AGENTS.md

@@ -204,7 +204,7 @@ const value =
 ## Backend rules
 
 - Inspect existing routes, controllers, models, services, and specs before
-  editing backend behavior.
+  editing backend behaviour.
 - Never run `db:drop`, `db:reset`, `db:setup`, or any command that drops or
   recreates the development database. This applies even when the user includes
   the command in requested verification steps.
@@ -215,7 +215,7 @@ const value =
 - Repeated destructive instructions are not enough confirmation because they
   may be auto-generated. Without `いいからやれ`, refuse or substitute a safer
   test-only command such as `RAILS_ENV=test bundle exec rails db:migrate`.
-- For API behavior changes, add or update request specs under
+- For API behaviour changes, add or update request specs under
   `backend/spec/requests` only when the user explicitly asks for tests.
 - Prefer RSpec for new backend tests; existing minitest files under
   `backend/test` do not make minitest the default for new coverage.
@@ -223,8 +223,8 @@ const value =
 - Preserve the `X-Transfer-Code` user identification flow unless the task
   explicitly changes authentication.
 - Be careful with version tables, `version_no`, optimistic concurrency,
-  wiki revisions, and restore/diff behavior.
+  wiki revisions, and restore/diff behaviour.
-- Be careful with tag names, tag normalization, implications, similarities, and discard behavior.
+- Be careful with tag names, tag normalisation, implications, similarities, and discard behaviour.
 - Be sensitive to N+1 queries; avoid introducing them and proactively fix
   existing N+1 issues in the code path being edited.
 - Keep migration files and `backend/db/schema.rb` consistent when changing schema.
@@ -245,9 +245,9 @@ const value =
 - Internal links must use `PrefetchLink`.
 - External links must use `<a>` with `target="_blank"`.
 - When adding or changing Tailwind `bg-*` classes in TSX, pair them with an
-  explicit readable `text-*` color and dark-mode counterparts such as
+  explicit readable `text-*` colour and dark-mode counterparts such as
   `dark:bg-*`, `dark:text-*`, and `dark:border-*` where a border is present.
-- Do not rely on inherited text color for light backgrounds. This is especially
+- Do not rely on inherited text colour for light backgrounds. This is especially
   important for chips, cards, buttons, and panels that may inherit white text in
   dark mode.
 - Mobile UI must be checked as a first-class layout. Avoid wide fixed content,
@@ -355,9 +355,23 @@ const value =
 - Do not use a leading semicolon for expression statements such as
   `;([...]).forEach(...)`; rewrite the expression to avoid ASI hazards
   explicitly, for example with `void`.
-- Spell British English identifiers correctly when the feature name uses
+- Use correct British English spelling for new identifiers, filenames,
-  British English. Use `Behaviour`, not `Behavior`, for new component and file
+  component names, helper names, comments, and developer-facing prose unless
-  names unless editing an already established American-English API.
+  editing an already established American-English API that must keep its
+  existing spelling for compatibility.
+- Prefer British English spellings such as `behaviour`, `colour`, `realise`,
+  `theatre`, `centre`, `favourite`, `optimise`, and `catalogue`.
+- Avoid American or Canadian spellings such as `behavior`, `color`, `realize`,
+  `theater`, `center`, `favorite`, `optimize`, and `catalog`.
+- Even when an external library or API uses the wrong spelling, prefer to
+  correct it at the local boundary and use proper British English names in this
+  codebase. For example, prefer `import { color: colour } from '@external-lib'`
+  over spreading `color` through local code.
+- Apply the same boundary correction to object destructuring, wrapper helpers,
+  adapter layers, and local variable names when doing so does not break the
+  external contract.
+- For this repository, prefer names such as `BehaviourSettingsSection.tsx`, not
+  `BehaviorSettingsSection.tsx`.
 
 ### Frontend delimiter decision table
 
@@ -740,7 +754,7 @@ function PostFormTagsArea ({ tags, setTags }: Props) {
 - Do not scan or summarize dependency/generated/runtime directories such as
   `node_modules`, `dist`, `tmp`, `log`, and `storage` unless explicitly needed.
 - Before touching wiki, tag, versioning, BAN, IP BAN, or authentication
-  behavior, inspect the related request specs and service objects.
+  behaviour, inspect the related request specs and service objects.
 - If frontend code changes, run only non-test verification commands that
   apply, such as `npm run build` and `npm run lint`. Run `npm run test:run`
   only when the user explicitly asks for tests.
@@ -757,4 +771,4 @@ A task is complete only when:
   explained,
 - unrelated files are not changed,
 - migrations and schema are consistent when schema changes are made,
-- user-facing behavior is documented when needed.
+- user-facing behaviour is documented when needed.

frontend/AGENTS.md

@@ -118,7 +118,7 @@ pass or the remaining failure is clearly blocked.
   short Japanese labels that fit the control.
 - Preserve existing Japanese tone and orthography in nearby UI text, including
   old-kana wording where the file already uses it.
-- When adding dynamic tag color classes, update `tailwind.config.js` safelist
+- When adding dynamic tag colour classes, update `tailwind.config.js` safelist
   if the class cannot be statically detected.
 - Do not introduce new UI libraries or production dependencies without approval.
 
@@ -192,9 +192,24 @@ pass or the remaining failure is clearly blocked.
   single physical line.
 - Always add braces around `if`, `else`, or `for` bodies when the body spans
   two or more physical lines, even if it is one statement.
-- Spell British English identifiers correctly when the feature name uses
+- Use correct British English spelling for new identifiers, filenames,
-  British English. Use `Behaviour`, not `Behavior`, for new component and file
+  component names, helper names, comments, and developer-facing prose unless
-  names unless editing an already established American-English API.
+  editing an already established American-English API that must keep its
+  existing spelling for compatibility.
+- Prefer British English spellings such as `behaviour`, `colour`, `realise`,
+  `theatre`, `centre`, `favourite`, `optimise`, and `catalogue`.
+- Avoid American or Canadian spellings such as `behavior`, `color`, `realize`,
+  `theater`, `center`, `favorite`, `optimize`, and `catalog`.
+- Even when an external library or API uses the wrong spelling, prefer to
+  correct it at the local boundary and use proper British English names in this
+  frontend codebase. For example, prefer
+  `import { color: colour } from '@external-lib'` over spreading `color`
+  through local code.
+- Apply the same boundary correction to object destructuring, wrapper helpers,
+  adapter layers, and local variable names when doing so does not break the
+  external contract.
+- In this frontend, prefer names such as `BehaviourSettingsSection.tsx`, not
+  `BehaviorSettingsSection.tsx`.
 - Avoid reformatting unrelated JSX.
 
 ### Delimiter decision table

frontend/src/App.tsx

@@ -1,5 +1,5 @@
-import { AnimatePresence, LayoutGroup, motion } from 'framer-motion'
+import { AnimatePresence, LayoutGroup, MotionConfig, motion } from 'framer-motion'
-import { useEffect, useState } from 'react'
+import { useEffect, useMemo, useState } from 'react'
 import { BrowserRouter,
 	 Navigate,
 	 Route,
@@ -12,12 +12,14 @@ import TopNav from '@/components/TopNav'
 import DialogueProvider from '@/components/dialogues/DialogueProvider'
 import { Toaster } from '@/components/ui/toaster'
 import { apiPost, isApiError } from '@/lib/api'
-import {
+import { applyClientAppearance,
-    applyClientAppearance,
+	 CLIENT_SETTINGS_EVENT,
-    fetchUserSettings,
+	 fetchUserSettings,
-    getClientThemeMode,
+	 getClientAnimationMode,
-    seedClientThemeMode,
+	 getClientThemeMode,
-} from '@/lib/settings'
+	 seedClientThemeMode } from '@/lib/settings'
+
+import type { ClientAnimationMode } from '@/lib/settings'
 import { KeyboardShortcutsProvider } from '@/lib/useKeyboardShortcuts'
 import DeerjikistDetailPage from '@/pages/deerjikists/DeerjikistDetailPage'
 import MaterialBasePage from '@/pages/materials/MaterialBasePage'
@@ -110,6 +112,19 @@ const PostDetailRoute = ({ user }: { user: User | null }) => {
 const App: FC = () => {
   const [user, setUser] = useState<User | null> (null)
   const [status, setStatus] = useState (200)
+  const [animationMode, setAnimationMode] =
+      useState<ClientAnimationMode> (() => getClientAnimationMode ())
+
+  const appLayoutTransition = useMemo (
+      () => (
+	  animationMode === 'off'
+	  ? { duration: 0 }
+	  : animationMode === 'reduced'
+	    ? { duration: .08, ease: 'linear' as const }
+	    : { duration: .2, ease: 'easeOut' as const }
+      ),
+      [animationMode],
+  )
 
   useEffect (() => {
     applyClientAppearance ()
@@ -124,6 +139,16 @@ const App: FC = () => {
     return () => mediaQuery.removeEventListener ('change', handleThemeChange)
   }, [])
 
+  useEffect (() => {
+    const handleClientSettingsChange = () => {
+      setAnimationMode (getClientAnimationMode ())
+    }
+
+    window.addEventListener (CLIENT_SETTINGS_EVENT, handleClientSettingsChange)
+    return () =>
+	window.removeEventListener (CLIENT_SETTINGS_EVENT, handleClientSettingsChange)
+  }, [])
+
   useEffect (() => {
     const createUser = async () => {
       const data = await apiPost<{ code: string; user: User }> ('/users')
@@ -170,6 +195,7 @@ const App: FC = () => {
       }
       catch
       {
+	return
       }
     }) ()
   }, [user])
@@ -188,15 +214,24 @@ const App: FC = () => {
 	<BrowserRouter>
 	  <KeyboardShortcutsProvider>
 	    <DialogueProvider>
-	      <LayoutGroup>
+	      <MotionConfig
-		<motion.div
+		  reducedMotion={
-		    layout="position"
+		      animationMode === 'normal'
-		    transition={{ layout: { duration: .2, ease: 'easeOut' } }}
+		      ? 'never'
-		    className="relative flex flex-col h-dvh w-full overflow-y-hidden">
+		      : animationMode === 'reduced'
-		  <TopNav user={user}/>
+			? 'user'
-		  <RouteTransitionWrapper user={user} setUser={setUser}/>
+			: 'always'
-		</motion.div>
+		  }>
-	      </LayoutGroup>
+		<LayoutGroup>
+		  <motion.div
+		      layout={animationMode === 'off' ? false : 'position'}
+		      transition={{ layout: appLayoutTransition }}
+		      className="relative flex flex-col h-dvh w-full overflow-y-hidden">
+		    <TopNav user={user}/>
+		    <RouteTransitionWrapper user={user} setUser={setUser}/>
+		  </motion.div>
+		</LayoutGroup>
+	      </MotionConfig>
 
 	      <Toaster/>
 	    </DialogueProvider>

frontend/src/components/PostEmbed.tsx

@@ -4,6 +4,8 @@ import YoutubeEmbed from 'react-youtube'
 import NicoViewer from '@/components/NicoViewer'
 import TwitterEmbed from '@/components/TwitterEmbed'
 import { useDialogue } from '@/components/dialogues/DialogueProvider'
+import { Button } from '@/components/ui/button'
+import { getClientEmbedAutoLoadMode } from '@/lib/settings'
 
 import type { FC, RefObject } from 'react'
 
@@ -24,16 +26,18 @@ type Props = {
     onError?:           (data: unknown) => void }
 
 
-const PostEmbed: FC<Props> = ({
+const PostEmbed: FC<Props> = (
-    ref,
+    { ref,
-    post,
+      post,
-    onLoadComplete,
+      onLoadComplete,
-    onMetadataChange,
+      onMetadataChange,
-    onVideoReady,
+      onVideoReady,
-    onPlaybackChange,
+      onPlaybackChange,
-    onError,
+      onError },
-}) => {
+) => {
   const dialogue = useDialogue ()
+  const embedAutoLoadMode = getClientEmbedAutoLoadMode ()
+  const [manualLoadRequested, setManualLoadRequested] = useState (false)
   const [framed, setFramed] = useState (false)
   const [youtubePlayer, setYoutubePlayer] = useState<YouTubePlayer | null> (null)
   const niconicoVideoReadyRef = useRef (false)
@@ -105,6 +109,10 @@ const PostEmbed: FC<Props> = ({
     niconicoVideoReadyRef.current = false
   }, [post.url])
 
+  useEffect (() => {
+    setManualLoadRequested (false)
+  }, [embedAutoLoadMode, post.url])
+
   useEffect (() => {
     if (!(youtubePlayer) || !(onPlaybackChange))
       return
@@ -117,6 +125,28 @@ const PostEmbed: FC<Props> = ({
   }, [onPlaybackChange, reportYoutubePlayback, youtubePlayer])
 
   const url = new URL (post.url)
+  const shouldLoadEmbed =
+      embedAutoLoadMode === 'auto'
+      || (
+	  embedAutoLoadMode === 'manual'
+	  && manualLoadRequested
+      )
+
+  const externalLink = (
+      <a href={post.url} target="_blank" rel="noreferrer">
+	外部ページを開く
+      </a>)
+
+  const manualLoadControl = (
+      <div className="flex flex-wrap items-center gap-3">
+	<Button
+	    type="button"
+	    variant="outline"
+	    onClick={() => setManualLoadRequested (true)}>
+	  埋め込みを読込
+	</Button>
+	{externalLink}
+      </div>)
 
   switch (url.hostname.split ('.').slice (-2).join ('.'))
   {
@@ -128,6 +158,11 @@ const PostEmbed: FC<Props> = ({
 
       const [videoId] = mVideoId
 
+      if (embedAutoLoadMode === 'off')
+	return externalLink
+      if (shouldLoadEmbed === false)
+	return manualLoadControl
+
       return (
 	  <NicoViewer
 	      ref={ref}
@@ -150,6 +185,11 @@ const PostEmbed: FC<Props> = ({
       const [userId] = mUserId
       const [statusId] = mStatusId
 
+      if (embedAutoLoadMode === 'off')
+	return externalLink
+      if (shouldLoadEmbed === false)
+	return manualLoadControl
+
       return <TwitterEmbed userId={userId} statusId={statusId}/>
     }
 
@@ -159,6 +199,11 @@ const PostEmbed: FC<Props> = ({
       if (!(videoId))
 	break
 
+      if (embedAutoLoadMode === 'off')
+	return externalLink
+      if (shouldLoadEmbed === false)
+	return manualLoadControl
+
       return (
 	  <YoutubeEmbed videoId={videoId} opts={{ playerVars: {
 	      playsinline:  1,
@@ -184,6 +229,11 @@ const PostEmbed: FC<Props> = ({
 		height={360}/>)
 	 : (
 	    <div>
+	      {embedAutoLoadMode === 'off'
+	       ? externalLink
+	       : embedAutoLoadMode === 'manual' && shouldLoadEmbed === false
+		 ? manualLoadControl
+		 : (
 	      <a href="#" onClick={async e => {
 		e.preventDefault ()
 
@@ -197,7 +247,7 @@ const PostEmbed: FC<Props> = ({
 		    confirmText: '表示' }))
 	      }}>
 		外部ページを表示
-	      </a>
+	      </a>)}
 	    </div>)}
       </>)
 }

frontend/src/components/PostList.tsx

@@ -3,6 +3,7 @@ import { useRef } from 'react'
 import { useLocation } from 'react-router-dom'
 
 import PrefetchLink from '@/components/PrefetchLink'
+import { getClientThumbnailMode } from '@/lib/settings'
 import { cn } from '@/lib/utils'
 import { useSharedTransitionStore } from '@/stores/sharedTransitionStore'
 
@@ -16,6 +17,7 @@ type Props = { posts:     Post[]
 
 const PostList: FC<Props> = ({ posts, onClick }) => {
   const location = useLocation ()
+  const thumbnailMode = getClientThumbnailMode ()
 
   const setForLocationKey = useSharedTransitionStore (s => s.setForLocationKey)
 
@@ -54,18 +56,32 @@ const PostList: FC<Props> = ({ posts, onClick }) => {
 		    }}
 		    onLayoutAnimationComplete={() => {
 		      if (!(cardRef.current))
-			return 
+			return
 
 		      cardRef.current.style.zIndex = ''
 		      cardRef.current.style.position = ''
 		    }}
 		    transition={{ layout: { duration: .2, ease: 'easeOut' } }}>
-		  <img src={post.thumbnail || post.thumbnailBase || undefined}
+		  {thumbnailMode === 'off'
-		       alt={post.title || post.url}
+		   ? (
-		       title={post.title || post.url || undefined}
+		      <div className="flex h-full w-full items-center justify-center
-		       loading={i < 12 ? 'eager' : 'lazy'}
+				      bg-muted text-center text-xs text-muted-foreground">
-		       decoding="async"
+			サムネイルなし
-		       className="object-cover w-full h-full"/>
+		      </div>)
+		   : (
+		      <img
+			  src={post.thumbnail || post.thumbnailBase || undefined}
+			  alt={post.title || post.url}
+			  title={post.title || post.url || undefined}
+			  loading={
+			      thumbnailMode === 'light'
+			      ? 'lazy'
+			      : i < 12
+				? 'eager'
+				: 'lazy'
+			  }
+			  decoding="async"
+			  className="object-cover w-full h-full"/>)}
 		</motion.div>
 	      </PrefetchLink>)
 	})}

frontend/src/components/users/BehaviorSettingsSection.tsx

@@ -1,143 +0,0 @@
-import { useEffect, useMemo, useState } from 'react'
-
-import FormField from '@/components/common/FormField'
-import { Button } from '@/components/ui/button'
-import { toast } from '@/components/ui/use-toast'
-import {
-    getClientBehaviorSettings,
-    setClientBehaviorSettings,
-} from '@/lib/settings'
-import { inputClass } from '@/lib/utils'
-
-import type {
-    ClientAnimationMode,
-    ClientBehaviorSettings,
-    ClientEmbedAutoLoadMode,
-    ClientThumbnailMode,
-} from '@/lib/settings'
-import type { FC } from 'react'
-
-type Props = {
-    sectionClassName: string
-}
-
-
-const BehaviorSettingsSection: FC<Props> = ({ sectionClassName }) => {
-  const [savedSettings, setSavedSettings] =
-      useState<ClientBehaviorSettings> (() => getClientBehaviorSettings ())
-  const [draftSettings, setDraftSettings] =
-      useState<ClientBehaviorSettings> (() => getClientBehaviorSettings ())
-
-  useEffect (() => {
-    const current = getClientBehaviorSettings ()
-    setSavedSettings (current)
-    setDraftSettings (current)
-  }, [])
-
-  const hasUnsavedChanges = useMemo (
-      () => JSON.stringify (draftSettings) !== JSON.stringify (savedSettings),
-      [draftSettings, savedSettings],
-  )
-
-  const updateDraft = <Key extends keyof ClientBehaviorSettings,> (
-      key:    Key,
-      value:  ClientBehaviorSettings[Key],
-  ) => {
-    setDraftSettings (current => ({ ...current, [key]: value }))
-  }
-
-  const handleSave = () => {
-    const next = setClientBehaviorSettings (draftSettings)
-    setSavedSettings (next)
-    setDraftSettings (next)
-    toast ({ title: '動作設定を保存しました' })
-  }
-
-  const handleDiscard = () => {
-    setDraftSettings (savedSettings)
-  }
-
-  return (
-      <section className={sectionClassName}>
-	<div className="flex flex-wrap items-start justify-between gap-3">
-	  <div className="space-y-1">
-	    <h2 className="text-xl font-bold">動作</h2>
-	    <p className="text-sm text-muted-foreground">
-	      この設定は現在のブラウザに保存されます。
-	    </p>
-	    <p className="text-sm text-muted-foreground">
-	      重い端末や通信量を抑えたい場合に調整します。
-	    </p>
-	    {hasUnsavedChanges && (
-		<p className="text-sm text-amber-700 dark:text-amber-300">
-		  未保存の変更があります
-		</p>)}
-	  </div>
-
-	  <div className="flex flex-wrap gap-2">
-	    <Button
-		type="button"
-		variant="outline"
-		disabled={!(hasUnsavedChanges)}
-		onClick={handleDiscard}>
-	      変更を破棄
-	    </Button>
-	    <Button
-		type="button"
-		disabled={!(hasUnsavedChanges)}
-		onClick={handleSave}>
-	      保存
-	    </Button>
-	  </div>
-	</div>
-
-	<div className="grid gap-4 md:grid-cols-2">
-	  <FormField label="アニメーション">
-	    {() => (
-		<select
-		    className={inputClass (false)}
-		    value={draftSettings.animation ?? 'normal'}
-		    onChange={ev => updateDraft (
-			'animation',
-			ev.target.value as ClientAnimationMode,
-		    )}>
-		  <option value="normal">通常</option>
-		  <option value="reduced">控えめ</option>
-		  <option value="off">なし</option>
-		</select>)}
-	  </FormField>
-
-	  <FormField label="埋め込み自動読込">
-	    {() => (
-		<select
-		    className={inputClass (false)}
-		    value={draftSettings.embedAutoLoad ?? 'auto'}
-		    onChange={ev => updateDraft (
-			'embedAutoLoad',
-			ev.target.value as ClientEmbedAutoLoadMode,
-		    )}>
-		  <option value="auto">自動</option>
-		  <option value="manual">クリック時</option>
-		  <option value="off">読み込まない</option>
-		</select>)}
-	  </FormField>
-
-	  <FormField label="サムネイル表示">
-	    {() => (
-		<select
-		    className={inputClass (false)}
-		    value={draftSettings.thumbnailMode ?? 'normal'}
-		    onChange={ev => updateDraft (
-			'thumbnailMode',
-			ev.target.value as ClientThumbnailMode,
-		    )}>
-		  <option value="normal">通常</option>
-		  <option value="light">軽量</option>
-		  <option value="off">非表示</option>
-		</select>)}
-	  </FormField>
-	</div>
-      </section>)
-}
-
-export default BehaviorSettingsSection

frontend/src/components/users/BehaviourSettingsSection.tsx

@@ -0,0 +1,192 @@
+import { useEffect, useMemo, useState } from 'react'
+
+import { Button } from '@/components/ui/button'
+import { toast } from '@/components/ui/use-toast'
+import { getClientBehaviorSettings,
+	 setClientBehaviorSettings } from '@/lib/settings'
+import { cn } from '@/lib/utils'
+
+import type {
+    ClientAnimationMode,
+    ClientBehaviorSettings,
+    ClientEmbedAutoLoadMode,
+    ClientThumbnailMode } from '@/lib/settings'
+import type { FC, ReactNode } from 'react'
+
+type Props = {
+    sectionClassName: string }
+
+type SegmentedOption<T extends string> = {
+    value: T
+    label: string }
+
+type SegmentedControlProps<T extends string> = {
+    value:     T
+    options:   SegmentedOption<T>[]
+    onChange:  (value: T) => void }
+
+type SettingBlockProps = {
+    title:       string
+    description: string
+    children:    ReactNode }
+
+const animationOptions: SegmentedOption<ClientAnimationMode>[] = [
+    { value: 'normal', label: '通常' },
+    { value: 'reduced', label: '控えめ' },
+    { value: 'off', label: 'なし' },
+]
+
+const embedAutoLoadOptions: SegmentedOption<ClientEmbedAutoLoadMode>[] = [
+    { value: 'auto', label: '自動' },
+    { value: 'manual', label: 'クリック時' },
+    { value: 'off', label: '読み込まない' },
+]
+
+const thumbnailModeOptions: SegmentedOption<ClientThumbnailMode>[] = [
+    { value: 'normal', label: '通常' },
+    { value: 'light', label: '軽量' },
+    { value: 'off', label: '非表示' },
+]
+
+
+const SegmentedControl = <T extends string,>(
+    { value, options, onChange }: SegmentedControlProps<T>,
+) => {
+  return (
+      <div
+	  role="radiogroup"
+	  className="flex flex-wrap gap-2 rounded-xl border border-border bg-muted/40 p-1">
+	{options.map (option => (
+	    <button
+		key={option.value}
+		type="button"
+		role="radio"
+		aria-checked={value === option.value}
+		className={cn (
+		    'min-w-0 flex-1 rounded-lg px-3 py-2 text-sm font-medium',
+		    'transition-colors focus-visible:outline-none focus-visible:ring-2',
+		    'focus-visible:ring-ring focus-visible:ring-offset-2',
+		    value === option.value
+		    ? 'bg-background text-foreground shadow-sm'
+		    : 'text-muted-foreground hover:bg-background/70 hover:text-foreground',
+		)}
+		onClick={() => onChange (option.value)}>
+	      {option.label}
+	    </button>))}
+      </div>)
+}
+
+
+const SettingBlock: FC<SettingBlockProps> = (
+    { title, description, children },
+) => (
+    <div className="space-y-3 rounded-xl border border-border/70 bg-background/70 p-4">
+      <div className="space-y-1">
+	<h3 className="text-sm font-semibold">{title}</h3>
+	<p className="text-sm text-muted-foreground">{description}</p>
+      </div>
+      {children}
+    </div>)
+
+
+const BehaviourSettingsSection: FC<Props> = ({ sectionClassName }) => {
+  const [savedSettings, setSavedSettings] =
+      useState<ClientBehaviorSettings> (() => getClientBehaviorSettings ())
+  const [draftSettings, setDraftSettings] =
+      useState<ClientBehaviorSettings> (() => getClientBehaviorSettings ())
+
+  useEffect (() => {
+    const current = getClientBehaviorSettings ()
+    setSavedSettings (current)
+    setDraftSettings (current)
+  }, [])
+
+  const hasUnsavedChanges = useMemo (
+      () => JSON.stringify (draftSettings) !== JSON.stringify (savedSettings),
+      [draftSettings, savedSettings],
+  )
+
+  const updateDraft = <Key extends keyof ClientBehaviorSettings,> (
+      key:    Key,
+      value:  ClientBehaviorSettings[Key],
+  ) => {
+    setDraftSettings (current => ({ ...current, [key]: value }))
+  }
+
+  const handleSave = () => {
+    const next = setClientBehaviorSettings (draftSettings)
+    setSavedSettings (next)
+    setDraftSettings (next)
+    toast ({ title: '動作設定を保存しました' })
+  }
+
+  const handleDiscard = () => {
+    setDraftSettings (savedSettings)
+  }
+
+  return (
+      <section className={sectionClassName}>
+	<div className="flex flex-wrap items-start justify-between gap-3">
+	  <div className="space-y-1">
+	    <h2 className="text-xl font-bold">動作</h2>
+	    <p className="text-sm text-muted-foreground">
+	      この設定は現在のブラウザに保存されます。
+	    </p>
+	    <p className="text-sm text-muted-foreground">
+	      重い端末や通信量を抑えたい場合に調整します。
+	    </p>
+	    {hasUnsavedChanges && (
+		<p className="text-sm text-amber-700 dark:text-amber-300">
+		  未保存の変更があります
+		</p>)}
+	  </div>
+
+	  <div className="flex flex-wrap gap-2">
+	    <Button
+		type="button"
+		variant="outline"
+		disabled={hasUnsavedChanges === false}
+		onClick={handleDiscard}>
+	      変更を破棄
+	    </Button>
+	    <Button
+		type="button"
+		disabled={hasUnsavedChanges === false}
+		onClick={handleSave}>
+	      保存
+	    </Button>
+	  </div>
+	</div>
+
+	<div className="space-y-4">
+	  <SettingBlock
+	      title="アニメーション"
+	      description="画面遷移やレイアウト移動の動きを調整します。">
+	    <SegmentedControl
+		value={draftSettings.animation ?? 'normal'}
+		options={animationOptions}
+		onChange={value => updateDraft ('animation', value)}/>
+	  </SettingBlock>
+
+	  <SettingBlock
+	      title="埋め込み自動読込"
+	      description="外部埋め込みを自動で読み込むかを切り替えます。">
+	    <SegmentedControl
+		value={draftSettings.embedAutoLoad ?? 'auto'}
+		options={embedAutoLoadOptions}
+		onChange={value => updateDraft ('embedAutoLoad', value)}/>
+	  </SettingBlock>
+
+	  <SettingBlock
+	      title="サムネイル表示"
+	      description="投稿一覧や詳細の画像表示を軽量化します。">
+	    <SegmentedControl
+		value={draftSettings.thumbnailMode ?? 'normal'}
+		options={thumbnailModeOptions}
+		onChange={value => updateDraft ('thumbnailMode', value)}/>
+	  </SettingBlock>
+	</div>
+      </section>)
+}
+
+export default BehaviourSettingsSection

frontend/src/lib/settings.ts

@@ -112,6 +112,7 @@ export type ClientSettings = {
     thumbnailMode?: string }
 
 export const CLIENT_SETTINGS_STORAGE_KEY = 'btrc_hub.client_settings'
+export const CLIENT_SETTINGS_EVENT = 'btrc-hub:client-settings-change'
 export const THEME_OPTIONS = ['system', 'light', 'dark'] as const
 export const LIST_LIMIT_OPTIONS = [20, 50, 100] as const
 export const POST_LIST_ORDER_OPTIONS = [
@@ -297,6 +298,13 @@ export const loadClientSettings = (): ClientSettings =>
 
 export const saveClientSettings = (settings: ClientSettings): void => {
   localStorage.setItem (CLIENT_SETTINGS_STORAGE_KEY, JSON.stringify (settings))
+
+  if (typeof window !== 'undefined')
+    {
+      window.dispatchEvent (new CustomEvent (CLIENT_SETTINGS_EVENT, {
+	  detail: settings,
+      }))
+    }
 }
 
 

frontend/src/pages/posts/PostDetailPage.tsx

@@ -15,6 +15,7 @@ import { SITE_TITLE } from '@/config'
 import { isApiError } from '@/lib/api'
 import { fetchPost, toggleViewedFlg } from '@/lib/posts'
 import { postsKeys, tagsKeys } from '@/lib/queryKeys'
+import { getClientThumbnailMode } from '@/lib/settings'
 import { canEditContent } from '@/lib/users'
 import { cn } from '@/lib/utils'
 import NotFound from '@/pages/NotFound'
@@ -29,6 +30,7 @@ type Props = { user: User | null }
 
 const PostDetailPage: FC<Props> = ({ user }) => {
   const editable = canEditContent (user)
+  const thumbnailMode = getClientThumbnailMode ()
   const { id } = useParams ()
   const postId = String (id ?? '')
   const postKey = postsKeys.show (postId)
@@ -134,7 +136,7 @@ const PostDetailPage: FC<Props> = ({ user }) => {
 		      </div>)
 		})}
 
-		{(post.thumbnail || post.thumbnailBase) && (
+		{thumbnailMode !== 'off' && (post.thumbnail || post.thumbnailBase) && (
 		    <motion.div
 			layoutId={`page-${ id }`}
 			className="absolute top-4 left-4 w-[min(640px,calc(100vw-2rem))] h-[360px]
@@ -145,6 +147,8 @@ const PostDetailPage: FC<Props> = ({ user }) => {
 		      <img src={post.thumbnail || post.thumbnailBase || undefined}
 			   alt={post.title || post.url}
 			   title={post.title || post.url || undefined}
+			   loading={thumbnailMode === 'light' ? 'lazy' : 'eager'}
+			   decoding="async"
 			   className="object-cover w-full h-full"/>
 		    </motion.div>)}
 

frontend/src/pages/users/SettingPage.tsx

@@ -8,7 +8,7 @@ import Label from '@/components/common/Label'
 import PageTitle from '@/components/common/PageTitle'
 import MainArea from '@/components/layout/MainArea'
 import TagLink from '@/components/TagLink'
-import BehaviorSettingsSection from '@/components/users/BehaviorSettingsSection'
+import BehaviourSettingsSection from '@/components/users/BehaviourSettingsSection'
 import KeyboardSettingsSection from '@/components/users/KeyboardSettingsSection'
 import InheritDialogue from '@/components/users/InheritDialogue'
 import UserCodeDialogue from '@/components/users/UserCodeDialogue'
@@ -259,7 +259,7 @@ const ThemeSection: FC<SharedSectionProps> = (
       <div className="space-y-3">
 	<div className="flex flex-wrap items-center justify-between gap-2">
 	  <div>
-	  <h3 className="font-medium">タグカテゴリ色</h3>
+	    <h3 className="font-medium">タグカテゴリ色</h3>
 	  </div>
 	  <Button
 	      type="button"
@@ -270,7 +270,7 @@ const ThemeSection: FC<SharedSectionProps> = (
 	  </Button>
 	</div>
 
-	<div className="grid gap-3 sm:grid-cols-2">
+	<div className="space-y-3">
 	  {CATEGORIES.map (category => {
 	      const resolvedDefaults =
 		  defaultThemeTagColoursFor (selectedTheme.baseTheme)
@@ -280,9 +280,19 @@ const ThemeSection: FC<SharedSectionProps> = (
 	      return (
 		  <div
 		      key={category}
-		      className="space-y-2 rounded-lg border border-border/70 px-3 py-2">
+		      className="flex flex-col gap-3 rounded-lg border border-border/70 px-3 py-3
-		    <div className="flex items-center justify-between gap-3">
+				 sm:flex-row sm:items-center sm:justify-between">
-		      <span className="text-sm">{CATEGORY_NAMES[category]}</span>
+		    <div className="space-y-2">
+		      <p className="text-sm font-medium">{CATEGORY_NAMES[category]}</p>
+		      <div style={themePreviewStyle (draftTagColours)}>
+			<TagLink
+			    tag={previewTags[category]}
+			    linkFlg={false}
+			    truncateOnMobile
+			    withWiki={false}/>
+		      </div>
+		    </div>
+		    <div className="flex flex-wrap items-center gap-3">
 		      <input
 			  type="color"
 			  value={draftTagColours[category]}
@@ -291,8 +301,6 @@ const ThemeSection: FC<SharedSectionProps> = (
 			  onChange={ev => onTagColourChange (category, ev.target.value)}
 			  className="h-10 w-16 cursor-pointer rounded border border-border
 				     bg-transparent p-1"/>
-		    </div>
-		    <div className="flex justify-end">
 		      <Button
 			  type="button"
 			  variant="ghost"
@@ -693,7 +701,7 @@ const SettingPage: FC<Props> = ({ user, setUser }) => {
 		{activeTab === 'keyboard' && (
 		    <KeyboardSettingsSection sectionClassName={sectionClassName}/>)}
 		{activeTab === 'behavior' && (
-		    <BehaviorSettingsSection sectionClassName={sectionClassName}/>)}
+		    <BehaviourSettingsSection sectionClassName={sectionClassName}/>)}
 	      </div>)}
 	</div>
 
@@ -742,7 +750,7 @@ const SettingPage: FC<Props> = ({ user, setUser }) => {
 			  {activeTab === 'keyboard' && (
 			      <KeyboardSettingsSection sectionClassName={sectionClassName}/>)}
 			  {activeTab === 'behavior' && (
-			      <BehaviorSettingsSection sectionClassName={sectionClassName}/>)}
+			      <BehaviourSettingsSection sectionClassName={sectionClassName}/>)}
 			</>)}
 		  </div>)}
 	    </div>