diff --git a/services/mailer/incoming/incoming_handler.go b/services/mailer/incoming/incoming_handler.go
index 7d6b2c6361..1317f5aad6 100644
--- a/services/mailer/incoming/incoming_handler.go
+++ b/services/mailer/incoming/incoming_handler.go
@@ -148,7 +148,7 @@ func (h *ReplyHandler) Handle(ctx context.Context, content *MailContent, doer *u
 				nil,
 				issue,
 				comment.Line,
-				0, // extraLinesCount: a reply to a comment inherit single line
+				comment.ExtraLinesCount, // inherit the parent's range so the reply threads under a multi-line comment
 				content.Content,
 				comment.TreePath,
 				false, // not pending review but a single review
diff --git a/templates/repo/diff/comment_form_datahandler.tmpl b/templates/repo/diff/comment_form_datahandler.tmpl
index d0e493488d..5b72aa5b18 100644
--- a/templates/repo/diff/comment_form_datahandler.tmpl
+++ b/templates/repo/diff/comment_form_datahandler.tmpl
@@ -1,5 +1,5 @@
 {{if $.comment}}
-	{{template "repo/diff/comment_form" dict "root" $.root "hidden" $.hidden "reply" $.reply "Line" $.comment.UnsignedLine "File" $.comment.TreePath "Side" $.comment.DiffSide "HasComments" true}}
+	{{template "repo/diff/comment_form" dict "root" $.root "hidden" $.hidden "reply" $.reply "Line" $.comment.UnsignedLine "ExtraLinesCount" $.comment.ExtraLinesCount "File" $.comment.TreePath "Side" $.comment.DiffSide "HasComments" true}}
 {{else if $.root}}
 	{{template "repo/diff/comment_form" $}}
 {{else}}
diff --git a/tests/integration/incoming_email_test.go b/tests/integration/incoming_email_test.go
index c9876a63dd..95f8867a36 100644
--- a/tests/integration/incoming_email_test.go
+++ b/tests/integration/incoming_email_test.go
@@ -206,6 +206,44 @@ func TestIncomingEmail(t *testing.T) {
 
 				checkReply(t, payload, issue, issues_model.CommentTypeComment)
 			})
+
+			t.Run("MultiLineCodeComment", func(t *testing.T) {
+				defer tests.PrintCurrentTest(t)()
+
+				// A reply to a multi-line code comment must inherit the parent's extra_lines_count
+				issue := unittest.AssertExistsAndLoadBean(t, &issues_model.Issue{ID: 2})
+				require.NoError(t, issue.LoadRepo(db.DefaultContext))
+				review, err := issues_model.CreateReview(db.DefaultContext, issues_model.CreateReviewOptions{
+					Type:     issues_model.ReviewTypeComment,
+					Issue:    issue,
+					Reviewer: user,
+				})
+				require.NoError(t, err)
+				// Anchor the parent with a stored patch so the reply reuses it (no git resolution needed).
+				parent, err := issues_model.CreateComment(db.DefaultContext, &issues_model.CreateCommentOptions{
+					Type:            issues_model.CommentTypeCode,
+					Doer:            user,
+					Repo:            issue.Repo,
+					Issue:           issue,
+					Content:         "multi-line parent",
+					LineNum:         4,
+					ExtraLinesCount: 2,
+					TreePath:        "README.md",
+					CommitSHA:       "0000000000000000000000000000000000000000",
+					ReviewID:        review.ID,
+					Patch:           "@@ -4,3 +4,3 @@",
+				})
+				require.NoError(t, err)
+
+				payload, err := incoming_payload.CreateReferencePayload(parent)
+				require.NoError(t, err)
+
+				handler := &incoming.ReplyHandler{}
+				require.NoError(t, handler.Handle(db.DefaultContext, &incoming.MailContent{Content: "multi-line reply by mail"}, user, payload))
+
+				reply := unittest.AssertExistsAndLoadBean(t, &issues_model.Comment{Content: "multi-line reply by mail", IssueID: issue.ID, Type: issues_model.CommentTypeCode})
+				assert.EqualValues(t, 2, reply.ExtraLinesCount)
+			})
 		})
 
 		t.Run("Unsubscribe", func(t *testing.T) {
diff --git a/tests/integration/pull_review_test.go b/tests/integration/pull_review_test.go
index b072c445bf..306e8fa8d9 100644
--- a/tests/integration/pull_review_test.go
+++ b/tests/integration/pull_review_test.go
@@ -1838,6 +1838,53 @@ func TestPullRequestCommentPlacement(t *testing.T) {
 			assert.False(t, comment.Invalidated)
 		})
 
+		t.Run("reply to a multi-line comment threads under it", func(t *testing.T) {
+			defer tests.PrintCurrentTest(t)()
+			tester := newPullRequestCommentPlacementTester(t)
+
+			content := tester.fileContent
+			content = strings.Replace(content, "Line 48\n", "Line 48--modified\n", 1)
+			content = strings.Replace(content, "Line 49\n", "Line 49--modified\n", 1)
+			content = strings.Replace(content, "Line 50\n", "Line 50--modified\n", 1)
+			tester.changeFile("file1.md", content)
+			tester.createPR()
+
+			comment := tester.multiLineCommentFromFilesChanged("file1.md", 48, 2)
+			assert.EqualValues(t, 2, comment.ExtraLinesCount)
+			assert.EqualValues(t, 50, comment.DisplayLine())
+
+			// Simulate a reply like the reply button does
+			req := NewRequest(t, "GET", fmt.Sprintf("/%s/%s/pulls/%d/files", tester.repo.OwnerName, tester.repo.Name, tester.pr.Index))
+			resp := tester.session.MakeRequest(t, req, http.StatusOK)
+			form := NewHTMLParser(t, resp.Body).Find(`.conversation-holder[data-extra-lines-count="2"] form`)
+			field := func(name string) string {
+				return form.Find(fmt.Sprintf(`input[name=%q]`, name)).AttrOr("value", "")
+			}
+			require.Equal(t, "2", field("extra_lines_count"), "reply form must carry the parent's extra_lines_count")
+
+			const replyContent = "reply through the multi-line comment reply form"
+			req = NewRequestWithValues(t, "POST", fmt.Sprintf("/%s/%s/pulls/%d/files/reviews/comments", tester.repo.OwnerName, tester.repo.Name, tester.pr.Index), map[string]string{
+				"origin":            field("origin"),
+				"before_commit_id":  field("before_commit_id"),
+				"latest_commit_id":  field("latest_commit_id"),
+				"side":              field("side"),
+				"line":              field("line"),
+				"extra_lines_count": field("extra_lines_count"),
+				"path":              field("path"),
+				"reply":             field("reply"),
+				"content":           replyContent,
+			})
+			tester.session.MakeRequest(t, req, http.StatusOK)
+
+			// The reply threads under the parent: it shares the conversation grouping key
+			// (ReviewID + TreePath + DisplayLine, the end of the range).
+			reply := unittest.AssertExistsAndLoadBean(t, &issues_model.Comment{Content: replyContent})
+			assert.EqualValues(t, 2, reply.ExtraLinesCount)
+			assert.Equal(t, comment.ReviewID, reply.ReviewID)
+			assert.Equal(t, comment.TreePath, reply.TreePath)
+			assert.Equal(t, comment.DisplayLine(), reply.DisplayLine())
+		})
+
 		t.Run("multi-line proposed comment over unmodified-then-modified lines stays visible", func(t *testing.T) {
 			defer tests.PrintCurrentTest(t)()
 			tester := newPullRequestCommentPlacementTester(t)