( // TODO review interaction with other slivers
           scrollExtent: geometry.scrollExtent,
           layoutExtent: childExtent,
-          paintExtent: math.max(childExtent, paintedHeaderSize),
+          paintExtent: math.max(childExtent, headerExtent),
+          paintOrigin: switch (headerGrowthPlacement) {
+            _HeaderGrowthPlacement.growthStart => 0,
+            _HeaderGrowthPlacement.growthEnd =>
+              math.min(0.0, childExtent - headerExtent),
+          },
           maxPaintExtent: math.max(geometry.maxPaintExtent, headerExtent),
           hasVisualOverflow: geometry.hasVisualOverflow
             || headerExtent > constraints.remainingPaintExtent,