fix: give inbox threads identity distinct from their grouping key

InboxThreadSummary.id returned threadGroupingKey, which is listRID plus the
subject with Re:/Fwd: stripped. Two unrelated threads on one list sharing a
subject therefore shared an id — common on sourcehut, where "[PATCH] test" is
an ordinary subject — which collides under Identifiable in every list that
renders these summaries.

Key id on the root Message-ID, which is unique per thread, and leave
threadGroupingKey subject-based so replies still collapse into one
conversation.

Read state moves to threadGroupingKey at each call site. It was already
keyed on that string via id, so persisted keys are unchanged and marking a
conversation read still covers the whole subject group, matching how
HomeViewModel already builds the key for isUnread.

5 files changed, 16 insertions, 13 deletions

diff --git a/Hutch/App/RootView.swift b/Hutch/App/RootView.swift
index e223766..1ae4651 100644
--- a/Hutch/App/RootView.swift
+++ b/Hutch/App/RootView.swift
@@ -476,15 +476,15 @@ private struct MoreNavigationRoot: View {
                     ThreadDetailView(
                         thread: thread,
                         onViewed: {
-                            InboxReadStateStore.markViewed(max(Date(), thread.lastActivityAt), for: thread.id, defaults: appState.accountDefaults)
+                            InboxReadStateStore.markViewed(max(Date(), thread.lastActivityAt), for: thread.threadGroupingKey, defaults: appState.accountDefaults)
                             NeedsAttentionSnapshotStore.adjustUnreadInboxThreads(by: -1, accountID: appState.activeAccountID)
                         },
                         onMarkRead: {
-                            InboxReadStateStore.markViewed(max(Date(), thread.lastActivityAt), for: thread.id, defaults: appState.accountDefaults)
+                            InboxReadStateStore.markViewed(max(Date(), thread.lastActivityAt), for: thread.threadGroupingKey, defaults: appState.accountDefaults)
                             NeedsAttentionSnapshotStore.adjustUnreadInboxThreads(by: -1, accountID: appState.activeAccountID)
                         },
                         onMarkUnread: {
-                            InboxReadStateStore.markUnread(for: thread.id, defaults: appState.accountDefaults)
+                            InboxReadStateStore.markUnread(for: thread.threadGroupingKey, defaults: appState.accountDefaults)
                             NeedsAttentionSnapshotStore.adjustUnreadInboxThreads(by: 1, accountID: appState.activeAccountID)
                         }
                     )
diff --git a/Hutch/Models/Inbox.swift b/Hutch/Models/Inbox.swift
index 071bcbc..be2e105 100644
--- a/Hutch/Models/Inbox.swift
+++ b/Hutch/Models/Inbox.swift
@@ -17,8 +17,11 @@ struct InboxThreadSummary: Identifiable, Hashable, Sendable {
     let containsPatch: Bool
     let isUnread: Bool
 
+    /// Identity is per-thread, keyed on the root Message-ID. It deliberately differs
+    /// from ``threadGroupingKey``, which is subject-based so replies collapse into
+    /// one conversation — two unrelated threads can share a subject on the same list.
     var id: String {
-        threadGroupingKey
+        "\(listRID)#\(rootMessageID)"
     }
 
     var listDisplayName: String {
diff --git a/Hutch/Views/Home/HomeViewModel.swift b/Hutch/Views/Home/HomeViewModel.swift
index 2cd4869..4436f0c 100644
--- a/Hutch/Views/Home/HomeViewModel.swift
+++ b/Hutch/Views/Home/HomeViewModel.swift
@@ -622,7 +622,7 @@ final class HomeViewModel {
     }
 
     func markInboxThreadRead(_ thread: InboxThreadSummary) {
-        InboxReadStateStore.markViewed(max(Date(), thread.lastActivityAt), for: thread.id, defaults: defaults)
+        InboxReadStateStore.markViewed(max(Date(), thread.lastActivityAt), for: thread.threadGroupingKey, defaults: defaults)
         unreadInboxThreads.removeAll { $0.id == thread.id }
         unreadInboxThreadCount = max((unreadInboxThreadCount ?? 1) - 1, 0)
         hasUnreadInboxThreads = (unreadInboxThreadCount ?? 0) > 0
@@ -634,7 +634,7 @@ final class HomeViewModel {
 
         let viewedAt = Date()
         for thread in unreadInboxThreads {
-            InboxReadStateStore.markViewed(max(viewedAt, thread.lastActivityAt), for: thread.id, defaults: defaults)
+            InboxReadStateStore.markViewed(max(viewedAt, thread.lastActivityAt), for: thread.threadGroupingKey, defaults: defaults)
         }
 
         unreadInboxThreads = []
@@ -644,7 +644,7 @@ final class HomeViewModel {
     }
 
     func markInboxThreadUnread(_ thread: InboxThreadSummary) {
-        InboxReadStateStore.markUnread(for: thread.id, defaults: defaults)
+        InboxReadStateStore.markUnread(for: thread.threadGroupingKey, defaults: defaults)
         if unreadInboxThreads.contains(where: { $0.id == thread.id }) == false {
             unreadInboxThreads.append(
                 InboxThreadSummary(
diff --git a/Hutch/Views/Lookup/LookupView.swift b/Hutch/Views/Lookup/LookupView.swift
index 6f7ebe1..2a26282 100644
--- a/Hutch/Views/Lookup/LookupView.swift
+++ b/Hutch/Views/Lookup/LookupView.swift
@@ -469,15 +469,15 @@ struct LookupView: View {
                     ThreadDetailView(
                         thread: thread,
                         onViewed: {
-                            InboxReadStateStore.markViewed(max(Date(), thread.lastActivityAt), for: thread.id, defaults: appState.accountDefaults)
+                            InboxReadStateStore.markViewed(max(Date(), thread.lastActivityAt), for: thread.threadGroupingKey, defaults: appState.accountDefaults)
                             NeedsAttentionSnapshotStore.adjustUnreadInboxThreads(by: -1, accountID: appState.activeAccountID)
                         },
                         onMarkRead: {
-                            InboxReadStateStore.markViewed(max(Date(), thread.lastActivityAt), for: thread.id, defaults: appState.accountDefaults)
+                            InboxReadStateStore.markViewed(max(Date(), thread.lastActivityAt), for: thread.threadGroupingKey, defaults: appState.accountDefaults)
                             NeedsAttentionSnapshotStore.adjustUnreadInboxThreads(by: -1, accountID: appState.activeAccountID)
                         },
                         onMarkUnread: {
-                            InboxReadStateStore.markUnread(for: thread.id, defaults: appState.accountDefaults)
+                            InboxReadStateStore.markUnread(for: thread.threadGroupingKey, defaults: appState.accountDefaults)
                             NeedsAttentionSnapshotStore.adjustUnreadInboxThreads(by: 1, accountID: appState.activeAccountID)
                         }
                     )
diff --git a/Hutch/Views/Projects/ProjectMailingListView.swift b/Hutch/Views/Projects/ProjectMailingListView.swift
index ca6066c..d466121 100644
--- a/Hutch/Views/Projects/ProjectMailingListView.swift
+++ b/Hutch/Views/Projects/ProjectMailingListView.swift
@@ -94,13 +94,13 @@ final class MailingListDetailViewModel {
 
     func markThreadRead(_ thread: InboxThreadSummary) {
         let viewedAt = max(Date(), thread.lastActivityAt)
-        InboxReadStateStore.markViewed(viewedAt, for: thread.id, defaults: defaults)
+        InboxReadStateStore.markViewed(viewedAt, for: thread.threadGroupingKey, defaults: defaults)
         updateThread(thread, isUnread: false)
         NeedsAttentionSnapshotStore.adjustUnreadInboxThreads(by: -1, accountID: accountID)
     }
 
     func markThreadUnread(_ thread: InboxThreadSummary) {
-        InboxReadStateStore.markUnread(for: thread.id, defaults: defaults)
+        InboxReadStateStore.markUnread(for: thread.threadGroupingKey, defaults: defaults)
         updateThread(thread, isUnread: true)
         NeedsAttentionSnapshotStore.adjustUnreadInboxThreads(by: 1, accountID: accountID)
     }
@@ -111,7 +111,7 @@ final class MailingListDetailViewModel {
 
         let viewedAt = Date()
         for thread in unreadThreads {
-            InboxReadStateStore.markViewed(max(viewedAt, thread.lastActivityAt), for: thread.id, defaults: defaults)
+            InboxReadStateStore.markViewed(max(viewedAt, thread.lastActivityAt), for: thread.threadGroupingKey, defaults: defaults)
         }
 
         threads = threads.map { thread in