Fix when unfollow a tag, my post also disappears from the home timeline (#27391)

spec/lib/feed_manager_spec.rb

@@ -525,6 +525,44 @@ RSpec.describe FeedManager do
     end
   end
 
+  describe '#unmerge_tag_from_home' do
+    let(:receiver) { Fabricate(:account) }
+    let(:tag) { Fabricate(:tag) }
+
+    it 'leaves a tagged status' do
+      status = Fabricate(:status)
+      status.tags << tag
+      described_class.instance.push_to_home(receiver, status)
+
+      described_class.instance.unmerge_tag_from_home(tag, receiver)
+
+      expect(redis.zrange("feed:home:#{receiver.id}", 0, -1)).to_not include(status.id.to_s)
+    end
+
+    it 'remains a tagged status written by receiver\'s followee' do
+      followee = Fabricate(:account)
+      receiver.follow!(followee)
+
+      status = Fabricate(:status, account: followee)
+      status.tags << tag
+      described_class.instance.push_to_home(receiver, status)
+
+      described_class.instance.unmerge_tag_from_home(tag, receiver)
+
+      expect(redis.zrange("feed:home:#{receiver.id}", 0, -1)).to include(status.id.to_s)
+    end
+
+    it 'remains a tagged status written by receiver' do
+      status = Fabricate(:status, account: receiver)
+      status.tags << tag
+      described_class.instance.push_to_home(receiver, status)
+
+      described_class.instance.unmerge_tag_from_home(tag, receiver)
+
+      expect(redis.zrange("feed:home:#{receiver.id}", 0, -1)).to include(status.id.to_s)
+    end
+  end
+
   describe '#clear_from_home' do
     let(:account) { Fabricate(:account) }
     let(:followed_account) { Fabricate(:account) }