scroll [nfc]: Cut CustomPaintOrderScrollView now that upstream PR merged

This functionality is now covered by [ScrollView.paintOrder],
after my PR adding that feature upstream.

Author: gnprice

lib/widgets/message_list.dart

@@ -21,7 +21,7 @@ import 'emoji_reaction.dart';
 import 'icons.dart';
 import 'page.dart';
 import 'profile.dart';
-import 'scrolling.dart' hide SliverPaintOrder;
+import 'scrolling.dart';
 import 'sticky_header.dart';
 import 'store.dart';
 import 'text.dart';

lib/widgets/scrolling.dart

@@ -43,212 +43,6 @@ class _SingleChildScrollViewWithScrollbarState
   }
 }
 
-/// Specifies an order in which to paint the slivers of a [CustomScrollView].
-///
-/// Whichever order the slivers are painted in,
-/// they will be hit-tested in the opposite order.
-///
-/// This can also be thought of as an ordering in the z-direction:
-/// whichever sliver is painted last (and hit-tested first) is on top,
-/// because it will paint over other slivers if there is overlap.
-/// Similarly, whichever sliver is painted first (and hit-tested last)
-/// is on the bottom.
-enum SliverPaintOrder {
-  /// The first sliver paints on top, and the last sliver on bottom.
-  ///
-  /// The slivers are painted in the reverse order of [CustomScrollView.slivers],
-  /// and hit-tested in the same order as [CustomScrollView.slivers].
-  firstIsTop,
-
-  /// The last sliver paints on top, and the first sliver on bottom.
-  ///
-  /// The slivers are painted in the same order as [CustomScrollView.slivers],
-  /// and hit-tested in the reverse order.
-  lastIsTop,
-
-  /// The default order for [CustomScrollView]: the center sliver paints on top,
-  /// and the first sliver paints on bottom.
-  ///
-  /// If [CustomScrollView.center] is null or corresponds to the first sliver
-  /// in [CustomScrollView.slivers], this order is equivalent to [firstIsTop].
-  /// Otherwise, the [CustomScrollView.center] sliver paints on top;
-  /// it's followed in the z-order by the slivers after it to the end
-  /// of the list, then the slivers before the center in reverse order,
-  /// with the first sliver in the list at the bottom in the z-direction.
-  centerTopFirstBottom,
-}
-
-/// A [CustomScrollView] with control over the paint order, or z-order,
-/// between slivers.
-///
-/// This is just like [CustomScrollView] except it adds the [paintOrder_] field.
-///
-/// (Actually there's one [CustomScrollView] feature this doesn't implement:
-/// [shrinkWrap] always has its default value of false.  That feature would be
-/// easy to add if desired.)
-// TODO(upstream): Pending PR: https://github.com/flutter/flutter/pull/164818
-//   Notes from before sending that PR:
-//   Add an option [ScrollView.zOrder]?  (An enum, or possibly
-//   a delegate.)  Or at minimum document on [ScrollView.center] the
-//   existing behavior, which is counterintuitive.
-//   Nearest related upstream feature requests I find are for a "z-index",
-//   for CustomScrollView, Column, Row, and Stack respectively:
-//     https://github.com/flutter/flutter/issues/121173#issuecomment-1712825747
-//     https://github.com/flutter/flutter/issues/121173
-//     https://github.com/flutter/flutter/issues/121173#issuecomment-1914959184
-//     https://github.com/flutter/flutter/issues/70836
-//   A delegate would give enough flexibility for that and much else,
-//   but I'm not sure how many use cases wouldn't be covered by a small enum.
-//
-//   Ah, and here's a more on-point issue (more recently):
-//     https://github.com/flutter/flutter/issues/145592
-//
-// TODO: perhaps sticky_header should configure a CustomPaintOrderScrollView automatically?
-class CustomPaintOrderScrollView extends CustomScrollView {
-  const CustomPaintOrderScrollView({
-    super.key,
-    super.scrollDirection,
-    super.reverse,
-    super.controller,
-    super.primary,
-    super.physics,
-    super.scrollBehavior,
-    // super.shrinkWrap, // omitted, always false
-    super.center,
-    super.anchor,
-    super.cacheExtent,
-    super.slivers,
-    super.semanticChildCount,
-    super.dragStartBehavior,
-    super.keyboardDismissBehavior,
-    super.restorationId,
-    super.clipBehavior,
-    super.hitTestBehavior,
-    SliverPaintOrder paintOrder = SliverPaintOrder.centerTopFirstBottom,
-  }) : paintOrder_ = paintOrder;
-
-  /// The order in which to paint the slivers;
-  /// equivalently, the order in which to arrange them in the z-direction.
-  ///
-  /// Whichever order the slivers are painted in,
-  /// they will be hit-tested in the opposite order.
-  ///
-  /// To think of this as an ordering in the z-direction:
-  /// whichever sliver is painted last (and hit-tested first) is on top,
-  /// because it will paint over other slivers if there is overlap.
-  /// Similarly, whichever sliver is painted first (and hit-tested last)
-  /// is on the bottom.
-  ///
-  /// This defaults to [SliverPaintOrder.centerTopFirstBottom],
-  /// the behavior of the [CustomScrollView] base class.
-  final SliverPaintOrder paintOrder_;
-
-  @override
-  Widget buildViewport(BuildContext context, ViewportOffset offset,
-      AxisDirection axisDirection, List<Widget> slivers) {
-    return CustomPaintOrderViewport(
-      axisDirection: axisDirection,
-      offset: offset,
-      slivers: slivers,
-      cacheExtent: cacheExtent,
-      center: center,
-      anchor: anchor,
-      clipBehavior: clipBehavior,
-      paintOrder_: paintOrder_,
-    );
-  }
-}
-
-/// The viewport configured by a [CustomPaintOrderScrollView].
-class CustomPaintOrderViewport extends Viewport {
-  CustomPaintOrderViewport({
-    super.key,
-    super.axisDirection,
-    super.crossAxisDirection,
-    super.anchor,
-    required super.offset,
-    super.center,
-    super.cacheExtent,
-    super.cacheExtentStyle,
-    super.slivers,
-    super.clipBehavior,
-    required this.paintOrder_,
-  });
-
-  final SliverPaintOrder paintOrder_;
-
-  @override
-  RenderViewport createRenderObject(BuildContext context) {
-    return RenderCustomPaintOrderViewport(
-      axisDirection: axisDirection,
-      crossAxisDirection: crossAxisDirection
-        ?? Viewport.getDefaultCrossAxisDirection(context, axisDirection),
-      anchor: anchor,
-      offset: offset,
-      cacheExtent: cacheExtent,
-      cacheExtentStyle: cacheExtentStyle,
-      clipBehavior: clipBehavior,
-      paintOrder_: paintOrder_,
-    );
-  }
-}
-
-/// The render object configured by a [CustomPaintOrderViewport].
-class RenderCustomPaintOrderViewport extends RenderViewport {
-  RenderCustomPaintOrderViewport({
-    super.axisDirection,
-    required super.crossAxisDirection,
-    required super.offset,
-    super.anchor,
-    super.children,
-    super.center,
-    super.cacheExtent,
-    super.cacheExtentStyle,
-    super.clipBehavior,
-    required this.paintOrder_,
-  });
-
-  final SliverPaintOrder paintOrder_;
-
-  Iterable<RenderSliver> get _lastToFirst {
-    final List<RenderSliver> children = <RenderSliver>[];
-    RenderSliver? child = lastChild;
-    while (child != null) {
-      children.add(child);
-      child = childBefore(child);
-    }
-    return children;
-  }
-
-  Iterable<RenderSliver> get _firstToLast {
-    final List<RenderSliver> children = <RenderSliver>[];
-    RenderSliver? child = firstChild;
-    while (child != null) {
-      children.add(child);
-      child = childAfter(child);
-    }
-    return children;
-  }
-
-  @override
-  Iterable<RenderSliver> get childrenInPaintOrder {
-    return switch (paintOrder_) {
-      SliverPaintOrder.firstIsTop => _lastToFirst,
-      SliverPaintOrder.lastIsTop => _firstToLast,
-      SliverPaintOrder.centerTopFirstBottom => super.childrenInPaintOrder,
-    };
-  }
-
-  @override
-  Iterable<RenderSliver> get childrenInHitTestOrder {
-    return switch (paintOrder_) {
-      SliverPaintOrder.firstIsTop => _firstToLast,
-      SliverPaintOrder.lastIsTop => _lastToFirst,
-      SliverPaintOrder.centerTopFirstBottom => super.childrenInHitTestOrder,
-    };
-  }
-}
-
 /// A version of [ScrollPosition] adapted for the Zulip message list,
 /// used by [MessageListScrollController].
 class MessageListScrollPosition extends ScrollPositionWithSingleContext {