diff --git a/apps/web/src/components/views/rooms/wysiwyg_composer/DocumentView.tsx b/apps/web/src/components/views/rooms/wysiwyg_composer/DocumentView.tsx
index 4df1269177..edb65cbbbb 100644
--- a/apps/web/src/components/views/rooms/wysiwyg_composer/DocumentView.tsx
+++ b/apps/web/src/components/views/rooms/wysiwyg_composer/DocumentView.tsx
@@ -287,8 +287,11 @@ function useDocumentSync(
         const applyDeltaEvent = (event: import("matrix-js-sdk/src/matrix").MatrixEvent): void => {
             if (event.getRoomId() !== room.roomId) return;
             if (event.getType() !== DOC_DELTA_EVENT_TYPE) return;
-            // Skip our own events — local model already has our changes.
-            if (event.getSender() === client.getUserId()) return;
+            // Skip events sent by this exact device — we already have those
+            // changes in our local model.  We must NOT skip events from the same
+            // user on a different device (e.g. two tabs open).
+            const senderDeviceId = event.getContent<{ device_id?: string }>().device_id;
+            if (event.getSender() === client.getUserId() && senderDeviceId === client.getDeviceId()) return;
             const data = event.getContent<{ data?: string }>().data;
             if (!data) return;
             logger.info(`[DocumentView] Matrix delta event from ${event.getSender()}, applying ${data.length}b (base64)`);
@@ -384,6 +387,7 @@ function useDocumentSync(
                 await client.sendEvent(room.roomId, DOC_DELTA_EVENT_TYPE as any, {
                     data: base64Encode(delta),
                     heads,
+                    device_id: client.getDeviceId(),
                 });
 
                 deltaSendCount.current++;
@@ -432,6 +436,7 @@ function useDocumentSync(
                     client.sendEvent(room.roomId, DOC_DELTA_EVENT_TYPE as any, {
                         data: base64Encode(delta),
                         heads,
+                        device_id: client.getDeviceId(),
                     }).catch((e) => logger.warn("[DocumentView] Failed to send final delta on unmount", e));
                 }
                 // Always save snapshot on close so next load starts fresh.
diff --git a/apps/web/src/components/views/rooms/wysiwyg_composer/useDocumentRTC.ts b/apps/web/src/components/views/rooms/wysiwyg_composer/useDocumentRTC.ts
index 1c76a0f539..8fb3195228 100644
--- a/apps/web/src/components/views/rooms/wysiwyg_composer/useDocumentRTC.ts
+++ b/apps/web/src/components/views/rooms/wysiwyg_composer/useDocumentRTC.ts
@@ -19,7 +19,7 @@ Please see LICENSE files in the repository root for full details.
  * `isConnected` stays false and the caller should fall back to Matrix events.
  */
 
-import { useCallback, useEffect, useRef, useState } from "react";
+import { useCallback, useEffect, useMemo, useRef, useState } from "react";
 import { type Room as MatrixRoom, type MatrixClient } from "matrix-js-sdk/src/matrix";
 import { isLivekitTransportConfig } from "matrix-js-sdk/src/matrixrtc";
 import { logger } from "matrix-js-sdk/src/logger";
@@ -245,5 +245,16 @@ export function useDocumentRTC(
             .catch((e: unknown) => logger.warn("[DocumentRTC] Failed to publish cursor", e));
     }, []);
 
-    return { publishDelta, publishCursor, onDeltaRef, onCursorRef, onPeerLeaveRef, isConnected };
+    // Wrap in useMemo so the returned object reference is stable across renders
+    // (only changes when isConnected changes).  Without this, DocumentView's
+    // wiring useEffect ([rtc, applyDeltaBytes]) fires on every render because
+    // the plain object literal `{}` produces a new reference each time.
+    const result = useMemo(
+        () => ({ publishDelta, publishCursor, onDeltaRef, onCursorRef, onPeerLeaveRef, isConnected }),
+        // publishDelta/publishCursor are useCallback stable; refs are always stable;
+        // only isConnected can change.
+        // eslint-disable-next-line react-hooks/exhaustive-deps
+        [isConnected],
+    );
+    return result;
 }