mark grouped notifs by oldest id - sort-of fix 1139

Misskey's code does the same, but our groups behave differently enough
that this may be not the best choice

for example, let's say we have:

- notifications 1-5 for reaction to note A
- notifications 6-8 for reaction to note B
- notifications 9-12 for reaction to note A
- notification 13-19 for non-groupable events
- notification 20 for reaction to note A

and that events happened one every minute (so the last notification is
from 20 minutes ago)

client requests the most recent 10 notifications; we fetch
notifications 1-10, and reply:

- grouped id 6 for reactions 6-8 to note B
- grouped id 10 for reactions 1-5, 9-10 to note A

then the client requests 10 more notifications, untilId=10; we fetch
notifications 11-20, and reply:

- non-grouped notifications 13-19
- grouped id 20 for reactions 11,12,20 to note A

because we sort by id, and also the `createdAt` marks the _newest_
event in each group, the client will then show:

  6 reactions to note B, 6 minutes ago
  4 reactions to note A, 1 minute ago
  notifications 13-19, 13 minutes to 19 minutes ago
  3 reactions to note A, 11 minutes ago

I don't know how to make this work better ☹

1 files changed, 8 insertions, 4 deletions

diff --git a/packages/backend/src/server/api/endpoints/i/notifications-grouped.ts b/packages/backend/src/server/api/endpoints/i/notifications-grouped.ts
index 444734070f..3821b5a20e 100644
--- a/packages/backend/src/server/api/endpoints/i/notifications-grouped.ts
+++ b/packages/backend/src/server/api/endpoints/i/notifications-grouped.ts
@@ -116,7 +116,7 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 			// user has many notifications, the pagination will break the
 			// groups
 
-			// scan `notifications` newest-to-oldest
+			// scan `notifications` newest-to-oldest (unless we have sinceId && !untilId, in which case it's oldest-to-newest)
 			for (let i = 0; i < notifications.length; i++) {
 				const notification = notifications[i];
 
@@ -135,7 +135,7 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 					if (prevReaction.type !== 'reaction:grouped') {
 						prevReaction = groupedNotifications[reactionIdx] = {
 							type: 'reaction:grouped',
-							id: prevReaction.id, // this will be the newest id in this group
+							id: '',
 							createdAt: prevReaction.createdAt,
 							noteId: prevReaction.noteId!,
 							reactions: [{
@@ -149,6 +149,7 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 						userId: notification.notifierId!,
 						reaction: notification.reaction!,
 					});
+					prevReaction.id = notification.id; // this will be the *oldest* id in this group (newest if sinceId && !untilId)
 					continue;
 				}
 
@@ -167,7 +168,7 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 					if (prevRenote.type !== 'renote:grouped') {
 						prevRenote = groupedNotifications[renoteIdx] = {
 							type: 'renote:grouped',
-							id: prevRenote.id, // this will be the newest id in this group
+							id: '',
 							createdAt: prevRenote.createdAt,
 							noteId: prevRenote.noteId!,
 							userIds: [prevRenote.notifierId!],
@@ -175,6 +176,7 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 					}
 					// add this new renote to the existing group
 					(prevRenote as FilterUnionByProperty<MiGroupedNotification, 'type', 'renote:grouped'>).userIds.push(notification.notifierId!);
+					prevRenote.id = notification.id; // this will be the *oldest* id in this group (newest if sinceId && !untilId)
 					continue;
 				}
 
@@ -182,10 +184,12 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 				groupedNotifications.push(notification);
 			}
 
-			// sort the groups by their id, newest first
+			// sort the groups by their id
 			groupedNotifications.sort(
 				(a, b) => a.id < b.id ? 1 : a.id > b.id ? -1 : 0,
 			);
+			// this matches the logic in NotificationService and it's what MkPagination expects
+			if (ps.sinceId && !ps.untilId) groupedNotifications.reverse();
 
 			return await this.notificationEntityService.packGroupedMany(groupedNotifications, me.id);
 		});