diff --git a/src/lotus/lotusActions.ts b/src/lotus/lotusActions.ts
index 7dd85df2..c3df969f 100644
--- a/src/lotus/lotusActions.ts
+++ b/src/lotus/lotusActions.ts
@@ -56,6 +56,12 @@ export enum LotusWidgetActions {
   SetAudioOutput = "io.lotus.set_audio_output",
   /** fromWidget: the currently selected output `{ deviceId }` (#119). */
   AudioOutputState = "io.lotus.audio_output_state",
+  /**
+   * fromWidget: local screenshare reminder `{ kind: "ended" | "no-frames" | "alone" }`
+   * — window closed, no frames for 15 s, or 30 min of sharing with nobody
+   * else in the call (#39). Each fires at most once per share.
+   */
+  ScreenshareNotice = "io.lotus.screenshare_notice",
 }
 
 /** toWidget Lotus actions that must be allow-listed in `initializeWidget`. */
diff --git a/src/lotus/lotusScreenshareWatch.test.ts b/src/lotus/lotusScreenshareWatch.test.ts
new file mode 100644
index 00000000..64cbee40
--- /dev/null
+++ b/src/lotus/lotusScreenshareWatch.test.ts
@@ -0,0 +1,154 @@
+/*
+Copyright 2026 Lotus Guild
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { RoomEvent, Track } from "livekit-client";
+import { BehaviorSubject } from "rxjs";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+import { type CallViewModel } from "../state/CallViewModel/CallViewModel";
+import {
+  ALONE_MS,
+  NO_FRAMES_MS,
+  startLotusScreenshareWatch,
+} from "./lotusScreenshareWatch";
+
+const sent: unknown[] = [];
+vi.mock("../widget", () => ({ widget: { api: {} } }));
+vi.mock("./lotusWidget", () => ({
+  lotusSendToHost: (action: string, data: unknown): boolean => {
+    sent.push({ action, data });
+    return true;
+  },
+}));
+
+class FakeTrack extends EventTarget {}
+
+const makeRoom = (): {
+  room: Record<string, unknown>;
+  emit: (event: string, ...args: unknown[]) => void;
+  remote: Map<string, unknown>;
+} => {
+  const handlers = new Map<string, ((...args: unknown[]) => void)[]>();
+  const remote = new Map<string, unknown>();
+  const room = {
+    remoteParticipants: remote,
+    localParticipant: { trackPublications: new Map() },
+    on: (event: string, h: (...args: unknown[]) => void): void => {
+      handlers.set(event, [...(handlers.get(event) ?? []), h]);
+    },
+    off: (event: string, h: (...args: unknown[]) => void): void => {
+      handlers.set(
+        event,
+        (handlers.get(event) ?? []).filter((x) => x !== h),
+      );
+    },
+  };
+  return {
+    room,
+    remote,
+    emit: (event, ...args): void => {
+      (handlers.get(event) ?? []).forEach((h) => h(...args));
+    },
+  };
+};
+
+describe("startLotusScreenshareWatch", () => {
+  let clock = 0;
+  beforeEach(() => {
+    sent.length = 0;
+    clock = 1_000_000;
+    vi.useFakeTimers();
+  });
+  afterEach(() => vi.useRealTimers());
+
+  const start = (): ReturnType<typeof makeRoom> & { stop: () => void } => {
+    const fake = makeRoom();
+    const connections = new BehaviorSubject({
+      getConnections: () => [{ livekitRoom: fake.room }],
+    });
+    const stop = startLotusScreenshareWatch(
+      { allConnections$: connections } as unknown as CallViewModel,
+      () => clock,
+    );
+    return { ...fake, stop };
+  };
+
+  it("reports a share whose track ended", () => {
+    const { emit, stop } = start();
+    const mst = new FakeTrack();
+    emit(RoomEvent.LocalTrackPublished, {
+      source: Track.Source.ScreenShare,
+      track: { mediaStreamTrack: mst },
+    });
+    mst.dispatchEvent(new Event("ended"));
+    expect(sent).toEqual([
+      { action: "io.lotus.screenshare_notice", data: { kind: "ended" } },
+    ]);
+    stop();
+  });
+
+  it("reports no frames after a sustained mute, once, and not after an unmute", () => {
+    const { emit, stop } = start();
+    const mst = new FakeTrack();
+    emit(RoomEvent.LocalTrackPublished, {
+      source: Track.Source.ScreenShare,
+      track: { mediaStreamTrack: mst },
+    });
+    mst.dispatchEvent(new Event("mute"));
+    vi.advanceTimersByTime(NO_FRAMES_MS / 2);
+    mst.dispatchEvent(new Event("unmute"));
+    vi.advanceTimersByTime(NO_FRAMES_MS);
+    expect(sent).toEqual([]);
+    mst.dispatchEvent(new Event("mute"));
+    vi.advanceTimersByTime(NO_FRAMES_MS);
+    mst.dispatchEvent(new Event("mute"));
+    vi.advanceTimersByTime(NO_FRAMES_MS);
+    expect(sent).toEqual([
+      { action: "io.lotus.screenshare_notice", data: { kind: "no-frames" } },
+    ]);
+    stop();
+  });
+
+  it("nudges after 30 min of sharing with nobody else, never while others are present", () => {
+    const { emit, remote, stop } = start();
+    emit(RoomEvent.LocalTrackPublished, {
+      source: Track.Source.ScreenShare,
+      track: { mediaStreamTrack: new FakeTrack() },
+    });
+    remote.set("bob", {});
+    clock += ALONE_MS + 60_000;
+    vi.advanceTimersByTime(60_000);
+    expect(sent).toEqual([]);
+    remote.clear();
+    vi.advanceTimersByTime(60_000);
+    vi.advanceTimersByTime(60_000);
+    expect(sent).toEqual([
+      { action: "io.lotus.screenshare_notice", data: { kind: "alone" } },
+    ]);
+    stop();
+  });
+
+  it("ignores non-screenshare publications and stops watching on unpublish", () => {
+    const { emit, stop } = start();
+    const cam = new FakeTrack();
+    emit(RoomEvent.LocalTrackPublished, {
+      source: Track.Source.Camera,
+      track: { mediaStreamTrack: cam },
+    });
+    cam.dispatchEvent(new Event("ended"));
+    const share = new FakeTrack();
+    const pub = {
+      source: Track.Source.ScreenShare,
+      track: { mediaStreamTrack: share },
+    };
+    emit(RoomEvent.LocalTrackPublished, pub);
+    emit(RoomEvent.LocalTrackUnpublished, pub);
+    share.dispatchEvent(new Event("ended"));
+    expect(sent).toEqual([]);
+    stop();
+  });
+});
diff --git a/src/lotus/lotusScreenshareWatch.ts b/src/lotus/lotusScreenshareWatch.ts
new file mode 100644
index 00000000..53a7e056
--- /dev/null
+++ b/src/lotus/lotusScreenshareWatch.ts
@@ -0,0 +1,123 @@
+/*
+Copyright 2026 Lotus Guild
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import {
+  type LocalTrackPublication,
+  type Room as LivekitRoom,
+  RoomEvent,
+  Track,
+} from "livekit-client";
+
+import { type CallViewModel } from "../state/CallViewModel/CallViewModel";
+import { widget } from "../widget";
+import { LotusWidgetActions } from "./lotusActions";
+import { lotusSendToHost } from "./lotusWidget";
+
+export type ScreenshareNoticeKind = "ended" | "no-frames" | "alone";
+
+/** A shared window minimised/occluded this long with no frames → tell the host. */
+export const NO_FRAMES_MS = 15_000;
+/** Sharing this long with nobody else in the call → one "still sharing?" nudge. */
+export const ALONE_MS = 30 * 60_000;
+const ALONE_CHECK_MS = 60_000;
+
+/**
+ * [lotus #39] Watch the local screenshare and tell the host about the two
+ * cases people miss: the share went black/ended (window closed or minimised)
+ * and a long share with nobody else in the call. Detection only — the host
+ * renders the notices. Each notice fires at most once per share.
+ */
+export function startLotusScreenshareWatch(
+  vm: CallViewModel,
+  now: () => number = () => Date.now(),
+): () => void {
+  if (!widget) return () => undefined;
+
+  const notify = (kind: ScreenshareNoticeKind): void => {
+    lotusSendToHost(LotusWidgetActions.ScreenshareNotice, { kind });
+  };
+
+  const perRoom = new Map<LivekitRoom, () => void>();
+
+  const attach = (room: LivekitRoom): void => {
+    let cleanupTrack: (() => void) | undefined;
+
+    const watchPublication = (pub: LocalTrackPublication): void => {
+      if (pub.source !== Track.Source.ScreenShare) return;
+      cleanupTrack?.();
+      const mst = pub.track?.mediaStreamTrack;
+      const startedAt = now();
+      let noFramesTimer: ReturnType<typeof setTimeout> | undefined;
+      let sentNoFrames = false;
+      let sentAlone = false;
+
+      const onEnded = (): void => notify("ended");
+      const onMute = (): void => {
+        if (sentNoFrames) return;
+        noFramesTimer = setTimeout(() => {
+          sentNoFrames = true;
+          notify("no-frames");
+        }, NO_FRAMES_MS);
+      };
+      const onUnmute = (): void => {
+        if (noFramesTimer !== undefined) clearTimeout(noFramesTimer);
+        noFramesTimer = undefined;
+      };
+      mst?.addEventListener("ended", onEnded);
+      mst?.addEventListener("mute", onMute);
+      mst?.addEventListener("unmute", onUnmute);
+
+      const aloneTimer = setInterval(() => {
+        if (sentAlone) return;
+        if (room.remoteParticipants.size > 0) return;
+        if (now() - startedAt < ALONE_MS) return;
+        sentAlone = true;
+        notify("alone");
+      }, ALONE_CHECK_MS);
+
+      cleanupTrack = (): void => {
+        mst?.removeEventListener("ended", onEnded);
+        mst?.removeEventListener("mute", onMute);
+        mst?.removeEventListener("unmute", onUnmute);
+        if (noFramesTimer !== undefined) clearTimeout(noFramesTimer);
+        clearInterval(aloneTimer);
+        cleanupTrack = undefined;
+      };
+    };
+
+    const onUnpublished = (pub: LocalTrackPublication): void => {
+      if (pub.source === Track.Source.ScreenShare) cleanupTrack?.();
+    };
+
+    room.on(RoomEvent.LocalTrackPublished, watchPublication);
+    room.on(RoomEvent.LocalTrackUnpublished, onUnpublished);
+    room.localParticipant.trackPublications.forEach(watchPublication);
+
+    perRoom.set(room, () => {
+      cleanupTrack?.();
+      room.off(RoomEvent.LocalTrackPublished, watchPublication);
+      room.off(RoomEvent.LocalTrackUnpublished, onUnpublished);
+    });
+  };
+
+  const sub = vm.allConnections$.subscribe((data) => {
+    const rooms = data.getConnections().map((c) => c.livekitRoom);
+    for (const [room, off] of perRoom) {
+      if (!rooms.includes(room)) {
+        off();
+        perRoom.delete(room);
+      }
+    }
+    for (const room of rooms) if (!perRoom.has(room)) attach(room);
+  });
+
+  return () => {
+    sub.unsubscribe();
+    for (const off of perRoom.values()) off();
+    perRoom.clear();
+  };
+}
diff --git a/src/room/InCallView.tsx b/src/room/InCallView.tsx
index eef6f987..c6ff7022 100644
--- a/src/room/InCallView.tsx
+++ b/src/room/InCallView.tsx
@@ -37,6 +37,7 @@ import { startLotusDecorations } from "../lotus/lotusDecorations";
 import { startLotusDenoise } from "../lotus/lotusDenoise";
 import { startLotusCallSummary } from "../lotus/lotusCallSummary";
 import { startLotusAudioOutput } from "../lotus/lotusAudioOutput";
+import { startLotusScreenshareWatch } from "../lotus/lotusScreenshareWatch";
 import { startLotusDeafen } from "../lotus/lotusDeafen";
 import styles from "./InCallView.module.css";
 import { GridTile } from "../tile/GridTile";
@@ -312,6 +313,8 @@ export const InCallView: FC<InCallViewProps> = ({
   // survives reconnects (#1 / A7). No-op unless lotusDenoiseSource=1.
   useEffect(() => startLotusDenoise(vm), [vm]);
   useEffect(() => startLotusCallSummary(vm), [vm]);
+  // [lotus #39] Screenshare reminders (window closed / black / sharing alone).
+  useEffect(() => startLotusScreenshareWatch(vm), [vm]);
   // [lotus] Handle the host's io.lotus.set_deafen action to silence remote
   // audio (and optionally screenshare audio) at the LiveKit source. No-op
   // unless the host sends the action.