feat: explain Following changes before review (Closes #1322)

tests/e2e/test_mobile_following_release.py

@@ -41,7 +41,8 @@ def test_following_queue_is_phone_usable_at_narrow_viewport(viewport):
             json:async () => ({revision:1,items:[{
               repository:'stackchain/api', kind:'pull', number:42,
               title:'Changed pull request with a long mobile title', state:'open',
-              updated_at:'2026-08-23T05:00:00Z', has_unseen_change:true
+              updated_at:'2026-08-23T05:00:00Z', has_unseen_change:true,
+              change_summary:'Closed since last review'
             }]})
           });
           globalThis.followingReleaseQueue = attachFollowing(() => {});
@@ -50,7 +51,7 @@ def test_following_queue_is_phone_usable_at_narrow_viewport(viewport):
 
         expect(page.locator("#following-sheet")).to_be_visible()
         expect(page.locator(".following-card")).to_be_visible()
-        expect(page.locator(".following-card")).to_contain_text("New activity")
+        expect(page.locator(".following-card")).to_contain_text("Closed since last review")
         expect(page.locator(".following-card")).to_contain_text("Pull request")
         expect(page.locator("#review-following")).to_have_text("Review new activity")
         assert page.locator("#review-following").bounding_box()["height"] >= 44

tests/test_following_frontend.py

@@ -113,6 +113,69 @@ def test_following_preview_wires_keep_for_later_action_and_retryable_failure():
     assert "keepForLater:feature.keepForLater" in following
 
 
+def test_following_conversation_marks_messages_newer_than_last_review():
+    preview_module = ROOT / "frontend" / "search-preview.js"
+    script = f"""
+require({json.dumps(str(preview_module))});
+const nodes={{
+  '#search-preview-comments':{{innerHTML:'',textContent:''}},
+  '#search-preview-conversation-status':{{textContent:''}},
+  '#retry-search-preview-conversation':{{hidden:true}},
+  '#load-older-search-preview-comments':{{hidden:true,disabled:false}},
+}};
+const document={{querySelector:selector=>nodes[selector]}};
+global.renderSearchPreviewConversation({{
+  status:'ready', reviewedAt:'2026-08-23T03:00:00Z', comments:[
+    {{id:1,author:'Timmy',created_at:'2026-08-23T02:00:00Z',body:'Earlier'}},
+    {{id:2,author:'Alex',created_at:'2026-08-23T04:00:00Z',body:'New decision'}},
+  ]
+}},document,String,String,String);
+process.stdout.write(JSON.stringify({{html:nodes['#search-preview-comments'].innerHTML,status:nodes['#search-preview-conversation-status'].textContent}}));
+"""
+
+    result = json.loads(subprocess.run(
+        ["node", "-e", script], text=True, capture_output=True, check=True
+    ).stdout)
+
+    assert result["html"].count("new-since-review") == 1
+    assert "New since last review" in result["html"]
+    assert result["status"] == "2 messages · 1 new since last review."
+
+
+def test_following_revision_is_acknowledged_only_after_conversation_context_loads():
+    preview_module = ROOT / "frontend" / "search-preview.js"
+    script = f"""
+const createSearchPreview=require({json.dumps(str(preview_module))});
+let unavailable=true;
+const opened=[];
+const preview=createSearchPreview({{
+  fetchJson:async item=>({{...item,title:'Changed issue'}}),
+  fetchConversation:async()=>{{if(unavailable)throw new Error('offline');return {{comments:[]}};}},
+  onOpened:async item=>opened.push(item.updated_at),
+  onState:()=>{{}},
+}});
+(async()=>{{
+  await preview.open({{
+    repository:'stackchain/api',kind:'issue',number:42,following:true,
+    reviewed_at:'2026-08-23T03:00:00Z',updated_at:'2026-08-23T04:00:00Z'
+  }});
+  const afterFailure=[...opened];
+  unavailable=false;
+  await preview.retryConversation();
+  process.stdout.write(JSON.stringify({{afterFailure,opened}}));
+}})().catch(error=>{{console.error(error);process.exit(1);}});
+"""
+
+    result = json.loads(subprocess.run(
+        ["node", "-e", script], text=True, capture_output=True, check=True
+    ).stdout)
+
+    assert result == {
+        "afterFailure": [],
+        "opened": ["2026-08-23T04:00:00Z"],
+    }
+
+
 def test_following_typed_identity_prevents_issue_pull_collisions():
     script = f"""
 const createFollowing = require({json.dumps(str(MODULE))});

tests/test_following_store.py

@@ -51,6 +51,34 @@ def test_refresh_marks_changed_items_unseen_and_sorts_them_first(tmp_path):
     assert snapshot["items"][0]["state"] == "closed"
 
 
+def test_refresh_explains_change_from_last_reviewed_snapshot(tmp_path):
+    store = FollowingStore(tmp_path / "following.sqlite3", encryption_key=KEY)
+    store.set_watching("timmy", ITEM, True)
+
+    closed = store.refresh("timmy", [{
+        **ITEM,
+        "title": "Make mobile review trustworthy",
+        "state": "closed",
+        "updated_at": "2026-08-23T04:00:00Z",
+    }])
+
+    assert closed["items"][0]["reviewed_at"] == ITEM["updated_at"]
+    assert closed["items"][0]["change_summary"] == "Closed since last review"
+
+    store.acknowledge(
+        "timmy", ITEM["repository"], ITEM["number"], "2026-08-23T04:00:00Z"
+    )
+    title_change = store.refresh("timmy", [{
+        **ITEM,
+        "title": "Make mobile review understandable",
+        "state": "closed",
+        "updated_at": "2026-08-23T05:00:00Z",
+    }])
+
+    assert title_change["items"][0]["reviewed_at"] == "2026-08-23T04:00:00Z"
+    assert title_change["items"][0]["change_summary"] == "Title changed since last review"
+
+
 def test_acknowledgement_is_revision_conditional_and_later_change_is_unseen(tmp_path):
     store = FollowingStore(tmp_path / "following.sqlite3", encryption_key=KEY)
     store.set_watching("timmy", ITEM, True)