PurchaseScreen: Allow it to be scrolled

Maybe for really small screens?

lib/screens/purchase_screen.dart

@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:math';
 
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
@@ -101,9 +102,8 @@ class _PurchaseScreenState extends State<PurchaseScreen> {
       mainAxisAlignment: MainAxisAlignment.spaceAround,
     );
 
-    w = Container(
+    w = _SingleChildScrollViewExpanded(
       child: SafeArea(child: w),
-      color: theme.scaffoldBackgroundColor,
       padding: const EdgeInsets.all(16.0),
     );
 
@@ -237,3 +237,26 @@ class EmptyAppBar extends StatelessWidget implements PreferredSizeWidget {
   @override
   Size get preferredSize => const Size(0.0, 0.0);
 }
+
+class _SingleChildScrollViewExpanded extends StatelessWidget {
+  final Widget child;
+  final EdgeInsetsGeometry padding;
+
+  _SingleChildScrollViewExpanded({this.child, this.padding});
+
+  @override
+  Widget build(BuildContext context) {
+    return LayoutBuilder(
+      builder: (BuildContext context, BoxConstraints constraints) {
+        return SingleChildScrollView(
+          child: ConstrainedBox(
+            constraints: BoxConstraints.tightFor(
+                height: max(100, constraints.maxHeight)),
+            child: child,
+          ),
+          padding: padding,
+        );
+      },
+    );
+  }
+}