diff --git a/apps/client/src/features/comment/components/comment-content-view.test.tsx b/apps/client/src/features/comment/components/comment-content-view.test.tsx
index 597c60d0..96e01961 100644
--- a/apps/client/src/features/comment/components/comment-content-view.test.tsx
+++ b/apps/client/src/features/comment/components/comment-content-view.test.tsx
@@ -72,6 +72,15 @@ describe("CommentContentView", () => {
     expect(container.querySelector("s")?.textContent).toBe("st");
   });
 
+  it("renders the underline mark as <u> (not the editor fallback)", () => {
+    const { container } = renderView(
+      doc([para([text("un", [{ type: "underline" }])])]),
+    );
+    expect(container.querySelector("u")?.textContent).toBe("un");
+    // Underline is a supported mark, so no degrade to the editor fallback.
+    expect(screen.queryByTestId("comment-editor-fallback")).toBeNull();
+  });
+
   it("renders the code mark as <code>", () => {
     const { container } = renderView(
       doc([para([text("co", [{ type: "code" }])])]),
diff --git a/apps/client/src/features/comment/components/comment-content-view.tsx b/apps/client/src/features/comment/components/comment-content-view.tsx
index 7e6feea9..3255cd24 100644
--- a/apps/client/src/features/comment/components/comment-content-view.tsx
+++ b/apps/client/src/features/comment/components/comment-content-view.tsx
@@ -62,6 +62,11 @@ function renderMarks(
         return <em key={key}>{acc}</em>;
       case "strike":
         return <s key={key}>{acc}</s>;
+      case "underline":
+        // StarterKit enables the Underline extension by default (Mod-u) and
+        // CommentEditor does not disable it, so real comments can carry this
+        // mark. Render it here rather than degrading the whole comment.
+        return <u key={key}>{acc}</u>;
       case "code":
         return <code key={key}>{acc}</code>;
       case "link": {
diff --git a/apps/client/src/features/comment/components/comment-list-item.test.tsx b/apps/client/src/features/comment/components/comment-list-item.test.tsx
index f41f952c..5b217520 100644
--- a/apps/client/src/features/comment/components/comment-list-item.test.tsx
+++ b/apps/client/src/features/comment/components/comment-list-item.test.tsx
@@ -1,5 +1,5 @@
-import { describe, it, expect, vi } from "vitest";
-import { render, screen, fireEvent } from "@testing-library/react";
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { render, screen, fireEvent, waitFor } from "@testing-library/react";
 import { MantineProvider } from "@mantine/core";
 import { IComment } from "@/features/comment/types/comment.types";
 
@@ -9,10 +9,11 @@ import { IComment } from "@/features/comment/types/comment.types";
 // component renders in isolation. We only assert the AI-badge rendering branch.
 const applyMutateAsync = vi.fn();
 const dismissMutateAsync = vi.fn();
+const updateMutateAsync = vi.fn();
 vi.mock("@/features/comment/queries/comment-query", () => ({
   useDeleteCommentMutation: () => ({ mutateAsync: vi.fn() }),
   useResolveCommentMutation: () => ({ mutateAsync: vi.fn() }),
-  useUpdateCommentMutation: () => ({ mutateAsync: vi.fn() }),
+  useUpdateCommentMutation: () => ({ mutateAsync: updateMutateAsync }),
   useApplySuggestionMutation: () => ({
     mutateAsync: applyMutateAsync,
     isPending: false,
@@ -23,10 +24,42 @@ vi.mock("@/features/comment/queries/comment-query", () => ({
   }),
 }));
 
+// The document the mocked editor emits via onUpdate when the edit form is open.
+// Duplicated inside the mock factory (below) to keep the factory self-contained.
+const EDITED_DOC = {
+  type: "doc",
+  content: [
+    { type: "paragraph", content: [{ type: "text", text: "edited via editor" }] },
+  ],
+};
+
 // CommentEditor pulls in the full TipTap editor stack; replace it with a stub.
-vi.mock("@/features/comment/components/comment-editor", () => ({
-  default: () => <div data-testid="comment-editor" />,
-}));
+// In edit mode the stub exposes buttons that fire the real onUpdate/onSave props
+// so the edit->save/cancel flow can be driven without a live editor.
+vi.mock("@/features/comment/components/comment-editor", () => {
+  const doc = {
+    type: "doc",
+    content: [
+      { type: "paragraph", content: [{ type: "text", text: "edited via editor" }] },
+    ],
+  };
+  return {
+    default: ({ onUpdate, onSave }: any) => (
+      <div data-testid="comment-editor">
+        <button
+          type="button"
+          data-testid="editor-emit-update"
+          onClick={() => onUpdate?.(doc)}
+        />
+        <button
+          type="button"
+          data-testid="editor-emit-save"
+          onClick={() => onSave?.()}
+        />
+      </div>
+    ),
+  };
+});
 
 // CommentContentView (used for the read-only body) imports the mention view,
 // which pulls page-query -> main.tsx (createRoot). Stub the queries so the item
@@ -297,6 +330,113 @@ describe("canShowDismiss predicate", () => {
   });
 });
 
+describe("CommentListItem — edit -> save/cancel flow (#340 F3)", () => {
+  const body = (t: string) =>
+    JSON.stringify({
+      type: "doc",
+      content: [{ type: "paragraph", content: [{ type: "text", text: t }] }],
+    });
+
+  // The edit menu item is gated on the viewer owning the comment
+  // (currentUser.id === creatorId). currentUserAtom is atomWithStorage-backed,
+  // so seed localStorage to make the viewer the owner (creatorId "user-1").
+  beforeEach(() => {
+    updateMutateAsync.mockClear();
+    localStorage.setItem(
+      "currentUser",
+      JSON.stringify({ user: { id: "user-1", name: "Owner" } }),
+    );
+  });
+  afterEach(() => {
+    localStorage.clear();
+  });
+
+  async function openEditor() {
+    // Open the comment menu, then click "Edit comment" to toggle into edit mode.
+    fireEvent.click(screen.getByLabelText("Comment menu"));
+    fireEvent.click(await screen.findByText("Edit comment"));
+    // Edit form (mocked editor + actions) is now mounted.
+    await screen.findByTestId("comment-editor");
+  }
+
+  it("saves the edited content and, on cache update, shows the new body", async () => {
+    const { rerender } = renderItem(
+      baseComment({ content: body("original body") }),
+    );
+    // Static body first.
+    expect(screen.getByText("original body")).toBeDefined();
+
+    await openEditor();
+
+    // Editor emits an update (populates editContentRef), then Save is clicked.
+    fireEvent.click(screen.getByTestId("editor-emit-update"));
+    fireEvent.click(screen.getByRole("button", { name: "Save" }));
+
+    // mutateAsync is called with the stringified edited doc.
+    expect(updateMutateAsync).toHaveBeenCalledWith({
+      commentId: "c-1",
+      content: JSON.stringify(EDITED_DOC),
+    });
+
+    // On success the form closes (isEditing -> false); the static body renders
+    // from the comment.content prop again.
+    await waitFor(() =>
+      expect(screen.queryByTestId("comment-editor")).toBeNull(),
+    );
+
+    // Simulate the cache invalidation swapping in a new comment object with the
+    // updated content — the static body reflects it.
+    rerender(
+      <MantineProvider>
+        <CommentListItem
+          comment={baseComment({ content: body("updated body after save") })}
+          pageId="page-1"
+          canComment={true}
+          canEdit={true}
+        />
+      </MantineProvider>,
+    );
+    expect(screen.getByText("updated body after save")).toBeDefined();
+    expect(screen.queryByText("original body")).toBeNull();
+  });
+
+  it("cancel restores the static body and does not call the update mutation", async () => {
+    renderItem(baseComment({ content: body("original body") }));
+    await openEditor();
+
+    // Type something (editContentRef set), then cancel.
+    fireEvent.click(screen.getByTestId("editor-emit-update"));
+    fireEvent.click(screen.getByRole("button", { name: "Cancel" }));
+
+    // Editor unmounts, static body restored, no save happened.
+    await waitFor(() =>
+      expect(screen.queryByTestId("comment-editor")).toBeNull(),
+    );
+    expect(screen.getByText("original body")).toBeDefined();
+    expect(updateMutateAsync).not.toHaveBeenCalled();
+  });
+
+  it("saving without editing sends the existing content (editContentRef cleared after cancel)", async () => {
+    renderItem(baseComment({ content: body("original body") }));
+
+    // Cancel path clears editContentRef...
+    await openEditor();
+    fireEvent.click(screen.getByTestId("editor-emit-update"));
+    fireEvent.click(screen.getByRole("button", { name: "Cancel" }));
+    await waitFor(() =>
+      expect(screen.queryByTestId("comment-editor")).toBeNull(),
+    );
+
+    // ...so re-opening and saving WITHOUT an update falls back to comment.content.
+    await openEditor();
+    fireEvent.click(screen.getByRole("button", { name: "Save" }));
+    expect(updateMutateAsync).toHaveBeenCalledWith({
+      commentId: "c-1",
+      content: JSON.stringify(body("original body")),
+    });
+  });
+});
+
 describe("CommentListItem — read-only body renders statically", () => {
   it("renders the comment body as static text without a TipTap editor", () => {
     renderItem(
diff --git a/apps/client/src/features/comment/components/comment-list-with-tabs.test.tsx b/apps/client/src/features/comment/components/comment-list-with-tabs.test.tsx
new file mode 100644
index 00000000..cdc3bcb4
--- /dev/null
+++ b/apps/client/src/features/comment/components/comment-list-with-tabs.test.tsx
@@ -0,0 +1,108 @@
+import { describe, it, expect, vi } from "vitest";
+import { render, screen, fireEvent } from "@testing-library/react";
+import { MantineProvider } from "@mantine/core";
+import { IComment } from "@/features/comment/types/comment.types";
+
+// matchMedia (read by MantineProvider) is stubbed globally in vitest.setup.ts.
+
+// CommentEditor pulls in the full TipTap editor stack; replace it with a stub so
+// the lazy reply editor's mount transition can be observed without the editor.
+vi.mock("@/features/comment/components/comment-editor", () => ({
+  default: () => <div data-testid="comment-editor" />,
+}));
+
+// page-query -> main.tsx (createRoot) is a module side effect; stub the queries
+// pulled in transitively so importing the module is side-effect free.
+vi.mock("@/features/page/queries/page-query.ts", () => ({
+  usePageQuery: () => ({ data: undefined, isLoading: false, isError: false }),
+}));
+vi.mock("@/features/share/queries/share-query.ts", () => ({
+  useSharePageQuery: () => ({ data: undefined }),
+}));
+// space-query -> main.tsx (createRoot) is another module side effect; stub it.
+vi.mock("@/features/space/queries/space-query.ts", () => ({
+  useGetSpaceBySlugQuery: () => ({ data: undefined }),
+}));
+
+import {
+  buildChildrenByParent,
+  CommentEditorWithActions,
+} from "./comment-list-with-tabs";
+
+const c = (id: string, parentCommentId: string | null = null): IComment =>
+  ({ id, parentCommentId }) as IComment;
+
+describe("buildChildrenByParent (childrenByParent grouping)", () => {
+  it("returns an empty map for undefined or empty input", () => {
+    expect(buildChildrenByParent(undefined).size).toBe(0);
+    expect(buildChildrenByParent([]).size).toBe(0);
+  });
+
+  it("does not index a top-level comment (parentCommentId null)", () => {
+    const map = buildChildrenByParent([c("p1", null)]);
+    expect(map.size).toBe(0);
+    expect(map.has("p1")).toBe(false);
+  });
+
+  it("groups replies under the correct parent, including reply-to-reply nesting", () => {
+    const p1 = c("p1", null);
+    const r1 = c("r1", "p1");
+    const r2 = c("r2", "r1"); // a reply to a reply
+    const map = buildChildrenByParent([p1, r1, r2]);
+    expect(map.get("p1")).toEqual([r1]);
+    expect(map.get("r1")).toEqual([r2]);
+    // The top-level comment itself is never a key.
+    expect(map.has("p1") && map.get("p1")?.length).toBe(1);
+  });
+
+  it("still groups a reply whose parent is not present in items", () => {
+    const orphan = c("o1", "missing-parent");
+    const map = buildChildrenByParent([orphan]);
+    expect(map.get("missing-parent")).toEqual([orphan]);
+  });
+
+  it("preserves insertion order among sibling replies", () => {
+    const map = buildChildrenByParent([
+      c("a", "p1"),
+      c("b", "p1"),
+      c("d", "p1"),
+    ]);
+    expect(map.get("p1")?.map((x) => x.id)).toEqual(["a", "b", "d"]);
+  });
+});
+
+function renderReplyEditor() {
+  return render(
+    <MantineProvider>
+      <CommentEditorWithActions commentId="c-1" onSave={vi.fn()} />
+    </MantineProvider>,
+  );
+}
+
+describe("CommentEditorWithActions — lazy reply editor activation", () => {
+  it("shows only the stub initially (no editor instance mounted)", () => {
+    renderReplyEditor();
+    expect(screen.getByRole("button")).toBeDefined();
+    expect(screen.queryByTestId("comment-editor")).toBeNull();
+  });
+
+  it("mounts the real editor when the stub is clicked and keeps it mounted", () => {
+    renderReplyEditor();
+    fireEvent.click(screen.getByRole("button"));
+    expect(screen.getByTestId("comment-editor")).toBeDefined();
+    // The stub button is replaced by the editor subtree.
+    expect(screen.queryByRole("button")).toBeNull();
+  });
+
+  it("mounts the editor when the stub receives focus", () => {
+    renderReplyEditor();
+    fireEvent.focus(screen.getByRole("button"));
+    expect(screen.getByTestId("comment-editor")).toBeDefined();
+  });
+
+  it("mounts the editor on Enter keydown of the stub", () => {
+    renderReplyEditor();
+    fireEvent.keyDown(screen.getByRole("button"), { key: "Enter" });
+    expect(screen.getByTestId("comment-editor")).toBeDefined();
+  });
+});
diff --git a/apps/client/src/features/comment/components/comment-list-with-tabs.tsx b/apps/client/src/features/comment/components/comment-list-with-tabs.tsx
index 5ee19a3e..b5e62e8e 100644
--- a/apps/client/src/features/comment/components/comment-list-with-tabs.tsx
+++ b/apps/client/src/features/comment/components/comment-list-with-tabs.tsx
@@ -35,6 +35,24 @@ interface CommentListWithTabsProps {
   onClose?: () => void;
 }
 
+// Index replies by their parent id once (O(n)), instead of an O(n^2) filter per
+// thread. Replies whose parent is not in `items` are still grouped under their
+// parentCommentId (they simply won't be reached by the top-level walk).
+// Exported for unit testing.
+export function buildChildrenByParent(
+  items: IComment[] | undefined,
+): Map<string, IComment[]> {
+  const m = new Map<string, IComment[]>();
+  for (const c of items ?? []) {
+    if (c.parentCommentId) {
+      const arr = m.get(c.parentCommentId);
+      if (arr) arr.push(c);
+      else m.set(c.parentCommentId, [c]);
+    }
+  }
+  return m;
+}
+
 function CommentListWithTabs({ onClose }: CommentListWithTabsProps) {
   const { t } = useTranslation();
   const { pageSlug } = useParams();
@@ -79,17 +97,10 @@ function CommentListWithTabs({ onClose }: CommentListWithTabsProps) {
   // Index replies by their parent once, instead of an O(n^2) filter per thread.
   // The map ref changes on any comments update, so MemoizedChildComments re-runs
   // (cheap) and re-looks-up, while memoized CommentListItems skip unchanged items.
-  const childrenByParent = useMemo(() => {
-    const m = new Map<string, IComment[]>();
-    for (const c of comments?.items ?? []) {
-      if (c.parentCommentId) {
-        const arr = m.get(c.parentCommentId);
-        if (arr) arr.push(c);
-        else m.set(c.parentCommentId, [c]);
-      }
-    }
-    return m;
-  }, [comments?.items]);
+  const childrenByParent = useMemo(
+    () => buildChildrenByParent(comments?.items),
+    [comments?.items],
+  );
 
   const [isPageCommentLoading, setIsPageCommentLoading] = useState(false);
 
@@ -217,8 +228,10 @@ function CommentListWithTabs({ onClose }: CommentListWithTabsProps) {
       <Tabs
         defaultValue="open"
         variant="default"
-        // Do not mount the inactive tab: the Resolved list (hundreds of items)
-        // stays unmounted while the Open tab is shown, and vice versa.
+        // Default to not mounting an inactive tab (the heavy Resolved list stays
+        // unmounted while Open is shown). The Open panel overrides this with its
+        // own keepMounted (below) so an in-progress reply/edit draft survives an
+        // Open -> Resolved -> Open switch.
         keepMounted={false}
         style={{
           flex: "1 1 auto",
@@ -278,7 +291,10 @@ function CommentListWithTabs({ onClose }: CommentListWithTabsProps) {
           type="scroll"
         >
           <div style={{ paddingBottom: "8px" }}>
-            <Tabs.Panel value="open" pt="xs">
+            {/* keepMounted keeps the Open panel alive even while Resolved is
+                active, so a lazily-mounted reply editor's draft (and an
+                in-progress edit) is not discarded on tab switch. */}
+            <Tabs.Panel value="open" pt="xs" keepMounted>
               {activeComments.length === 0 ? (
                 <Center py="xl">
                   <Stack align="center" gap="xs">
@@ -368,7 +384,7 @@ const ChildComments = ({
 
 const MemoizedChildComments = memo(ChildComments);
 
-const CommentEditorWithActions = ({
+export const CommentEditorWithActions = ({
   commentId,
   onSave,
   placeholder = undefined,