FIX: improves unread state precision (#20615)

- Will consider a message read only one the bottom of the message has been read
- Will allow to mark a message bigger than the view port as read
- Code should be more performant as the scroll is doing less (albeit more often)
- Gives us a very precise scroll state. Problem with throttling scroll is that you could end up never getting the even where scrollTop is at 0, opening a whole range of edge cases to handle

plugins/chat/spec/system/update_last_read.rb

@@ -0,0 +1,79 @@
+# frozen_string_literal: true
+
+RSpec.describe "Update last read", type: :system, js: true do
+  fab!(:current_user) { Fabricate(:user) }
+  fab!(:channel_1) { Fabricate(:chat_channel) }
+  fab!(:first_unread) { Fabricate(:chat_message, chat_channel: channel_1) }
+
+  let(:chat_page) { PageObjects::Pages::Chat.new }
+  let(:channel_page) { PageObjects::Pages::ChatChannel.new }
+  let(:membership) { Chat::ChatChannelMembershipManager.new(channel_1).find_for_user(current_user) }
+
+  before do
+    chat_system_bootstrap
+    channel_1.add(current_user)
+    membership.update!(last_read_message_id: first_unread.id)
+    25.times { |i| Fabricate(:chat_message, chat_channel: channel_1) }
+    sign_in(current_user)
+  end
+
+  context "when the full message is not visible" do
+    it "doesn’t mark it as read" do
+      before_last_message = Fabricate(:chat_message, chat_channel: channel_1)
+      last_message = Fabricate(:chat_message, chat_channel: channel_1)
+      chat_page.visit_channel(channel_1)
+
+      page.execute_script("document.querySelector('.chat-messages-scroll').scrollTo(0, -5)")
+
+      try_until_success(timeout: 5) do
+        expect(membership.reload.last_read_message_id).to eq(before_last_message.id)
+      end
+    end
+  end
+
+  context "when the full message is visible" do
+    it "marks it as read" do
+      last_message = Fabricate(:chat_message, chat_channel: channel_1)
+      chat_page.visit_channel(channel_1)
+
+      page.execute_script("document.querySelector('.chat-messages-scroll').scrollTo(0, 0)")
+
+      try_until_success(timeout: 5) do
+        expect(membership.reload.last_read_message_id).to eq(last_message.id)
+      end
+    end
+  end
+
+  context "when user had not previous last read" do
+    before { membership.update!(last_read_message_id: nil) }
+
+    it "marks last message as read" do
+      last_message = Fabricate(:chat_message, chat_channel: channel_1)
+      chat_page.visit_channel(channel_1)
+
+      try_until_success(timeout: 5) do
+        expect(membership.reload.last_read_message_id).to eq(last_message.id)
+      end
+    end
+  end
+
+  context "when scrolling from not visible to bottom" do
+    it "marks last message as read" do
+      before_last_message = Fabricate(:chat_message, chat_channel: channel_1)
+      last_message = Fabricate(:chat_message, chat_channel: channel_1)
+      chat_page.visit_channel(channel_1)
+
+      page.execute_script("document.querySelector('.chat-messages-scroll').scrollTo(0, -15)")
+
+      try_until_success(timeout: 5) do
+        expect(membership.reload.last_read_message_id).to eq(before_last_message.id)
+      end
+
+      page.execute_script("document.querySelector('.chat-messages-scroll').scrollTo(0, -1)")
+
+      try_until_success(timeout: 5) do
+        expect(membership.reload.last_read_message_id).to eq(last_message.id)
+      end
+    end
+  end
+end