#34

AGENTS.md

@@ -362,6 +362,37 @@ 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`.
+- Do not insert `void` mechanically before ordinary event-handler or callback
+  calls merely because the callee returns a `Promise`. If the surrounding code
+  already uses a plain expression statement such as
+  `onClick={() => requestActiveTab ('theme')}`, preserve that style unless a
+  local rule or type requirement truly forces `void`. Adding unnecessary
+  `void` is a medium-severity style violation.
+- When fixing formatting, never change operator binding, evaluation order,
+  tuple/comma behaviour, or any other expression structure. A formatting pass
+  that changes how `!==`, `??`, `?:`, `&&`, `||`, or `,` binds is a capital
+  offence.
+- In TypeScript and TSX, do not mechanically verticalise short call tails,
+  short object literals, or short callback bodies that already fit the local
+  line-length and delimiter rules. Expanding compact local code without a
+  readability need is a minor style violation.
+- In TypeScript and TSX, treat short object literals, short returned objects,
+  short typed object shapes, and short function-call object arguments as
+  compact associative syntax by default. If they fit locally, keep the opening
+  `{` on the same line as the first property and keep the closing `}` on the
+  same line as the final property. Do not explode them into vertical
+  Prettier-style blocks without a concrete readability or line-length reason.
+- In TypeScript and TSX, when a compact associative form is broken across two
+  lines, keep the opening `{` with the first property and the closing `}` with
+  the final property. A shape like `{\n    key: value,\n}` is wrong even when
+  line length would permit it.
+- In TypeScript and TSX, when a short call such as `apiGet<...> (...)`,
+  `apiPatch<...> (...)`, `setState (...)`, `updateClientSettings (...)`, or
+  `window.dispatchEvent (...)` already fits within the local style, do not
+  add extra vertical layers around the object argument or callback body.
+- In TypeScript and TSX, after editing a file, inspect the file tail and
+  remove meaningless blank lines before `export default` or the end of file.
+  A stray empty line before `export default` is a minor style offence.
 - Use correct British English spelling for new identifiers, filenames,
   component names, helper names, comments, and developer-facing prose unless
   editing an already established American-English API that must keep its
@@ -380,6 +411,168 @@ const value =
 - For this repository, prefer names such as `BehaviourSettingsSection.tsx`, not
   `BehaviorSettingsSection.tsx`.
 
+### Additional style offences and penalties
+
+Use this section for recurrent failure modes that are easy to miss when
+applying the delimiter rules mechanically.
+
+#### Unnecessary `void` before ordinary callbacks
+
+Bad:
+
+```ts
+onClick={() => void requestActiveTab ('theme')}
+```
+
+Good:
+
+```ts
+onClick={() => requestActiveTab ('theme')}
+```
+
+Penalty: medium-severity offence. The code still works, but it pollutes the
+local style and usually signals that the edit was made mechanically rather than
+by reading the surrounding code.
+
+#### Formatting that changes expression structure
+
+Bad:
+
+```ts
+const hasUnsavedChanges =
+    serialiseThemeDraft (draftThemeSlots)
+    !== (serialiseThemeDraft (savedThemeSlots),
+	 draftActiveThemeMode !== savedThemeMode)
+```
+
+Good:
+
+```ts
+const hasUnsavedChanges =
+    serialiseThemeDraft (draftThemeSlots)
+    !== serialiseThemeDraft (savedThemeSlots)
+    || draftThemeMode !== savedThemeMode
+```
+
+Penalty: capital offence. Formatting must never corrupt the meaning of an
+expression.
+
+#### Mechanical vertical expansion of compact local code
+
+Bad:
+
+```ts
+const nextState = {
+    open,
+    activeTab }
+
+setDirtyStates (
+    current => ({
+	current,
+	key:  value }),
+)
+```
+
+Good:
+
+```ts
+const nextState = { open, activeTab }
+
+setDirtyStates (
+    current => ({
+	current,
+	key:  value }))
+```
+
+Penalty: minor offence when semantics stay intact. It is not necessarily
+broken, but it ignores the repository's compact associative style.
+
+#### Prettier-style exploded object literals that should stay compact
+
+Bad:
+
+```ts
+const next = {
+    enabled: keyboard?.enabled !== false,
+    bindings: normaliseKeyboardBindings (keyboard?.bindings),
+}
+```
+
+Good:
+
+```ts
+const next = {
+    enabled: keyboard?.enabled !== false,
+    bindings: normaliseKeyboardBindings (keyboard?.bindings) }
+```
+
+Also good when short enough:
+
+```ts
+const next = { enabled: keyboard?.enabled !== false,
+	       bindings: normaliseKeyboardBindings (keyboard?.bindings) }
+```
+
+Rule: for compact associative syntax, keep `{` with the first property and `}`
+with the final property unless line length truly forces a different shape.
+Do not mechanically apply the common Prettier block form.
+Penalty: medium-severity offence. The code still runs, but it violates a core
+repository formatting habit that must be preserved consistently.
+
+#### Exploded object arguments inside short calls
+
+Bad:
+
+```ts
+updateClientSettings (settings => ({
+    ...settings,
+    keyboard: next,
+}))
+```
+
+Good:
+
+```ts
+updateClientSettings (settings => ({
+    ...settings,
+    keyboard: next }))
+```
+
+Rule: a call-expression closing `)` must not drift onto its own line, and a
+short object argument should not be exploded more than necessary. Keep the
+callback's returned object compact when it fits.
+Penalty: medium-severity offence. This usually means the edit was formatted
+mechanically instead of by reading the local style.
+
+#### Stray blank line before `export default`
+
+Bad:
+
+```ts
+const Component: FC = () => {
+  return <div/>
+}
+
+
+export default Component
+```
+
+Good:
+
+```ts
+const Component: FC = () => {
+  return <div/>
+}
+
+export default Component
+```
+
+Rule: do not leave an extra empty line between the last top-level declaration
+and `export default` unless some surrounding file structure genuinely requires
+visual separation.
+Penalty: minor offence. It is small, but it is easy to avoid and should not
+recur.
+
 ### Frontend delimiter decision table
 
 Use this table before accepting any edited TypeScript or TSX hunk. The table is
@@ -412,6 +605,7 @@ import { Button, Card } from '@/components/ui'
 Rule: named-binding `}` is associative-array-style syntax. It must not be alone
 at the beginning of a line. Prefer keeping `{` with the first binding and `}`
 with the final binding when this fits the line limit.
+Penalty: capital offence. This is one of the core associative-brace rules.
 
 #### Type literals
 
@@ -434,6 +628,8 @@ type Props = {
 
 Rule: type-literal `}` is not a block close. It must stay on the same line as
 the final property unless that would break the hard line limit.
+Penalty: capital offence. Mistaking a type-literal `}` for a block `}` is one
+of the recurring death-class errors.
 
 #### Object literals
 
@@ -474,6 +670,8 @@ const value = useMemo (() => ({
 Rule: object-literal `}` is associative-array-style syntax. It must not be on a
 line by itself. Keep it with the final property, and keep call `)` off the
 beginning of a line.
+Penalty: capital offence. Wrong object-literal `}` placement and wrong call
+`)` placement are both death-class violations here.
 
 #### Destructuring parameters
 
@@ -504,6 +702,8 @@ that spans multiple lines, do not use the Prettier-style `= ({ ... }) =>`
 shape. Put the function parameter list on its own lines. The destructuring `}`
 stays with the final binding. The parameter-list `)` is then allowed and
 required at the beginning of its own line.
+Penalty: capital offence. This pattern is a primary source of repeated TSX
+style regressions.
 
 #### Inline typed destructuring parameter
 
@@ -531,6 +731,8 @@ const RouteTransitionWrapper = ({ user, setUser }: {
 Rule: this is not a separately split parameter-list block. The line break is
 inside the inline type. Keep the type-literal `}` and parameter-list `)` on the
 same line as the final type property.
+Penalty: capital offence. Misclassifying this case means the delimiter review
+has failed at the syntax-role level.
 
 #### Multi-line normal parameter list
 
@@ -558,6 +760,8 @@ const updateDraft = <Key extends keyof Settings,> (
 Rule: when the parameter list itself is split across multiple parameter lines,
 the closing parameter `)` goes at the beginning of its own line before `=>` or
 the return type.
+Penalty: capital offence. This is the main allowed exception for a leading `)`,
+so getting it wrong destroys the whole rule set.
 
 #### Function and callback blocks
 
@@ -586,6 +790,8 @@ const handleSave = () => {
 Rule: block `}` closes executable code, not associative data. Multi-line
 function, lambda, callback, `if`, `for`, `switch`, and similar block braces
 belong on their own line.
+Penalty: capital offence. Confusing block `}` with associative `}` is a
+death-class delimiter error.
 
 #### Function and method calls
 
@@ -609,6 +815,8 @@ const value = compute (
 Rule: call-expression `)` must not be at the beginning of a line. The exception
 for leading `)` applies only to function declaration parameter lists, never to
 calls.
+Penalty: capital offence. A leading call `)` is one of the clearest death-class
+violations in this codebase.
 
 #### JSX closing markers
 
@@ -653,6 +861,8 @@ Good:
 
 Rule: keep `>` or `/>` with the final prop, and keep JSX closing parentheses in
 the local compact form such as `</div>)`.
+Penalty: medium-severity offence. It is usually not semantically broken, but it
+is a conspicuous TSX-style failure.
 
 #### Arrays and tuples
 
@@ -681,6 +891,8 @@ const items = [first, second]
 
 Rule: array and tuple `]` must not be at the beginning of a line. Keep it with
 the final element unless that would break the hard line limit.
+Penalty: capital offence. A leading `]` is forbidden by the same class of hard
+delimiter rule as leading associative `}` and leading call `)`.
 
 #### Single-line braces
 
@@ -705,6 +917,8 @@ Good:
 
 Rule: JavaScript object braces on one line get one inner space. JSX expression
 braces do not get inner spaces.
+Penalty: minor offence. This is not normally semantic breakage, but it still
+counts as a style miss.
 
 #### Final TypeScript/TSX self-review checklist
 

backend/db/schema.rb

@@ -371,12 +371,6 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_05_000000) do
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
     t.string "theme", default: "system", null: false
-    t.string "display_density", default: "comfortable", null: false
-    t.string "font_size", default: "normal", null: false
-    t.integer "post_list_limit", default: 50, null: false
-    t.string "post_list_order", default: "created_at_desc", null: false
-    t.string "viewed_post_display", default: "show", null: false
-    t.boolean "tag_autocomplete_nico", default: true, null: false
     t.string "auto_fetch_title", default: "manual", null: false
     t.string "auto_fetch_thumbnail", default: "manual", null: false
     t.string "wiki_editor_mode", default: "split", null: false

frontend/src/App.tsx

@@ -21,6 +21,7 @@ import { applyClientAppearance,
 	 setCachedUserThemeSlots,
 	 seedClientThemeMode } from '@/lib/settings'
 import { useClientBehaviourSettings } from '@/lib/useClientBehaviourSettings'
+import { UnsavedChangesGuardProvider } from '@/lib/useUnsavedChangesGuard'
 
 import { KeyboardShortcutsProvider } from '@/lib/useKeyboardShortcuts'
 import DeerjikistDetailPage from '@/pages/deerjikists/DeerjikistDetailPage'
@@ -210,8 +211,9 @@ const App: FC = () => {
 	{import.meta.env.DEV && <DevModeWatermark/>}
 
 	<BrowserRouter>
-	  <KeyboardShortcutsProvider>
-	    <DialogueProvider>
+	  <DialogueProvider>
+	    <UnsavedChangesGuardProvider>
+	      <KeyboardShortcutsProvider>
 	      <MotionConfig
 		  reducedMotion={
 		      animationMode === 'normal'
@@ -232,8 +234,9 @@ const App: FC = () => {
 	      </MotionConfig>
 
 	      <Toaster/>
-	    </DialogueProvider>
-	  </KeyboardShortcutsProvider>
+	      </KeyboardShortcutsProvider>
+	    </UnsavedChangesGuardProvider>
+	  </DialogueProvider>
 	</BrowserRouter>
       </>)
 }

frontend/src/components/PrefetchLink.tsx

@@ -6,6 +6,7 @@ import { createPath, useNavigate } from 'react-router-dom'
 import { useOverlayStore } from '@/components/RouteBlockerOverlay'
 import { prefetchForURL } from '@/lib/prefetchers'
 import { useClientBehaviourSettings } from '@/lib/useClientBehaviourSettings'
+import { useUnsavedChangesGuard } from '@/lib/useUnsavedChangesGuard'
 import { cn } from '@/lib/utils'
 
 import type { AnchorHTMLAttributes, MouseEvent, TouchEvent } from 'react'
@@ -35,11 +36,19 @@ export default forwardRef<HTMLAnchorElement, Props> (({
   const navigate = useNavigate ()
   const qc = useQueryClient ()
   const behaviourSettings = useClientBehaviourSettings ()
+  const { confirmDiscardNavigation } = useUnsavedChangesGuard ()
   const linkPreloadMode = behaviourSettings.linkPreload ?? 'intent'
+  const path = useMemo (
+      () => typeof to === 'string' ? to : createPath (to),
+      [to],
+  )
   const url = useMemo (() => {
-    const path = (typeof to === 'string') ? to : createPath (to)
-    return (new URL (path, location.origin)).toString ()
-  }, [to])
+    return (new URL (path, window.location.origin)).toString ()
+  }, [path])
+  const nextPathname = useMemo (
+      () => (new URL (path, window.location.origin)).pathname,
+      [path],
+  )
   const setOverlay = useOverlayStore (s => s.setActive)
 
   const doPrefetch = async () => {
@@ -84,6 +93,13 @@ export default forwardRef<HTMLAnchorElement, Props> (({
 
       ev.preventDefault ()
 
+      if (nextPathname !== window.location.pathname)
+	{
+	  const confirmed = await confirmDiscardNavigation ()
+	  if (!(confirmed))
+	    return
+	}
+
       flushSync (() => {
 	setOverlay (true)
       })
@@ -106,7 +122,7 @@ export default forwardRef<HTMLAnchorElement, Props> (({
 
   return (
       <a ref={ref}
-	 href={typeof to === 'string' ? to : createPath (to)}
+	 href={path}
 	 onMouseEnter={handleMouseEnter}
 	 onTouchStart={handleTouchStart}
 	 onClick={handleClick}

frontend/src/components/users/BehaviourSettingsSection.tsx

@@ -158,7 +158,7 @@ const BehaviourSettingsSection: FC<Props> = (
 	  <div className="flex flex-wrap gap-2">
 	    <Button
 		type="button"
-		variant="outline"
+		variant="destructive"
 		disabled={hasUnsavedChanges === false}
 		onClick={handleDiscard}>
 	      変更を破棄

frontend/src/components/users/KeyboardSettingsSection.tsx

@@ -167,7 +167,7 @@ const KeyboardSettingsSection: FC<Props> = (
 	    </Button>
 	    <Button
 		type="button"
-		variant="outline"
+		variant="destructive"
 		disabled={!(hasUnsavedChanges)}
 		onClick={handleDiscard}>
 	      変更を破棄

frontend/src/components/users/UserSettingsProvider.tsx

@@ -1,97 +0,0 @@
-import { createContext, useContext, useEffect, useMemo, useState } from 'react'
-
-import {
-    applyClientAppearance,
-    DEFAULT_USER_SETTINGS,
-    fetchUserSettings,
-} from '@/lib/settings'
-
-import type { Dispatch, FC, ReactNode, SetStateAction } from 'react'
-
-import type { User } from '@/types'
-import type { UserSettings } from '@/lib/settings'
-
-type ContextValue = {
-    loaded:       boolean
-    error:        string | null
-    settings:     UserSettings
-    setSettings:  Dispatch<SetStateAction<UserSettings>> }
-
-const UserSettingsContext = createContext<ContextValue | null> (null)
-
-
-export const UserSettingsProvider: FC<{
-    children:  ReactNode
-    user:      User | null }> = ({ children, user }) => {
-  const [error, setError] = useState<string | null> (null)
-  const [loaded, setLoaded] = useState (false)
-  const [settings, setSettings] = useState<UserSettings> (DEFAULT_USER_SETTINGS)
-
-  useEffect (() => {
-    let cancelled = false
-
-    if (!(user))
-      {
-	setError (null)
-	setSettings (DEFAULT_USER_SETTINGS)
-	setLoaded (true)
-	return
-      }
-
-    setLoaded (false)
-    setError (null)
-
-    void (async () => {
-      try
-      {
-	const next = await fetchUserSettings ()
-	if (!(cancelled))
-	  {
-	    setError (null)
-	    setSettings (next)
-	    setLoaded (true)
-	  }
-      }
-      catch
-      {
-	if (!(cancelled))
-	  {
-	    setError ('設定を読み込めませんでした．既定値で表示しています．')
-	    setSettings (DEFAULT_USER_SETTINGS)
-	    setLoaded (true)
-	  }
-      }
-    }) ()
-
-    return () => {
-      cancelled = true
-    }
-  }, [user])
-
-  useEffect (() => {
-    void settings.theme
-    applyClientAppearance ()
-  }, [settings.theme])
-
-  const value = useMemo<ContextValue> (() => ({
-      error,
-      loaded,
-      settings,
-      setSettings,
-  }), [error, loaded, settings])
-
-  return (
-      <UserSettingsContext.Provider value={value}>
-	{children}
-      </UserSettingsContext.Provider>)
-}
-
-
-export const useUserSettings = (): ContextValue => {
-  const value = useContext (UserSettingsContext)
-
-  if (value == null)
-    throw new Error ('UserSettingsProvider is missing')
-
-  return value
-}

frontend/src/index.css

@@ -38,6 +38,7 @@
     --top-nav-root-bg-desktop: #fefce8;
     --top-nav-active-bg: #fef08a;
     --top-nav-submenu-bg: #fef08a;
+    --top-nav-mobile-menu-bg: #fef08a;
     --top-nav-mobile-active-bg: #fefce8;
     --top-nav-brand-link: #db2777;
     --top-nav-menu-link: #1d4ed8;
@@ -89,6 +90,7 @@
     --top-nav-root-bg-desktop: #230505;
     --top-nav-active-bg: #450a0a;
     --top-nav-submenu-bg: #450a0a;
+    --top-nav-mobile-menu-bg: #230505;
     --top-nav-mobile-active-bg: #450a0a;
     --top-nav-brand-link: #f9a8d4;
     --top-nav-menu-link: #93c5fd;
@@ -105,7 +107,7 @@
   }
   a:hover
   {
-    opacity: .85;
+    color: color-mix(in oklab, hsl(var(--theme-link)), white 18%);
   }
 }
 
@@ -125,29 +127,6 @@
   -moz-osx-font-smoothing: grayscale;
 }
 
-:root[data-font-size='small']
-{
-  font-size: 14px;
-}
-
-:root[data-font-size='normal']
-{
-  font-size: 16px;
-}
-
-:root[data-font-size='large']
-{
-  font-size: 18px;
-}
-
-:root[data-display-density='compact'] input,
-:root[data-display-density='compact'] textarea,
-:root[data-display-density='compact'] select,
-:root[data-display-density='compact'] button
-{
-  line-height: 1.25;
-}
-
 :root[data-animation='reduced'] *,
 :root[data-animation='off'] *
 {
@@ -260,7 +239,7 @@ body
 
 .top-nav-mobile-menu
 {
-  background: var(--top-nav-root-bg-mobile);
+  background: var(--top-nav-mobile-menu-bg);
 }
 
 .top-nav-mobile-active

frontend/src/lib/useKeyboardShortcuts.tsx

@@ -25,6 +25,7 @@ import {
     getEffectiveKeyBindings,
     setClientKeyboardSettings,
 } from '@/lib/settings'
+import { useUnsavedChangesGuard } from '@/lib/useUnsavedChangesGuard'
 
 import type {
     KeyBinding,
@@ -96,6 +97,7 @@ const focusSearchTarget = (): void => {
 export const KeyboardShortcutsProvider = ({ children }: PropsWithChildren) => {
   const location = useLocation ()
   const navigate = useNavigate ()
+  const { confirmDiscardNavigation } = useUnsavedChangesGuard ()
 
   const [keyboardSettings, setKeyboardSettingsState] =
       useState<ClientKeyboardSettings> (() => getClientKeyboardSettings ())
@@ -146,6 +148,13 @@ export const KeyboardShortcutsProvider = ({ children }: PropsWithChildren) => {
     }
   }, [])
 
+  const guardedNavigate = useCallback ((path: string) => {
+    confirmDiscardNavigation ().then (confirmed => {
+      if (confirmed)
+	navigate (path)
+    })
+  }, [confirmDiscardNavigation, navigate])
+
   const builtinHandlers = useMemo<ShortcutHandlers> (
       () => ({
 	  'global.openShortcutHelp': () => {
@@ -154,16 +163,16 @@ export const KeyboardShortcutsProvider = ({ children }: PropsWithChildren) => {
 	  'global.focusSearch': () => {
 	    focusSearchTarget ()
 	  },
-	  'navigation.posts': () => navigate ('/posts'),
-	  'navigation.tags': () => navigate ('/tags'),
-	  'navigation.materials': () => navigate ('/materials'),
-	  'navigation.wiki': () => navigate ('/wiki'),
-	  'settings.account': () => navigate ('/users/settings?tab=account'),
-	  'settings.theme': () => navigate ('/users/settings?tab=theme'),
-	  'settings.keyboard': () => navigate ('/users/settings?tab=keyboard'),
-	  'settings.behavior': () => navigate ('/users/settings?tab=behavior'),
+	  'navigation.posts': () => guardedNavigate ('/posts'),
+	  'navigation.tags': () => guardedNavigate ('/tags'),
+	  'navigation.materials': () => guardedNavigate ('/materials'),
+	  'navigation.wiki': () => guardedNavigate ('/wiki'),
+	  'settings.account': () => guardedNavigate ('/users/settings?tab=account'),
+	  'settings.theme': () => guardedNavigate ('/users/settings?tab=theme'),
+	  'settings.keyboard': () => guardedNavigate ('/users/settings?tab=keyboard'),
+	  'settings.behavior': () => guardedNavigate ('/users/settings?tab=behavior'),
       }),
-      [navigate],
+      [guardedNavigate],
   )
 
   const availableActionIds = useMemo<Set<ShortcutActionId>> (

frontend/src/lib/useUnsavedChangesGuard.tsx

@@ -0,0 +1,69 @@
+import { createContext, useCallback, useContext, useMemo, useState } from 'react'
+
+import { useDialogue } from '@/components/dialogues/DialogueProvider'
+
+import type { FC, PropsWithChildren } from 'react'
+
+type UnsavedChangesSource = {
+    dirty:    boolean
+    discard:  () => void | Promise<void> }
+
+type UnsavedChangesGuardContextValue = {
+    hasUnsavedChanges:             boolean
+    registerUnsavedChangesSource:  (
+	source: UnsavedChangesSource | null,
+    ) => void
+    confirmDiscardNavigation:      () => Promise<boolean> }
+
+const UnsavedChangesGuardContext =
+    createContext<UnsavedChangesGuardContextValue | null> (null)
+
+
+export const UnsavedChangesGuardProvider: FC<PropsWithChildren> = ({ children }) => {
+  const dialogue = useDialogue ()
+  const [source, setSource] = useState<UnsavedChangesSource | null> (null)
+
+  const registerUnsavedChangesSource = useCallback ((
+      nextSource: UnsavedChangesSource | null,
+  ) => {
+    setSource (nextSource)
+  }, [])
+
+  const confirmDiscardNavigation = useCallback (async (): Promise<boolean> => {
+    if (!(source?.dirty))
+      return true
+
+    const confirmed = await dialogue.confirm ({
+	title: '未保存の変更があります',
+	description: 'このまま移動すると、保存していない変更は失われます。',
+	cancelText: 'このページに残る',
+	confirmText: '変更を破棄して移動',
+	variant: 'danger' })
+    if (!(confirmed))
+      return false
+
+    await source.discard ()
+    return true
+  }, [dialogue, source])
+
+  const value = useMemo<UnsavedChangesGuardContextValue> (() => ({
+      hasUnsavedChanges: source?.dirty === true,
+      registerUnsavedChangesSource,
+      confirmDiscardNavigation,
+  }), [confirmDiscardNavigation, registerUnsavedChangesSource, source?.dirty])
+
+  return (
+      <UnsavedChangesGuardContext.Provider value={value}>
+	{children}
+      </UnsavedChangesGuardContext.Provider>)
+}
+
+
+export const useUnsavedChangesGuard = (): UnsavedChangesGuardContextValue => {
+  const context = useContext (UnsavedChangesGuardContext)
+
+  if (context == null)
+    throw new Error ('UnsavedChangesGuardProvider が必要です．')
+
+  return context
+}