frontend/src/components/NicoViewer.test.tsx

@@ -0,0 +1,83 @@
+import { act, fireEvent, render } from '@testing-library/react'
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { createRef } from 'react'
+
+import NicoViewer from '@/components/NicoViewer'
+
+import type { NiconicoViewerHandle } from '@/types'
+
+
+describe ('NicoViewer', () => {
+  afterEach (() => {
+    vi.useRealTimers ()
+  })
+
+  it ('does not time out after metadata reports a playable duration', () => {
+    vi.useFakeTimers ()
+
+    const onError = vi.fn ()
+    const onMetadataChange = vi.fn ()
+    const { container } = render (
+        <NicoViewer
+            id="sm12345"
+            width={640}
+            height={360}
+            onMetadataChange={onMetadataChange}
+            onError={onError}/>,
+    )
+    const iframe = container.querySelector ('iframe')
+    expect (iframe).not.toBeNull ()
+
+    fireEvent.load (iframe!)
+    act (() => {
+      window.dispatchEvent (new MessageEvent ('message', {
+        origin: 'https://embed.nicovideo.jp',
+        source: iframe!.contentWindow,
+        data:   {
+          eventName: 'playerMetadataChange',
+          data:      {
+            currentTime:           7,
+            duration:              120,
+            isVideoMetaDataLoaded: true,
+            maximumBuffered:       30,
+            muted:                 false,
+            showComment:           true,
+            volume:                1,
+          },
+        },
+      }))
+    })
+
+    act (() => {
+      vi.advanceTimersByTime (8_000)
+    })
+
+    expect (onMetadataChange).toHaveBeenCalled ()
+    expect (onError).not.toHaveBeenCalled ()
+  })
+
+  it ('seeks with milliseconds', () => {
+    const ref = createRef<NiconicoViewerHandle> ()
+    const { container } = render (
+        <NicoViewer
+            ref={ref}
+            id="sm12345"
+            width={640}
+            height={360}/>,
+    )
+    const iframe = container.querySelector ('iframe')!
+    const postMessage = vi.spyOn (iframe.contentWindow!, 'postMessage')
+
+    act (() => {
+      ref.current!.seek (7_000)
+    })
+
+    expect (postMessage).toHaveBeenCalledWith (
+        expect.objectContaining ({
+          eventName: 'seek',
+          data:      { time: 7_000 },
+        }),
+        'https://embed.nicovideo.jp',
+    )
+  })
+})

frontend/src/components/NicoViewer.tsx

@@ -129,7 +129,9 @@ export default forwardRef ((props: Props, ref: ForwardedRef<NiconicoViewerHandle
   }, [playerId, postToPlayer])
 
   const seek = useCallback ((time: number) => {
-    postToPlayer ({ eventName: 'seek', sourceConnectorType: 1, playerId, data: { time } })
+    postToPlayer (
+	{ eventName: 'seek', sourceConnectorType: 1, playerId,
+	  data: { time } })
   }, [playerId, postToPlayer])
 
   const mute = useCallback (() => {
@@ -202,6 +204,9 @@ export default forwardRef ((props: Props, ref: ForwardedRef<NiconicoViewerHandle
 
       if (data.eventName === 'playerMetadataChange')
 	{
+	  if (Number.isFinite (data.data.duration) && data.data.duration > 0)
+	    clearLoadCompleteTimer ()
+
 	  onMetadataChange?.(data.data)
 	  return
 	}

frontend/src/components/PostEmbed.test.tsx

@@ -8,12 +8,19 @@ const dialogue = vi.hoisted (() => ({
   confirm: vi.fn (),
 }))
 
+const nicoViewer = vi.hoisted (() => ({
+  props: vi.fn (),
+}))
+
 vi.mock ('@/components/dialogues/DialogueProvider', () => ({
   useDialogue: () => dialogue,
 }))
 
 vi.mock ('@/components/NicoViewer', () => ({
-  default: ({ id }: { id: string }) => <div>Nico:{id}</div>,
+  default: (props: { id: string }) => {
+    nicoViewer.props (props)
+    return <div>Nico:{props.id}</div>
+  },
 }))
 
 vi.mock ('react-youtube', () => ({
@@ -31,6 +38,64 @@ describe ('PostEmbed', () => {
     expect (screen.getByText ('Nico:sm12345')).toBeInTheDocument ()
   })
 
+  it ('reports niconico metadata as milliseconds', () => {
+    const onVideoReady = vi.fn ()
+    const onPlaybackChange = vi.fn ()
+    render (
+        <PostEmbed
+            post={buildPost ({ url: 'https://www.nicovideo.jp/watch/sm12345' })}
+            onVideoReady={onVideoReady}
+            onPlaybackChange={onPlaybackChange}/>,
+    )
+
+    nicoViewer.props.mock.calls[0][0].onMetadataChange ({
+      currentTime:            7_000,
+      duration:               120_000,
+      isVideoMetaDataLoaded:  true,
+      maximumBuffered:        30,
+      muted:                  false,
+      showComment:            true,
+      volume:                 1,
+    })
+
+    expect (onVideoReady).toHaveBeenCalledWith (120_000)
+    expect (onPlaybackChange).toHaveBeenCalledWith (7_000)
+  })
+
+  it ('reports niconico video readiness only once', () => {
+    const onVideoReady = vi.fn ()
+    render (
+        <PostEmbed
+            post={buildPost ({ url: 'https://www.nicovideo.jp/watch/sm12345' })}
+            onVideoReady={onVideoReady}/>,
+    )
+
+    nicoViewer.props.mock.calls[0][0].onLoadComplete ({
+      title:           '動画',
+      videoId:         'sm12345',
+      lengthInSeconds: 120,
+      thumbnailUrl:    'https://example.com/thumb.jpg',
+      description:     '',
+      viewCount:       1,
+      commentCount:    2,
+      mylistCount:     3,
+      postedAt:        '2026-01-02T03:04:05.000Z',
+      watchId:         12345,
+    })
+    nicoViewer.props.mock.calls[0][0].onMetadataChange ({
+      currentTime:            7_000,
+      duration:               120_000,
+      isVideoMetaDataLoaded:  true,
+      maximumBuffered:        30,
+      muted:                  false,
+      showComment:            true,
+      volume:                 1,
+    })
+
+    expect (onVideoReady).toHaveBeenCalledTimes (1)
+    expect (onVideoReady).toHaveBeenCalledWith (120_000)
+  })
+
   it ('embeds x/twitter status URLs', () => {
     render (<PostEmbed post={buildPost ({ url: 'https://x.com/someone/status/12345' })}/>)
 

frontend/src/components/PostEmbed.tsx

@@ -1,4 +1,4 @@
-import { useCallback, useEffect, useState } from 'react'
+import { useCallback, useEffect, useRef, useState } from 'react'
 import YoutubeEmbed from 'react-youtube'
 
 import NicoViewer from '@/components/NicoViewer'
@@ -36,6 +36,17 @@ const PostEmbed: FC<Props> = ({
   const dialogue = useDialogue ()
   const [framed, setFramed] = useState (false)
   const [youtubePlayer, setYoutubePlayer] = useState<YouTubePlayer | null> (null)
+  const niconicoVideoReadyRef = useRef (false)
+
+  const notifyNiconicoVideoReady = useCallback ((durationMs: number) => {
+    if (niconicoVideoReadyRef.current
+	|| !(Number.isFinite (durationMs))
+	|| durationMs <= 0)
+      return
+
+    niconicoVideoReadyRef.current = true
+    onVideoReady?.(durationMs)
+  }, [onVideoReady])
 
   const reportYoutubePlayback = useCallback (async (player: YouTubePlayer) => {
     const currentTime = await player.getCurrentTime ()
@@ -80,15 +91,20 @@ const PostEmbed: FC<Props> = ({
   }
 
   const handleNiconicoLoadComplete = (info: NiconicoVideoInfo) => {
-    onVideoReady?.(info.lengthInSeconds * 1_000)
+    notifyNiconicoVideoReady (info.lengthInSeconds * 1_000)
     onLoadComplete?.(info)
   }
 
   const handleNiconicoMetadataChange = (meta: NiconicoMetadata) => {
+    notifyNiconicoVideoReady (meta.duration)
     onPlaybackChange?.(meta.currentTime)
     onMetadataChange?.(meta)
   }
 
+  useEffect (() => {
+    niconicoVideoReadyRef.current = false
+  }, [post.url])
+
   useEffect (() => {
     if (!(youtubePlayer) || !(onPlaybackChange))
       return

frontend/src/pages/theatres/TheatreDetailPage.test.tsx

@@ -1,4 +1,4 @@
-import { fireEvent, screen, waitFor } from '@testing-library/react'
+import { act, fireEvent, screen, waitFor } from '@testing-library/react'
 import { Route, Routes } from 'react-router-dom'
 import { beforeEach, describe, expect, it, vi } from 'vitest'
 
@@ -31,15 +31,31 @@ const dialogue = vi.hoisted (() => ({
   confirm: vi.fn (),
 }))
 
+const postEmbed = vi.hoisted (() => ({
+  props: vi.fn (),
+  play:  vi.fn (),
+  seek:  vi.fn (),
+}))
+
 vi.mock ('@/lib/api', () => api)
 vi.mock ('@/lib/posts', () => postsApi)
 vi.mock ('@/components/dialogues/DialogueProvider', () => ({
   useDialogue: () => dialogue,
 }))
 vi.mock ('@/components/PostEmbed', () => ({
-  default: ({ post }: {
+  default: (props: {
+    ref?: { current: unknown }
     post: { title: string | null; url: string }
-  }) => <div>Embed:{post.title || post.url}</div>,
+  }) => {
+    postEmbed.props (props)
+    if (props.ref)
+      props.ref.current = {
+        play: postEmbed.play,
+        seek: postEmbed.seek,
+      }
+
+    return <div>Embed:{props.post.title || props.post.url}</div>
+  },
 }))
 vi.mock ('@/components/PostEditForm', () => ({
   default: () => <div>Post edit form</div>,
@@ -153,6 +169,7 @@ const mockDefaultApi = () => {
 
 describe ('TheatreDetailPage', () => {
   beforeEach (() => {
+    vi.useRealTimers ()
     vi.clearAllMocks ()
     mockDefaultApi ()
   })
@@ -188,6 +205,83 @@ describe ('TheatreDetailPage', () => {
       .toBeInTheDocument ()
   })
 
+  it ('does not seek to zero while applying video length from the player', async () => {
+    api.apiPut.mockImplementation ((path: string) => {
+      switch (path)
+      {
+      case '/theatres/7/watching':
+        return Promise.resolve (buildTheatreInfo ({
+          hostFlg:       true,
+          postId:        currentPost.id,
+          postStartedAt: '2026-01-02T03:04:05.000Z',
+          postElapsedMs: 7_000,
+          watchingUsers: [{ id: 1, name: 'tester' }],
+          skipVote:      {
+            votesCount:         0,
+            requiredCount:      2,
+            watchingUsersCount: 1,
+            voted:              false,
+          },
+        }))
+
+      default:
+        return Promise.reject (new Error (`Unexpected PUT ${ path }`))
+      }
+    })
+
+    renderPage ()
+    await screen.findByText ('Embed:上映中の投稿')
+
+    const props = postEmbed.props.mock.calls.at (-1)![0]
+
+    act (() => {
+      props.onVideoReady (120_000)
+      props.onPlaybackChange (0)
+    })
+
+    const seekMs = postEmbed.seek.mock.calls[0][0]
+    expect (seekMs).toBeGreaterThanOrEqual (7_000)
+    expect (seekMs).toBeLessThan (10_000)
+    expect (postEmbed.seek).not.toHaveBeenCalledWith (0)
+  })
+
+  it ('does not advance host post while video length is unknown', async () => {
+    api.apiPut.mockImplementation ((path: string) => {
+      switch (path)
+      {
+      case '/theatres/7/watching':
+        return Promise.resolve (buildTheatreInfo ({
+          hostFlg:       true,
+          postId:        currentPost.id,
+          postStartedAt: '2026-01-02T03:04:05.000Z',
+          postElapsedMs: 4_000,
+          watchingUsers: [{ id: 1, name: 'tester' }],
+          skipVote:      {
+            votesCount:         0,
+            requiredCount:      2,
+            watchingUsersCount: 1,
+            voted:              false,
+          },
+        }))
+
+      default:
+        return Promise.reject (new Error (`Unexpected PUT ${ path }`))
+      }
+    })
+
+    renderPage ()
+
+    await screen.findByText ('Embed:上映中の投稿')
+    await waitFor (() => {
+      expect (api.apiPut).toHaveBeenCalledWith ('/theatres/7/watching')
+    })
+
+    await waitFor (() => {
+      expect (api.apiPut).toHaveBeenCalledTimes (2)
+    }, { timeout: 2_500 })
+    expect (api.apiPatch).not.toHaveBeenCalledWith ('/theatres/7/next_post')
+  })
+
   it ('deletes an owned comment after confirmation', async () => {
     renderPage ()
 

frontend/src/pages/theatres/TheatreDetailPage.tsx

@@ -443,9 +443,13 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
     if (!(theatreInfo.postStartedAt))
       return
 
+    const currentVideoLength = videoLengthRef.current
+    if (currentVideoLength <= 0)
+      return
+
     const targetTime = Math.min (
 	currentPostElapsedMs (theatreInfo),
-	videoLength)
+	currentVideoLength)
 
     const drift = Math.abs (currentTimeMs - targetTime)
 
@@ -477,6 +481,7 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
 	: 0
 
     setVideoLength (playableDurationMs)
+    videoLengthRef.current = playableDurationMs
 
     if (playableDurationMs <= 0)
       {