frontend/src/components/MaterialSidebar.tsx

@@ -489,7 +489,7 @@ const MaterialSidebar: FC = () => {
 	</div>
 
 	<div className="hidden md:block">
-	  <SidebarComponent>
+	  <SidebarComponent sidebarKey="materials">
 	    <div className="space-y-4">
 	      <MaterialFilterButtons
 		  materialFilter={materialFilter}

frontend/src/components/TagDetailSidebar.tsx

@@ -278,7 +278,7 @@ const TagDetailSidebar: FC<Props> = ({ post, sp }) => {
   }, [baseTags])
 
   return (
-      <SidebarComponent>
+      <SidebarComponent sidebarKey="post-detail-tags">
 	<TagSearch/>
 	<DndContext
 	    sensors={sensors}

frontend/src/components/TagSidebar.tsx

@@ -96,7 +96,7 @@ const TagSidebar: FC<Props> = ({ posts, onClick }) => {
       </>)
 
   return (
-      <SidebarComponent>
+      <SidebarComponent sidebarKey="posts-index-tags">
 	<TagSearch/>
 
 	<div className="hidden md:block mt-4">

frontend/src/components/layout/SidebarComponent.tsx

@@ -1,16 +1,304 @@
 import { motion } from 'framer-motion'
 import { Helmet } from 'react-helmet-async'
 
-import type { FC, ReactNode } from 'react'
+import { cn } from '@/lib/utils'
 
-type Props = { children: ReactNode }
+import type { CSSProperties, FC, MouseEvent, PointerEvent, ReactNode } from 'react'
+import { useCallback, useEffect, useRef, useState } from 'react'
+
+type SidebarSide = 'left' | 'right'
+
+type Props = {
+    children:      ReactNode
+    className?:    string
+    maxWidth?:     number
+    onWidthChange?: (width: number) => void
+    sidebarKey:    string
+    side?:         SidebarSide }
+
+type ResizeState = {
+    pointerId:  number
+    startWidth: number
+    startX:     number
+    side:       SidebarSide }
+
+type PendingPressState = ResizeState & { startY: number }
+
+const DEFAULT_SIDEBAR_WIDTH = 256
+const MIN_SIDEBAR_WIDTH = 192
+const MIN_MAIN_WIDTH = 360
+const MAX_VIEWPORT_WIDTH_RATIO = .8
+const LONG_PRESS_DELAY_MS = 350
+const TOUCH_MOVE_THRESHOLD_PX = 8
 
 
-const SidebarComponent: FC<Props> = ({ children }) => (
+const storageKeyForSidebar = (sidebarKey: string, side: SidebarSide): string =>
+    `sidebar:${ sidebarKey }:${ side }:width`
+
+
+const getSidebarMaxWidth = (containerWidth: number): number => {
+  const viewportWidth =
+      typeof window === 'undefined' ? containerWidth : window.innerWidth
+  const viewportMaxWidth = Math.floor (viewportWidth * MAX_VIEWPORT_WIDTH_RATIO)
+  const containerMaxWidth = Math.floor (containerWidth - MIN_MAIN_WIDTH)
+  const dynamicMaxWidth = Math.min (viewportMaxWidth, containerMaxWidth)
+
+  return Math.max (MIN_SIDEBAR_WIDTH, dynamicMaxWidth)
+}
+
+
+const clampSidebarWidth = (width: number, containerWidth: number): number =>
+    Math.min (getSidebarMaxWidth (containerWidth), Math.max (MIN_SIDEBAR_WIDTH, width))
+
+
+const readStoredSidebarWidth = (
+    sidebarKey: string,
+    side: SidebarSide,
+    containerWidth: number): number => {
+  if (typeof window === 'undefined')
+    return DEFAULT_SIDEBAR_WIDTH
+
+  const value = window.localStorage.getItem (storageKeyForSidebar (sidebarKey, side))
+  const width = value == null ? Number.NaN : Number (value)
+
+  return Number.isFinite (width)
+	 ? clampSidebarWidth (width, containerWidth)
+	 : clampSidebarWidth (DEFAULT_SIDEBAR_WIDTH, containerWidth)
+}
+
+
+const SidebarComponent: FC<Props> = ({
+    children,
+    className,
+    maxWidth,
+    onWidthChange,
+    sidebarKey,
+    side = 'left',
+}) => {
+  const rootRef = useRef<HTMLDivElement | null> (null)
+  const getContainerWidth = useCallback (() => {
+    const containerWidth = rootRef.current
+	?.closest ('[data-sidebar-container]')
+	?.clientWidth
+    ?? window.innerWidth
+    return containerWidth
+  }, [])
+  const getClampedWidth = useCallback ((
+      nextWidth: number,
+      containerWidth: number): number => {
+    const dynamicMaxWidth = getSidebarMaxWidth (containerWidth)
+    const maxAllowedWidth =
+	maxWidth == null
+	? dynamicMaxWidth
+	: Math.max (MIN_SIDEBAR_WIDTH, Math.min (dynamicMaxWidth, maxWidth))
+
+    return Math.min (maxAllowedWidth, Math.max (MIN_SIDEBAR_WIDTH, nextWidth))
+  }, [maxWidth])
+
+  const [width, setWidth] = useState (() => (
+      typeof window === 'undefined'
+      ? DEFAULT_SIDEBAR_WIDTH
+      : getClampedWidth (
+	  readStoredSidebarWidth (sidebarKey, side, window.innerWidth),
+	  window.innerWidth)))
+  const [resizing, setResizing] = useState (false)
+  const widthRef = useRef (width)
+  const longPressTimerRef = useRef<ReturnType<typeof setTimeout> | null> (null)
+  const pendingPressRef = useRef<PendingPressState | null> (null)
+  const resizeRef = useRef<ResizeState | null> (null)
+  const bodyStyleRef = useRef<{ cursor: string, userSelect: string } | null> (null)
+
+  const clearLongPressTimer = useCallback (() => {
+    if (longPressTimerRef.current == null)
+      return
+
+    clearTimeout (longPressTimerRef.current)
+    longPressTimerRef.current = null
+  }, [])
+
+  const restoreBodyStyle = useCallback (() => {
+    if (bodyStyleRef.current == null)
+      return
+
+    document.body.style.cursor = bodyStyleRef.current.cursor
+    document.body.style.userSelect = bodyStyleRef.current.userSelect
+    bodyStyleRef.current = null
+  }, [])
+
+  const startResize = useCallback ((state: ResizeState) => {
+    clearLongPressTimer ()
+    pendingPressRef.current = null
+    resizeRef.current = state
+    setResizing (true)
+
+    if (bodyStyleRef.current == null)
+    {
+      bodyStyleRef.current = {
+	  cursor: document.body.style.cursor,
+	  userSelect: document.body.style.userSelect }
+    }
+
+    document.body.style.cursor = 'col-resize'
+    document.body.style.userSelect = 'none'
+  }, [clearLongPressTimer])
+
+  const finishResize = useCallback (() => {
+    clearLongPressTimer ()
+    pendingPressRef.current = null
+
+    const state = resizeRef.current
+    resizeRef.current = null
+
+    if (state != null)
+      window.localStorage.setItem (
+	  storageKeyForSidebar (sidebarKey, state.side),
+	  String (widthRef.current))
+
+    setResizing (false)
+    restoreBodyStyle ()
+  }, [clearLongPressTimer, restoreBodyStyle, sidebarKey])
+
+  const updateWidth = useCallback ((clientX: number) => {
+    const state = resizeRef.current
+    if (state == null)
+      return
+
+    const containerWidth = getContainerWidth ()
+    const delta = state.side === 'left'
+		  ? clientX - state.startX
+		  : state.startX - clientX
+    const nextWidth = getClampedWidth (state.startWidth + delta, containerWidth)
+    widthRef.current = nextWidth
+    setWidth (nextWidth)
+  }, [getClampedWidth, getContainerWidth])
+
+  const handlePointerDown = (ev: PointerEvent<HTMLDivElement>) => {
+    if (ev.pointerType !== 'mouse' && ev.pointerType !== 'touch')
+      return
+
+    if (ev.pointerType === 'mouse' && ev.button !== 0)
+      return
+
+    ev.preventDefault ()
+    ev.stopPropagation ()
+    ev.currentTarget.setPointerCapture (ev.pointerId)
+
+    const state = {
+	pointerId: ev.pointerId,
+	side,
+	startWidth: widthRef.current,
+	startX: ev.clientX }
+
+    if (ev.pointerType === 'mouse')
+    {
+      startResize (state)
+      return
+    }
+
+    pendingPressRef.current = { ...state, startY: ev.clientY }
+    clearLongPressTimer ()
+    longPressTimerRef.current = setTimeout (() => {
+      if (pendingPressRef.current != null)
+	startResize (pendingPressRef.current)
+    }, LONG_PRESS_DELAY_MS)
+  }
+
+  const handlePointerMove = (ev: PointerEvent<HTMLDivElement>) => {
+    ev.stopPropagation ()
+
+    if (resizeRef.current != null)
+    {
+      ev.preventDefault ()
+      updateWidth (ev.clientX)
+      return
+    }
+
+    const pendingPress = pendingPressRef.current
+    if (pendingPress == null || pendingPress.pointerId !== ev.pointerId)
+      return
+
+    const deltaX = ev.clientX - pendingPress.startX
+    const deltaY = ev.clientY - pendingPress.startY
+    const distance = Math.hypot (deltaX, deltaY)
+
+    if (distance >= TOUCH_MOVE_THRESHOLD_PX)
+    {
+      clearLongPressTimer ()
+      pendingPressRef.current = null
+    }
+  }
+
+  const handlePointerEnd = (ev: PointerEvent<HTMLDivElement>) => {
+    ev.stopPropagation ()
+
+    if (ev.currentTarget.hasPointerCapture (ev.pointerId))
+      ev.currentTarget.releasePointerCapture (ev.pointerId)
+
+    finishResize ()
+  }
+
+  const resetWidth = useCallback (() => {
+    const containerWidth = getContainerWidth ()
+    const nextWidth = getClampedWidth (DEFAULT_SIDEBAR_WIDTH, containerWidth)
+    widthRef.current = nextWidth
+    setWidth (nextWidth)
+    window.localStorage.removeItem (storageKeyForSidebar (sidebarKey, side))
+  }, [getClampedWidth, getContainerWidth, sidebarKey, side])
+
+  const handleDoubleClick = (ev: MouseEvent<HTMLDivElement>) => {
+    ev.preventDefault ()
+    ev.stopPropagation ()
+    resetWidth ()
+  }
+
+  useEffect (() => {
+    const containerWidth = getContainerWidth ()
+    const nextWidth = getClampedWidth (
+	readStoredSidebarWidth (sidebarKey, side, containerWidth),
+	containerWidth)
+    widthRef.current = nextWidth
+    setWidth (nextWidth)
+  }, [getClampedWidth, getContainerWidth, sidebarKey, side])
+
+  useEffect (() => {
+    if (typeof window === 'undefined')
+      return
+
+    const handleResize = () => {
+      const containerWidth = getContainerWidth ()
+      const nextWidth = getClampedWidth (widthRef.current, containerWidth)
+      if (nextWidth === widthRef.current)
+	return
+
+      widthRef.current = nextWidth
+      setWidth (nextWidth)
+    }
+
+    window.addEventListener ('resize', handleResize)
+    return () => window.removeEventListener ('resize', handleResize)
+  }, [getClampedWidth, getContainerWidth])
+
+  useEffect (() => {
+    onWidthChange?.(width)
+  }, [onWidthChange, width])
+
+  useEffect (() => () => {
+    clearLongPressTimer ()
+    restoreBodyStyle ()
+  }, [clearLongPressTimer, restoreBodyStyle])
+
+  const style = {
+      '--sidebar-width': `${ width }px` } as CSSProperties
+
+  return (
       <motion.div
+	  ref={rootRef}
 	  layout="position"
 	  transition={{ layout: { duration: .2, ease: 'easeOut' } }}
-	className="p-4 w-full md:w-64 md:h-full md:overflow-y-auto sidebar">
+	  style={style}
+	  className={cn (
+	      'relative w-full md:w-[var(--sidebar-width)] md:shrink-0 md:h-full',
+	      className)}>
 	<Helmet>
 	  <style>
 	    {`
@@ -26,7 +314,31 @@ const SidebarComponent: FC<Props> = ({ children }) => (
 	  </style>
 	</Helmet>
 
+	<div className="h-full overflow-y-auto p-4 sidebar">
 	  {children}
+	</div>
+
+	<div
+	    role="separator"
+	    aria-orientation="vertical"
+	    aria-label="サイドバー幅"
+	    onPointerDown={handlePointerDown}
+	    onPointerMove={handlePointerMove}
+	    onPointerUp={handlePointerEnd}
+	    onPointerCancel={handlePointerEnd}
+	    onDoubleClick={handleDoubleClick}
+	    className={cn (
+		'hidden md:block absolute top-0 bottom-0 z-20 w-4 cursor-col-resize',
+		'touch-none select-none',
+		side === 'left' ? '-right-2' : '-left-2',
+		'before:absolute before:top-0 before:bottom-0 before:w-px',
+		'before:bg-neutral-500/0 hover:before:bg-neutral-500/30',
+		'before:transition-colors before:duration-150',
+		side === 'left'
+		? 'before:left-1/2'
+		: 'before:right-1/2',
+		resizing && 'before:bg-neutral-500/40')}/>
       </motion.div>)
+}
 
 export default SidebarComponent

frontend/src/pages/theatres/TheatreDetailPage.tsx

@@ -10,6 +10,7 @@ import PrefetchLink from '@/components/PrefetchLink'
 import TagLink from '@/components/TagLink'
 import FieldError from '@/components/common/FieldError'
 import { useDialogue } from '@/components/dialogues/DialogueProvider'
+import SidebarComponent from '@/components/layout/SidebarComponent'
 import { Button } from '@/components/ui/button'
 import { SITE_TITLE } from '@/config'
 import { CATEGORIES, CATEGORY_NAMES } from '@/consts'
@@ -52,6 +53,8 @@ const INITIAL_WEIGHTS: TheatrePostSelectionWeights =
 
 const LAYOUT_STORAGE_KEY = 'theatre-layout-mode'
 const TAG_FLOW_STORAGE_KEY = 'theatre-tag-flow'
+const MIN_MAIN_WIDTH = 360
+const SIDEBAR_GAP_WIDTH = 16
 
 const LAYOUT_LABELS: Record<TheatreLayoutMode, string> = {
     threeColumns:   '3 列',
@@ -211,11 +214,16 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
   const commentsRef = useRef<HTMLDivElement> (null)
   const embedRef = useRef<NiconicoViewerHandle> (null)
   const loadingRef = useRef (false)
+  const sidebarContainerRef = useRef<HTMLDivElement> (null)
   const theatreInfoRef = useRef<TheatreInfo> (INITIAL_THEATRE_INFO)
   const theatreInfoReceivedAtRef = useRef (performance.now ())
   const videoLengthRef = useRef (0)
   const lastCommentNoRef = useRef (0)
 
+  const [leftSidebarWidth, setLeftSidebarWidth] = useState (256)
+  const [rightSidebarWidth, setRightSidebarWidth] = useState (256)
+  const [sidebarContainerWidth, setSidebarContainerWidth] = useState (
+      () => typeof window === 'undefined' ? 0 : window.innerWidth)
   const [comments, setComments] = useState<TheatreComment[]> ([])
   const [content, setContent] = useState ('')
   const [editingPost, setEditingPost] = useState<Post | null> (null)
@@ -324,6 +332,17 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
     lastCommentNoRef.current = comments[0]?.no ?? 0
   }, [comments])
 
+  useEffect (() => {
+    const updateSidebarContainerWidth = () => {
+      const nextWidth = sidebarContainerRef.current?.clientWidth ?? window.innerWidth
+      setSidebarContainerWidth (nextWidth)
+    }
+
+    updateSidebarContainerWidth ()
+    window.addEventListener ('resize', updateSidebarContainerWidth)
+    return () => window.removeEventListener ('resize', updateSidebarContainerWidth)
+  }, [])
+
   useEffect (() => {
     if (!(id))
       return
@@ -639,6 +658,26 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
   if (status >= 400)
     return <ErrorScreen status={status}/>
 
+  const singleSidebarMaxWidth = Math.max (
+      192,
+      sidebarContainerWidth - MIN_MAIN_WIDTH - SIDEBAR_GAP_WIDTH)
+  const leftSidebarMaxWidth = layoutMode === 'threeColumns'
+			      ? Math.max (
+				  192,
+				  sidebarContainerWidth
+				  - rightSidebarWidth
+				  - MIN_MAIN_WIDTH
+				  - SIDEBAR_GAP_WIDTH * 2)
+			      : singleSidebarMaxWidth
+  const rightSidebarMaxWidth = layoutMode === 'threeColumns'
+			       ? Math.max (
+				   192,
+				   sidebarContainerWidth
+				   - leftSidebarWidth
+				   - MIN_MAIN_WIDTH
+				   - SIDEBAR_GAP_WIDTH * 2)
+			       : singleSidebarMaxWidth
+
   const tagPanel = (
       <section className="rounded border-zinc-300 p-4 dark:border-zinc-800">
 	<div className="mb-3 flex items-center justify-between gap-3">
@@ -806,30 +845,26 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
 	  {theatre && <title>{`${ theatreTitle } | ${ SITE_TITLE }`}</title>}
 	</Helmet>
 
-	<div className={cn (
-	    'grid min-h-full gap-4 overflow-visible md:h-full md:overflow-hidden',
-	    (layoutMode === 'threeColumns'
-	     && ['md:grid-cols-[16rem_minmax(0,1fr)_22rem]',
-		 'xl:grid-cols-[18rem_minmax(0,1fr)_24rem]']),
-	    (layoutMode === 'tagsBottom'
-	     && 'md:grid-cols-[minmax(0,1fr)_22rem] xl:grid-cols-[minmax(0,1fr)_24rem]'),
-	    (layoutMode === 'commentsBottom'
-	     && 'md:grid-cols-[16rem_minmax(0,1fr)] xl:grid-cols-[18rem_minmax(0,1fr)]'))}>
+	<div
+	    ref={sidebarContainerRef}
+	    data-sidebar-container
+	    className="flex min-h-full flex-col gap-4 overflow-visible md:h-full md:flex-row
+		       md:overflow-hidden">
 	  {layoutMode !== 'tagsBottom' && (
-	      <motion.aside
-		  layout="position"
-		  className="hidden min-w-0 space-y-4 md:order-none md:block md:overflow-y-auto
-			     md:[direction:rtl]">
-		<div className="md:[direction:ltr]">
+	      <SidebarComponent
+		  sidebarKey="theatre-tags"
+		  side="left"
+		  maxWidth={leftSidebarMaxWidth}
+		  onWidthChange={setLeftSidebarWidth}
+		  className="hidden md:block">
 		{tagPanel}
-		</div>
-	      </motion.aside>)}
+	      </SidebarComponent>)}
 
 	  <motion.main
 	      layout="position"
-	      className={cn ('order-1 min-w-0 space-y-4 md:order-none md:overflow-y-auto',
-			     layoutMode === 'tagsBottom' && 'md:[direction:rtl]')}>
-	    <div className={cn ('space-y-4', layoutMode === 'tagsBottom' && 'md:[direction:ltr]')}>
+	      className="order-1 min-w-0 flex-1 space-y-4 md:order-none
+			 md:min-w-[360px] md:overflow-y-auto">
+	    <div className="space-y-4">
 	      <section className="overflow-hidden rounded border-zinc-300
 				  dark:border-zinc-800">
 		<div className="flex flex-wrap items-center justify-between gap-3
@@ -965,12 +1000,17 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
 	  </motion.main>
 
 	  {layoutMode !== 'commentsBottom' && (
-	      <motion.aside
-		  layout="position"
-		  className="hidden min-w-0 space-y-4 md:order-none md:block md:overflow-y-auto">
+	      <SidebarComponent
+		  sidebarKey="theatre-comments"
+		  side="right"
+		  maxWidth={rightSidebarMaxWidth}
+		  onWidthChange={setRightSidebarWidth}
+		  className="hidden md:block">
+		<div className="space-y-4">
 		  {commentsPanel}
 		  {participantsPanel}
-	      </motion.aside>)}
+		</div>
+	      </SidebarComponent>)}
 	</div>
       </motion.div>)
 }