Timeline date headers

ts/models/conversations.ts

@@ -213,6 +213,9 @@ export class ConversationModel extends window.Backbone
 
   private isInReduxBatch = false;
 
+  // This number is recorded as an optimization and may be out of date.
+  private newestReceivedAtMarkedRead?: number;
+
   override defaults(): Partial<ConversationAttributesType> {
     return {
       unreadCount: 0,
@@ -4572,6 +4575,15 @@ export class ConversationModel extends window.Backbone
       sendReadReceipts: true,
     }
   ): Promise<void> {
+    // This early return is an optimization, not a guarantee.
+    const { newestReceivedAtMarkedRead } = this;
+    if (
+      typeof newestReceivedAtMarkedRead === 'number' &&
+      newestUnreadAt <= newestReceivedAtMarkedRead
+    ) {
+      return;
+    }
+
     await markConversationRead(this.attributes, newestUnreadAt, options);
 
     const unreadCount = await window.Signal.Data.getTotalUnreadForConversation(
@@ -4583,6 +4595,8 @@ export class ConversationModel extends window.Backbone
       this.set({ unreadCount });
       window.Signal.Data.updateConversation(this.attributes);
     }
+
+    this.newestReceivedAtMarkedRead = newestUnreadAt;
   }
 
   // This is an expensive operation we use to populate the message request hero row. It