inbox: Fix bug where channel unread badge text color was slightly wrong

UnreadCountBadge has a needlessly loose API for specifying a
background color, with three options:

1. Channel-colorized: callers are supposed to pass a
   ChannelColorSwatch and the implementation picks
   `unreadCountBadgeBackground` off of that.
2. Any other Color. (ChannelColorSwatch is a Color.)
3. A default color, chosen by the implementation, if `null` is
   passed.

We don't actually have any use for option 2, at least currently, so
we'll remove it next.

The buggy caller here was using option 2 when it should have used
option 1: it wanted a channel-colored background, but it itself
picked `unreadCountBadgeBackground` off of the swatch instead of
passing the whole swatch to UnreadCountBadge for it to do that. That
would have been fine except that the channel-colorized case has its
own associated *text* color, and that's conditioned on whether the
passed background color is a ChannelColorSwatch.

Author: chrisbobbe

lib/widgets/inbox.dart

@@ -463,7 +463,7 @@ class _StreamHeaderItem extends _HeaderItem with _LongPressable {
   @override Color uncollapsedBackgroundColor(context) =>
     colorSwatchFor(context, subscription).barBackground;
   @override Color? unreadCountBadgeBackgroundColor(context) =>
-    colorSwatchFor(context, subscription).unreadCountBadgeBackground;
+    colorSwatchFor(context, subscription);
 
   @override Future<void> onCollapseButtonTap() async {
     await super.onCollapseButtonTap();

test/widgets/inbox_test.dart

@@ -9,6 +9,7 @@ import 'package:zulip/widgets/color.dart';
 import 'package:zulip/widgets/home.dart';
 import 'package:zulip/widgets/icons.dart';
 import 'package:zulip/widgets/channel_colors.dart';
+import 'package:zulip/widgets/theme.dart';
 import 'package:zulip/widgets/unread_count_badge.dart';
 
 import '../example_data.dart' as eg;
@@ -206,6 +207,28 @@ void main() {
       await setupVarious(tester);
     });
 
+    testWidgets('UnreadCountBadge text color for a channel', (tester) async {
+      // Regression test for a bug where
+      // DesignVariables.labelCounterUnread was used for the text instead of
+      // DesignVariables.unreadCountBadgeTextForChannel.
+      final channel = eg.stream();
+      final subscription  = eg.subscription(channel);
+      await setupPage(tester,
+        streams: [channel],
+        subscriptions: [subscription],
+        unreadMessages: generateStreamMessages(stream: channel, count: 1, flags: []));
+
+      final text = tester.widget<Text>(
+        find.descendant(
+          of: find.byWidget(findRowByLabel(tester, channel.name)!),
+          matching: find.descendant(
+            of: find.byType(UnreadCountBadge),
+            matching: find.text('1'))));
+
+      final expectedTextColor = DesignVariables.light.unreadCountBadgeTextForChannel;
+      check(text).style.isNotNull().color.isNotNull().isSameColorAs(expectedTextColor);
+    });
+
     // TODO test that tapping a conversation row opens the message list
     //   for the conversation