Generalize post reply notifications (#6)

## Summary
- Notify the direct parent post author for replies throughout nested conversations
- Skip root creation, self-replies, edits, disabled mail, and recipients without email
- Link directly to each reply and use per-reply Resend idempotency

Co-authored-by: codegirl-007 <s.raide@gmail.com>

.env.example

@@ -18,7 +18,7 @@ SECURE_COOKIE=0
 # RESEND_API_KEY=re_xxxxxxxxx
 # RESEND_FROM=Ask a Plumber <notify@yourdomain.com>
 # Public site origin used in email links (required when Resend is enabled):
-# APP_BASE_URL=https://askaplumber.example
+# APP_BASE_URL=https://www.askaplumberfirst.com
 # DigitalOcean Spaces (profile avatars). Leave unset to disable uploads.
 # SPACES_KEY=
 # SPACES_SECRET=

internal/mail/mail.go

@@ -5,6 +5,7 @@ import (
 	_ "embed"
 	"fmt"
 	"html"
+	"net/url"
 	"os"
 	"strings"
 
@@ -14,24 +15,26 @@ import (
 //go:embed mark.png
 var markPNG []byte
 
-// QuestionAnswered is the payload for notifying a question author of a reply.
-type QuestionAnswered struct {
+// PostReply is the payload for notifying a post author of a direct reply.
+type PostReply struct {
 	ToEmail         string
 	ToName          string
-	QuestionID    string
-	QuestionTitle string
-	AnswerBody    string
+	RootID          string
+	RootTitle       string
+	ReplyID         string
+	ReplyBody       string
+	ReplyAuthorName string
 }
 
-// Notifier sends transactional email about answered questions.
+// Notifier sends transactional email about post replies.
 type Notifier interface {
-	NotifyQuestionAnswered(ctx context.Context, msg QuestionAnswered) error
+	NotifyPostReply(ctx context.Context, msg PostReply) error
 }
 
 // Nop is a no-op Notifier used when Resend is not configured.
 type Nop struct{}
 
-func (Nop) NotifyQuestionAnswered(context.Context, QuestionAnswered) error { return nil }
+func (Nop) NotifyPostReply(context.Context, PostReply) error { return nil }
 
 // Resend sends via the Resend HTTP API.
 type Resend struct {
@@ -62,7 +65,7 @@ func FromEnv() (Notifier, error) {
 	}, nil
 }
 
-func (r *Resend) NotifyQuestionAnswered(ctx context.Context, msg QuestionAnswered) error {
+func (r *Resend) NotifyPostReply(ctx context.Context, msg PostReply) error {
 	if r == nil || r.client == nil {
 		return nil
 	}
@@ -70,59 +73,72 @@ func (r *Resend) NotifyQuestionAnswered(ctx context.Context, msg QuestionAnswere
 	if to == "" {
 		return nil
 	}
-	text, htmlBody := questionAnsweredContent(r.baseURL, msg)
+	text, htmlBody := postReplyContent(r.baseURL, msg)
 	params := &resend.SendEmailRequest{
 		From:    r.from,
 		To:      []string{to},
-		Subject: "Your question was answered",
+		Subject: "New reply to your post",
 		Text:    text,
 		Html:    htmlBody,
 		Attachments: []*resend.Attachment{{
 			Content:     markPNG,
 			Filename:    "ask-a-plumber-first.png",
 			ContentType: "image/png",
-			ContentId:   "answer-notification-mark",
+			ContentId:   "reply-notification-mark",
 		}},
 	}
 	opts := &resend.SendEmailOptions{
-		IdempotencyKey: "answer-notify:" + msg.QuestionID,
+		IdempotencyKey: "post-reply:" + msg.ReplyID,
 	}
 	_, err := r.client.Emails.SendWithOptions(ctx, params, opts)
 	return err
 }
 
-func questionAnsweredContent(baseURL string, msg QuestionAnswered) (string, string) {
-	link := strings.TrimRight(baseURL, "/") + "/questions/" + msg.QuestionID
-	title := strings.TrimSpace(msg.QuestionTitle)
-	if title == "" {
-		title = "your question"
+func postReplyContent(baseURL string, msg PostReply) (string, string) {
+	link := strings.TrimRight(baseURL, "/") +
+		"/questions/" + url.PathEscape(msg.RootID) +
+		"#post-" + url.PathEscape(msg.ReplyID)
+	title := replyRootTitle(msg.RootTitle)
+	author := strings.TrimSpace(msg.ReplyAuthorName)
+	if author == "" {
+		author = "Someone"
 	}
 	text := fmt.Sprintf(
-		"Hi%s,\n\nYour question %q has an answer from a plumber:\n\n%s\n\nView it here:\n%s\n",
+		"Hi%s,\n\n%s replied in %q:\n\n%s\n\nView the reply:\n%s\n",
 		greetingName(msg.ToName),
+		author,
 		title,
-		msg.AnswerBody,
+		msg.ReplyBody,
 		link,
 	)
 	htmlBody := strings.NewReplacer(
-		"{{PREHEADER}}", html.EscapeString("A plumber answered "+title+"."),
+		"{{PREHEADER}}", html.EscapeString(author+" replied in "+title+"."),
 		"{{GREETING}}", html.EscapeString(greetingName(msg.ToName)),
 		"{{TITLE}}", html.EscapeString(title),
-		"{{ANSWER}}", html.EscapeString(msg.AnswerBody),
+		"{{AUTHOR}}", html.EscapeString(author),
+		"{{REPLY}}", html.EscapeString(msg.ReplyBody),
 		"{{LINK}}", html.EscapeString(link),
-		"{{MARK}}", "cid:answer-notification-mark",
-	).Replace(questionAnsweredHTML)
+		"{{MARK}}", "cid:reply-notification-mark",
+	).Replace(postReplyHTML)
 	return text, htmlBody
 }
 
-const questionAnsweredHTML = `<!doctype html>
+func replyRootTitle(title string) string {
+	title = strings.TrimSpace(title)
+	if title == "" {
+		return "your conversation"
+	}
+	return title
+}
+
+const postReplyHTML = `<!doctype html>
 <html lang="en">
 <head>
   <meta charset="utf-8">
   <meta name="viewport" content="width=device-width, initial-scale=1">
   <meta name="color-scheme" content="dark">
   <meta name="supported-color-schemes" content="dark">
-  <title>Your question was answered</title>
+  <title>New reply to your conversation</title>
 </head>
 <body style="margin:0;padding:0;background:#161719;color:#ecebe7;font-family:Arial,'Helvetica Neue',sans-serif;">
   <div style="display:none;max-height:0;overflow:hidden;opacity:0;color:transparent;">{{PREHEADER}}</div>
@@ -147,22 +163,22 @@ const questionAnsweredHTML = `<!doctype html>
           </tr>
           <tr>
             <td style="padding:30px 28px 32px;">
-              <div style="margin:0 0 10px;color:#e96a26;font-family:'Courier New',monospace;font-size:11px;font-weight:700;line-height:1.4;letter-spacing:1.8px;text-transform:uppercase;">Shop response</div>
-              <h1 style="margin:0;color:#ecebe7;font-size:28px;font-weight:600;line-height:1.2;letter-spacing:-0.4px;">Your question has an answer.</h1>
-              <p style="margin:18px 0 0;color:#b8babf;font-size:16px;line-height:1.6;">Hi{{GREETING}}, a plumber replied to:</p>
+              <div style="margin:0 0 10px;color:#e96a26;font-family:'Courier New',monospace;font-size:11px;font-weight:700;line-height:1.4;letter-spacing:1.8px;text-transform:uppercase;">New reply</div>
+              <h1 style="margin:0;color:#ecebe7;font-size:28px;font-weight:600;line-height:1.2;letter-spacing:-0.4px;">The conversation has a new reply.</h1>
+              <p style="margin:18px 0 0;color:#b8babf;font-size:16px;line-height:1.6;">Hi{{GREETING}}, {{AUTHOR}} replied in:</p>
               <p style="margin:8px 0 0;color:#ecebe7;font-size:17px;font-weight:600;line-height:1.45;">“{{TITLE}}”</p>
               <table role="presentation" width="100%" cellspacing="0" cellpadding="0" border="0" style="width:100%;margin-top:24px;background:#161719;border:1px solid #2e3136;border-radius:3px;">
                 <tr>
                   <td style="padding:20px 18px;">
-                    <div style="margin:0 0 10px;color:#8d9096;font-family:'Courier New',monospace;font-size:10px;font-weight:700;line-height:1.4;letter-spacing:1.5px;text-transform:uppercase;">The answer</div>
-                    <div style="margin:0;color:#ecebe7;font-size:16px;line-height:1.65;white-space:pre-wrap;">{{ANSWER}}</div>
+                    <div style="margin:0 0 10px;color:#8d9096;font-family:'Courier New',monospace;font-size:10px;font-weight:700;line-height:1.4;letter-spacing:1.5px;text-transform:uppercase;">The reply</div>
+                    <div style="margin:0;color:#ecebe7;font-size:16px;line-height:1.65;white-space:pre-wrap;">{{REPLY}}</div>
                   </td>
                 </tr>
               </table>
               <table role="presentation" cellspacing="0" cellpadding="0" border="0" style="margin-top:26px;">
                 <tr>
                   <td bgcolor="#e96a26" style="border-radius:3px;">
-                    <a href="{{LINK}}" style="display:inline-block;padding:13px 18px;color:#161719;font-family:'Courier New',monospace;font-size:12px;font-weight:700;line-height:1;text-decoration:none;letter-spacing:0.8px;text-transform:uppercase;">View the answer&nbsp;&rarr;</a>
+                    <a href="{{LINK}}" style="display:inline-block;padding:13px 18px;color:#161719;font-family:'Courier New',monospace;font-size:12px;font-weight:700;line-height:1;text-decoration:none;letter-spacing:0.8px;text-transform:uppercase;">View the reply&nbsp;&rarr;</a>
                   </td>
                 </tr>
               </table>
@@ -170,7 +186,7 @@ const questionAnsweredHTML = `<!doctype html>
           </tr>
           <tr>
             <td style="padding:18px 28px;border-top:1px solid #2e3136;color:#8d9096;font-family:'Courier New',monospace;font-size:10px;line-height:1.6;letter-spacing:0.4px;">
-              You received this because you asked a question on Ask a Plumber First.
+              You received this because someone replied to your post on Ask a Plumber First.
             </td>
           </tr>
         </table>

internal/mail/mail_test.go

@@ -14,24 +14,27 @@ func TestEmbeddedMarkIsPNG(t *testing.T) {
 	}
 }
 
-func TestQuestionAnsweredContent(t *testing.T) {
+func TestPostReplyContent(t *testing.T) {
 	t.Parallel()
 
-	text, htmlBody := questionAnsweredContent("https://plumber.example/", QuestionAnswered{
+	text, htmlBody := postReplyContent("https://www.askaplumberfirst.com/", PostReply{
 		ToName:          `<Sam & Pat>`,
-		QuestionID:    "question-123",
-		QuestionTitle: `<b>Leaky sink</b>`,
-		AnswerBody:    "Replace the cartridge.\nThen test the handle. <script>alert('x')</script>",
+		RootID:          "question-123",
+		RootTitle:       `<b>Leaky sink</b>`,
+		ReplyID:         "reply-456",
+		ReplyBody:       "Replace the cartridge.\nThen test the handle. <script>alert('x')</script>",
+		ReplyAuthorName: `<Jo & Co>`,
 	})
 
 	for _, want := range []string{
 		"Ask a Plumber First",
-		"Shop response",
-		"cid:answer-notification-mark",
-		"https://plumber.example/questions/question-123",
+		"New reply",
+		"cid:reply-notification-mark",
+		"https://www.askaplumberfirst.com/questions/question-123#post-reply-456",
 		"white-space:pre-wrap",
 		"&lt;Sam &amp; Pat&gt;",
 		"&lt;b&gt;Leaky sink&lt;/b&gt;",
+		"&lt;Jo &amp; Co&gt;",
 		"&lt;script&gt;alert(&#39;x&#39;)&lt;/script&gt;",
 	} {
 		if !strings.Contains(htmlBody, want) {
@@ -41,6 +44,7 @@ func TestQuestionAnsweredContent(t *testing.T) {
 	for _, unsafe := range []string{
 		"<Sam & Pat>",
 		"<b>Leaky sink</b>",
+		"<Jo & Co>",
 		"<script>alert('x')</script>",
 	} {
 		if strings.Contains(htmlBody, unsafe) {
@@ -52,9 +56,9 @@ func TestQuestionAnsweredContent(t *testing.T) {
 	}
 	for _, want := range []string{
 		`Hi <Sam & Pat>,`,
-		`Your question "<b>Leaky sink</b>"`,
+		`<Jo & Co> replied in "<b>Leaky sink</b>"`,
 		"Replace the cartridge.\nThen test the handle.",
-		"https://plumber.example/questions/question-123",
+		"https://www.askaplumberfirst.com/questions/question-123#post-reply-456",
 	} {
 		if !strings.Contains(text, want) {
 			t.Errorf("text missing %q", want)
@@ -62,15 +66,15 @@ func TestQuestionAnsweredContent(t *testing.T) {
 	}
 }
 
-func TestQuestionAnsweredContentUsesFallbackTitle(t *testing.T) {
+func TestPostReplyContentUsesFallbacks(t *testing.T) {
 	t.Parallel()
 
-	text, htmlBody := questionAnsweredContent("https://plumber.example", QuestionAnswered{})
-	if !strings.Contains(text, `"your question"`) {
+	text, htmlBody := postReplyContent("https://www.askaplumberfirst.com", PostReply{})
+	if !strings.Contains(text, `Someone replied in "your conversation"`) {
 		t.Errorf("text missing fallback title")
 	}
-	if !strings.Contains(htmlBody, "a plumber replied to:</p>") ||
-		!strings.Contains(htmlBody, "“your question”") {
-		t.Errorf("HTML missing fallback title")
+	if !strings.Contains(htmlBody, "Someone replied in:</p>") ||
+		!strings.Contains(htmlBody, "“your conversation”") {
+		t.Errorf("HTML missing fallbacks")
 	}
 }

internal/mail/recording.go

@@ -8,10 +8,10 @@ import (
 // Recording is a test Notifier that records calls.
 type Recording struct {
 	mu   sync.Mutex
-	Msgs []QuestionAnswered
+	Msgs []PostReply
 }
 
-func (r *Recording) NotifyQuestionAnswered(_ context.Context, msg QuestionAnswered) error {
+func (r *Recording) NotifyPostReply(_ context.Context, msg PostReply) error {
 	r.mu.Lock()
 	defer r.mu.Unlock()
 	r.Msgs = append(r.Msgs, msg)
@@ -25,10 +25,10 @@ func (r *Recording) Len() int {
 }
 
 // Snapshot returns a copy of recorded messages.
-func (r *Recording) Snapshot() []QuestionAnswered {
+func (r *Recording) Snapshot() []PostReply {
 	r.mu.Lock()
 	defer r.mu.Unlock()
-	out := make([]QuestionAnswered, len(r.Msgs))
+	out := make([]PostReply, len(r.Msgs))
 	copy(out, r.Msgs)
 	return out
 }

internal/web/posts.go

@@ -5,12 +5,15 @@ import (
 	"database/sql"
 	"errors"
 	"fmt"
+	"log"
 	"net/http"
 	"net/url"
 	"strings"
+	"time"
 
 	"github.com/go-chi/chi/v5"
 
+	"plumber/internal/mail"
 	"plumber/internal/store"
 )
 
@@ -37,7 +40,7 @@ func (s *Server) handleCreatePost(w http.ResponseWriter, r *http.Request) {
 		AuthorID: user.ID,
 		Body:     truncateRunes(body, 12000),
 	}
-	var root *store.Post
+	var parent, root *store.Post
 	if parentID == "" {
 		post.Title = truncateRunes(strings.TrimSpace(r.PostFormValue("title")), 120)
 		post.City = truncateRunes(strings.TrimSpace(r.PostFormValue("city")), 80)
@@ -46,7 +49,7 @@ func (s *Server) handleCreatePost(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 	} else {
-		parent, threadRoot, err := s.postAndRoot(r.Context(), parentID)
+		loadedParent, threadRoot, err := s.postAndRoot(r.Context(), parentID)
 		if err != nil {
 			if errors.Is(err, sql.ErrNoRows) {
 				http.NotFound(w, r)
@@ -63,6 +66,7 @@ func (s *Server) handleCreatePost(w http.ResponseWriter, r *http.Request) {
 			http.Error(w, "forbidden", http.StatusForbidden)
 			return
 		}
+		parent = loadedParent
 		post.ParentID = &parent.ID
 		root = threadRoot
 	}
@@ -78,6 +82,9 @@ func (s *Server) handleCreatePost(w http.ResponseWriter, r *http.Request) {
 	if root == nil {
 		root = post
 	}
+	if parent != nil {
+		s.notifyPostReply(parent, root, post, user)
+	}
 	http.Redirect(
 		w,
 		r,
@@ -86,6 +93,53 @@ func (s *Server) handleCreatePost(w http.ResponseWriter, r *http.Request) {
 	)
 }
 
+// notifyPostReply asynchronously emails the direct parent post's author.
+func (s *Server) notifyPostReply(
+	parent *store.Post,
+	root *store.Post,
+	reply *store.Post,
+	replyAuthor *store.User,
+) {
+	if parent == nil ||
+		root == nil ||
+		reply == nil ||
+		replyAuthor == nil ||
+		s.cfg.Mail == nil ||
+		parent.AuthorID == replyAuthor.ID {
+		return
+	}
+	if _, disabled := s.cfg.Mail.(mail.Nop); disabled {
+		return
+	}
+	msg := mail.PostReply{
+		RootID:          root.ID,
+		RootTitle:       root.Title,
+		ReplyID:         reply.ID,
+		ReplyBody:       reply.Body,
+		ReplyAuthorName: replyAuthor.Name,
+	}
+	recipientID := parent.AuthorID
+	go func() {
+		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+		defer cancel()
+		recipient, err := s.store.UserByID(ctx, recipientID)
+		if err != nil {
+			log.Printf("notify reply %s: load recipient: %v", msg.ReplyID, err)
+			return
+		}
+		if recipient == nil || strings.TrimSpace(recipient.Email) == "" {
+			return
+		}
+		msg.ToEmail = recipient.Email
+		msg.ToName = recipient.Name
+		if err := s.cfg.Mail.NotifyPostReply(ctx, msg); err != nil {
+			log.Printf("notify reply %s: %v", msg.ReplyID, err)
+			return
+		}
+		log.Printf("notify reply %s: accepted", msg.ReplyID)
+	}()
+}
+
 // handleEditPost updates only a post's body after verifying that the current
 // homeowner owns it or that an admin is editing an admin-authored post.
 func (s *Server) handleEditPost(w http.ResponseWriter, r *http.Request) {

internal/web/posts_test.go

@@ -7,7 +7,9 @@ import (
 	"net/url"
 	"strings"
 	"testing"
+	"time"
 
+	"plumber/internal/mail"
 	"plumber/internal/pacific"
 	"plumber/internal/store"
 )
@@ -258,6 +260,127 @@ func TestEditPostRoutePermissions(t *testing.T) {
 	}
 }
 
+func TestPostReplyNotifications(t *testing.T) {
+	t.Parallel()
+
+	recording := &mail.Recording{}
+	srv, mem := newTestServer(t, Config{Mail: recording})
+	handler := srv.Handler()
+	homeowner := seedUser(t, mem, uniq("homeowner"), "hunter22", store.RoleUser)
+	admin := seedUser(t, mem, uniq("admin"), "hunter22", store.RoleAdmin)
+	homeownerCookies := loginUser(t, handler, homeowner.Username, "hunter22")
+	adminCookies := loginUser(t, handler, admin.Username, "hunter22")
+	homeownerCSRF := csrfForCookies(t, handler, homeownerCookies)
+	adminCSRF := csrfForCookies(t, handler, adminCookies)
+
+	rec := postForm(handler, "/posts", url.Values{
+		"_csrf": {homeownerCSRF},
+		"title": {"Leaky sink"},
+		"body":  {"Water under the cabinet."},
+	}, homeownerCookies)
+	if rec.Code != http.StatusSeeOther {
+		t.Fatalf("root create status = %d: %s", rec.Code, rec.Body.String())
+	}
+	if recording.Len() != 0 {
+		t.Fatalf("root create sent %d notifications", recording.Len())
+	}
+	roots, err := mem.ListRootPosts(context.Background(), pacific.Today(), homeowner.ID)
+	if err != nil || len(roots) != 1 {
+		t.Fatalf("created roots = %+v, %v", roots, err)
+	}
+	root := roots[0]
+
+	rec = postForm(handler, "/posts", url.Values{
+		"_csrf":     {adminCSRF},
+		"parent_id": {root.ID},
+		"body":      {"Replace the cartridge."},
+	}, adminCookies)
+	if rec.Code != http.StatusSeeOther {
+		t.Fatalf("admin reply status = %d: %s", rec.Code, rec.Body.String())
+	}
+	thread, err := mem.GetPostThread(context.Background(), root.ID)
+	if err != nil || len(thread.Replies) != 1 {
+		t.Fatalf("admin reply thread = %+v, %v", thread, err)
+	}
+	adminReply := thread.Replies[0]
+	msgs := waitForMail(t, recording, 1)
+	if msg := msgs[0]; msg.ToEmail != homeowner.Email ||
+		msg.RootID != root.ID ||
+		msg.RootTitle != root.Title ||
+		msg.ReplyID != adminReply.ID ||
+		msg.ReplyBody != adminReply.Body ||
+		msg.ReplyAuthorName != admin.Name {
+		t.Fatalf("admin reply notification = %+v", msg)
+	}
+
+	rec = postForm(handler, "/posts", url.Values{
+		"_csrf":     {homeownerCSRF},
+		"parent_id": {adminReply.ID},
+		"body":      {"That fixed the drip."},
+	}, homeownerCookies)
+	if rec.Code != http.StatusSeeOther {
+		t.Fatalf("homeowner reply status = %d: %s", rec.Code, rec.Body.String())
+	}
+	thread, err = mem.GetPostThread(context.Background(), root.ID)
+	if err != nil || len(thread.Replies[0].Replies) != 1 {
+		t.Fatalf("homeowner nested reply thread = %+v, %v", thread, err)
+	}
+	homeownerReply := thread.Replies[0].Replies[0]
+	msgs = waitForMail(t, recording, 2)
+	if msg := msgs[1]; msg.ToEmail != admin.Email ||
+		msg.RootID != root.ID ||
+		msg.ReplyID != homeownerReply.ID ||
+		msg.ReplyAuthorName != homeowner.Name {
+		t.Fatalf("homeowner reply notification = %+v", msg)
+	}
+
+	rec = postForm(handler, "/posts", url.Values{
+		"_csrf":     {homeownerCSRF},
+		"parent_id": {root.ID},
+		"body":      {"A note to myself."},
+	}, homeownerCookies)
+	if rec.Code != http.StatusSeeOther {
+		t.Fatalf("self reply status = %d: %s", rec.Code, rec.Body.String())
+	}
+	rec = postForm(handler, "/posts/"+adminReply.ID+"/edit", url.Values{
+		"_csrf": {adminCSRF},
+		"body":  {"Replace the ceramic cartridge."},
+	}, adminCookies)
+	if rec.Code != http.StatusSeeOther {
+		t.Fatalf("edit status = %d: %s", rec.Code, rec.Body.String())
+	}
+
+	noEmail := &store.User{
+		Username:     uniq("no-email"),
+		PasswordHash: homeowner.PasswordHash,
+		Role:         store.RoleUser,
+	}
+	if err := mem.CreateUser(context.Background(), noEmail); err != nil {
+		t.Fatal(err)
+	}
+	noEmailRoot := &store.Post{
+		AuthorID: noEmail.ID,
+		Title:    "Quiet thread",
+		Body:     "No email configured.",
+		PostDate: pacific.Today(),
+	}
+	if err := mem.CreatePost(context.Background(), noEmailRoot); err != nil {
+		t.Fatal(err)
+	}
+	rec = postForm(handler, "/posts", url.Values{
+		"_csrf":     {adminCSRF},
+		"parent_id": {noEmailRoot.ID},
+		"body":      {"This should not send."},
+	}, adminCookies)
+	if rec.Code != http.StatusSeeOther {
+		t.Fatalf("no-email reply status = %d: %s", rec.Code, rec.Body.String())
+	}
+	time.Sleep(50 * time.Millisecond)
+	if recording.Len() != 2 {
+		t.Fatalf("self, edit, or no-email action sent a notification: %+v", recording.Snapshot())
+	}
+}
+
 func TestQuestionPageRendersNestedPostControls(t *testing.T) {
 	t.Parallel()
 
@@ -345,6 +468,19 @@ func TestQuestionPageRendersNestedPostControls(t *testing.T) {
 	}
 }
 
+func waitForMail(t *testing.T, recording *mail.Recording, want int) []mail.PostReply {
+	t.Helper()
+	deadline := time.Now().Add(2 * time.Second)
+	for time.Now().Before(deadline) {
+		if recording.Len() >= want {
+			return recording.Snapshot()
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+	t.Fatalf("recorded %d notifications, want %d", recording.Len(), want)
+	return nil
+}
+
 func csrfForCookies(t *testing.T, handler http.Handler, cookies []*http.Cookie) string {
 	t.Helper()
 	req := httptest.NewRequest(http.MethodGet, "/submit", nil)

internal/web/server.go

@@ -510,6 +510,7 @@ func (s *Server) handleAnswer(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "could not save answer", http.StatusInternalServerError)
 		return
 	}
+	s.notifyPostReply(root, root, reply, u)
 	location := "/questions/" + url.PathEscape(id) + "#post-" + url.PathEscape(reply.ID)
 	if isHTMX(r) {
 		w.Header().Set("HX-Redirect", location)

internal/web/server_test.go

@@ -19,6 +19,7 @@ import (
 
 	"plumber"
 	"plumber/internal/blob"
+	"plumber/internal/mail"
 	"plumber/internal/pacific"
 	"plumber/internal/store"
 )
@@ -483,7 +484,8 @@ func TestProfileAdminAnsweredListAndAvatarUpload(t *testing.T) {
 }
 
 func TestMutationsVoteAnswerHideAndCSRF(t *testing.T) {
-	srv, mem := newTestServer(t, Config{})
+	recording := &mail.Recording{}
+	srv, mem := newTestServer(t, Config{Mail: recording})
 	h := srv.Handler()
 	adminName := uniq("admin")
 	userName := uniq("user")
@@ -611,6 +613,13 @@ func TestMutationsVoteAnswerHideAndCSRF(t *testing.T) {
 	if got := rec.Header().Get("HX-Redirect"); got != "/questions/"+q.ID+"#post-"+adminReply.ID {
 		t.Fatalf("admin answer redirect = %q", got)
 	}
+	msgs := waitForMail(t, recording, 1)
+	if msg := msgs[0]; msg.ToEmail != user.Email ||
+		msg.RootID != q.ID ||
+		msg.ReplyID != adminReply.ID ||
+		msg.ReplyBody != adminReply.Body {
+		t.Fatalf("compatibility reply notification = %+v", msg)
+	}
 
 	rec = httptest.NewRecorder()
 	req = httptest.NewRequest(http.MethodGet, "/questions/"+q.ID, nil)