scroll [nfc]: Give SingleChildScrollViewWithScrollbar docs and a new home

Author: gnprice

lib/widgets/content.dart

@@ -22,6 +22,7 @@ import 'inset_shadow.dart';
 import 'lightbox.dart';
 import 'message_list.dart';
 import 'poll.dart';
+import 'scrolling.dart';
 import 'store.dart';
 import 'text.dart';
 
@@ -796,33 +797,6 @@ class _CodeBlockContainer extends StatelessWidget {
   }
 }
 
-class SingleChildScrollViewWithScrollbar extends StatefulWidget {
-  const SingleChildScrollViewWithScrollbar(
-    {super.key, required this.scrollDirection, required this.child});
-
-  final Axis scrollDirection;
-  final Widget child;
-
-  @override
-  State<SingleChildScrollViewWithScrollbar> createState() =>
-    _SingleChildScrollViewWithScrollbarState();
-}
-
-class _SingleChildScrollViewWithScrollbarState
-    extends State<SingleChildScrollViewWithScrollbar> {
-  final ScrollController controller = ScrollController();
-
-  @override
-  Widget build(BuildContext context) {
-    return Scrollbar(
-      controller: controller,
-      child: SingleChildScrollView(
-        controller: controller,
-        scrollDirection: widget.scrollDirection,
-        child: widget.child));
-  }
-}
-
 class MathBlock extends StatelessWidget {
   const MathBlock({super.key, required this.node});
 

lib/widgets/scrolling.dart

@@ -0,0 +1,39 @@
+import 'package:flutter/material.dart';
+
+/// A [SingleChildScrollView] that always shows a Material [Scrollbar].
+///
+/// This differs from the behavior provided by [MaterialScrollBehavior] in that
+/// (a) the scrollbar appears even when [scrollDirection] is [Axis.horizontal],
+/// and (b) the scrollbar appears on all platforms, rather than only on
+/// desktop platforms.
+// TODO(upstream): SingleChildScrollView should have a scrollBehavior field
+//   and pass it on to Scrollable, just like ScrollView does; then this would
+//   be covered by using that.
+// TODO: Maybe show scrollbar only on mobile platforms, like MaterialScrollBehavior
+//   and the base ScrollBehavior do?
+class SingleChildScrollViewWithScrollbar extends StatefulWidget {
+  const SingleChildScrollViewWithScrollbar(
+    {super.key, required this.scrollDirection, required this.child});
+
+  final Axis scrollDirection;
+  final Widget child;
+
+  @override
+  State<SingleChildScrollViewWithScrollbar> createState() =>
+    _SingleChildScrollViewWithScrollbarState();
+}
+
+class _SingleChildScrollViewWithScrollbarState
+    extends State<SingleChildScrollViewWithScrollbar> {
+  final ScrollController controller = ScrollController();
+
+  @override
+  Widget build(BuildContext context) {
+    return Scrollbar(
+      controller: controller,
+      child: SingleChildScrollView(
+        controller: controller,
+        scrollDirection: widget.scrollDirection,
+        child: widget.child));
+  }
+}