perf(media): defer image/video decryption until near-viewport (P5-5)

Creates useNearViewport hook (IntersectionObserver, 200px rootMargin,
one-shot disconnect after first trigger). ImageContent and VideoContent
now gate loadSrc() on nearViewport — when autoPlay is enabled, encrypted
media is not decrypted until the element is within 200px of the visible
area, reducing initial page load cost on long timelines.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

LOTUS_TODO.md

@@ -278,7 +278,7 @@ Themes:
 
 ---
 
-### [ ] P5-5 · Intersection-Based Lazy Loading
+### [x] P5-5 · Intersection-Based Lazy Loading ⚠️ UNTESTED — needs verification in timeline with many images
 
 **What:** Use `IntersectionObserver` to trigger media decryption and loading only when components approach the viewport.  
 **Approach:** Reduce initial memory footprint and improve timeline load times by deferring decryption of images/videos until they are visible.

src/app/components/message/content/ImageContent.tsx

@@ -1,4 +1,4 @@
-import React, { ReactNode, useCallback, useEffect, useState } from 'react';
+import React, { ReactNode, useCallback, useEffect, useRef, useState } from 'react';
 import {
   Badge,
   Box,
@@ -31,6 +31,7 @@ import { decryptFile, downloadEncryptedMedia, mxcUrlToHttp } from '../../../util
 import { useMediaAuthentication } from '../../../hooks/useMediaAuthentication';
 import { ModalWide } from '../../../styles/Modal.css';
 import { validBlurHash } from '../../../utils/blurHash';
+import { useNearViewport } from '../../../hooks/useNearViewport';
 
 type RenderViewerProps = {
   src: string;
@@ -85,6 +86,9 @@ export const ImageContent = as<'div', ImageContentProps>(
     const [viewer, setViewer] = useState(false);
     const [blurred, setBlurred] = useState(markedAsSpoiler ?? false);
 
+    const sentinelRef = useRef<HTMLDivElement>(null);
+    const nearViewport = useNearViewport(sentinelRef);
+
     const [srcState, loadSrc] = useAsyncCallback(
       useCallback(async () => {
         const mediaUrl = mxcUrlToHttp(mx, url, useAuthentication);
@@ -113,11 +117,12 @@ export const ImageContent = as<'div', ImageContentProps>(
     };
 
     useEffect(() => {
-      if (autoPlay) loadSrc().catch(() => undefined);
-    }, [autoPlay, loadSrc]);
+      if (autoPlay && nearViewport) loadSrc().catch(() => undefined);
+    }, [autoPlay, nearViewport, loadSrc]);
 
     return (
       <Box className={classNames(css.RelativeBase, className)} {...props} ref={ref}>
+        <div ref={sentinelRef} style={{ position: 'absolute', inset: 0, pointerEvents: 'none' }} />
         {srcState.status === AsyncStatus.Success && (
           <Overlay open={viewer} backdrop={<OverlayBackdrop />}>
             <OverlayCenter>

src/app/components/message/content/VideoContent.tsx

@@ -1,4 +1,4 @@
-import React, { ReactNode, useCallback, useEffect, useState } from 'react';
+import React, { ReactNode, useCallback, useEffect, useRef, useState } from 'react';
 import {
   Badge,
   Box,
@@ -32,6 +32,7 @@ import {
 } from '../../../utils/matrix';
 import { useMediaAuthentication } from '../../../hooks/useMediaAuthentication';
 import { validBlurHash } from '../../../utils/blurHash';
+import { useNearViewport } from '../../../hooks/useNearViewport';
 
 type RenderVideoProps = {
   title: string;
@@ -79,6 +80,9 @@ export const VideoContent = as<'div', VideoContentProps>(
     const [error, setError] = useState(false);
     const [blurred, setBlurred] = useState(markedAsSpoiler ?? false);
 
+    const sentinelRef = useRef<HTMLDivElement>(null);
+    const nearViewport = useNearViewport(sentinelRef);
+
     const [srcState, loadSrc] = useAsyncCallback(
       useCallback(async () => {
         const mediaUrl = mxcUrlToHttp(mx, url, useAuthentication);
@@ -106,11 +110,12 @@ export const VideoContent = as<'div', VideoContentProps>(
     };
 
     useEffect(() => {
-      if (autoPlay) loadSrc().catch(() => undefined);
-    }, [autoPlay, loadSrc]);
+      if (autoPlay && nearViewport) loadSrc().catch(() => undefined);
+    }, [autoPlay, nearViewport, loadSrc]);
 
     return (
       <Box className={classNames(css.RelativeBase, className)} {...props} ref={ref}>
+        <div ref={sentinelRef} style={{ position: 'absolute', inset: 0, pointerEvents: 'none' }} />
         {typeof blurHash === 'string' && !load && (
           <BlurhashCanvas
             style={{ width: '100%', height: '100%' }}

src/app/hooks/useNearViewport.ts

@@ -0,0 +1,30 @@
+import { RefObject, useEffect, useState } from 'react';
+
+/**
+ * Returns true once the observed element has come within `margin` pixels of
+ * the viewport. Disconnects the observer after the first intersection so there
+ * is no ongoing overhead.
+ */
+export function useNearViewport(ref: RefObject<Element | null>, margin = 200): boolean {
+  const [triggered, setTriggered] = useState(false);
+
+  useEffect(() => {
+    const el = ref.current;
+    if (!el || triggered) return undefined;
+
+    const observer = new IntersectionObserver(
+      (entries) => {
+        if (entries[0]?.isIntersecting) {
+          setTriggered(true);
+          observer.disconnect();
+        }
+      },
+      { rootMargin: `${margin}px` },
+    );
+
+    observer.observe(el);
+    return () => observer.disconnect();
+  }, [ref, margin, triggered]);
+
+  return triggered;
+}