feat(web): draw the sidebar's scrollbar only under the pointer

The session list overflows after a handful of sessions, and its scrollbar
was drawn permanently in a column that is at rest most of the time.

SidebarRoot now tracks the pointer over the whole column and rebinds
ui-theme's scrollbar indirection pair to `transparent` while it is
outside, keeping the thumb for 2s after the pointer leaves so it does not
blink out on the way past. Rebinding colour leaves the list's
`scrollbar-gutter: stable` reservation in force, so revealing the bar
moves no row.

ui-theme's gate now states the widened contract: a rebind targets an -l2
token pair or `transparent`, and nothing else.

packages/client/ui-sidebar/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/client/ui-sidebar/README.md
-README.md: 19c2d1033de4475816249aa8429f4a589eeb6481
-README.zh.md: b8c154586570cf1b9fd4bf776bc09b36ab5ee7d2
+README.md: 5bb697b3d2f9b5eaea9c382765d2510fa24806ce
+README.zh.md: 302f66c540774b1f209fc797201e41c56b849310

packages/client/ui-sidebar/README.md

@@ -8,6 +8,8 @@ New Session starts the runtime's page-local frontend Session Intent; a real Work
 
 `SidebarRootComponentProps` composes the layout owner share, the global `useSessions` and `useWorkspaces` hooks, the declared `sidebar.workspace` and `sidebar.settings` child slots, and injected `startSession`, `open`, and sidebar-toggle callbacks. There is no plugin store: `deriveGroups` consumes object-layer snapshots and component-local expansion/search state.
 
+Scrollbars in the column are a pointer affordance: the shell rebinds ui-theme's [scrollbar indirection](../ui-theme/README.md) to `transparent` whenever the pointer is outside it, and keeps the thumb drawn for 2s after the pointer leaves, so a list nobody is pointing at carries no bar. The reservation that keeps rows from moving belongs to the scrolling region ([ui-workspace](../ui-workspace/README.md)), so revealing a thumb never reflows.
+
 The foot is the `sidebar.settings` seat: the sidebar renders only the bottom-pinned layout slot and shares its column state (`wide`); ui-settings registers the trigger row and settings panel there.
 
 The `/client` export surface is the plugin body (`apply`/`inject`) plus the contract types only — SidebarRoot, the row components, and the tree derivation are internal (the slot registration closes over them; tests import src paths directly).

packages/client/ui-sidebar/README.zh.md

@@ -8,6 +8,8 @@ New Session 会启动运行时的页面局部前端 Session Intent；真实 Work
 
 `SidebarRootComponentProps` 组合布局 owner share、全局 `useSessions` 和 `useWorkspaces` 钩子、已声明的 `sidebar.workspace` 与 `sidebar.settings` 子 slot，以及注入的 `startSession`、`open` 和侧边栏切换回调。这里没有插件 store：`deriveGroups` 消费对象层快照与组件局部的展开／搜索状态。
 
+栏内的滚动条是一种指针可供性：只要指针不在栏内，外壳就把 ui-theme 的[滚动条间接层](../ui-theme/README.md)重新绑定为 `transparent`；指针离开后滑块再保留 2 秒，因此没人指向的列表不会带着滚动条。避免行位移的空间预留属于滚动区域本身（[ui-workspace](../ui-workspace/README.md)），所以显示滑块不会引起重排。
+
 页脚承载 `sidebar.settings`：侧边栏只渲染固定在底部的布局 slot，并共享其栏状态（`wide`）；ui-settings 在此注册触发行和设置面板。
 
 `/client` 导出表层只包含插件主体（`apply`／`inject`）及契约类型：SidebarRoot、行组件和树派生均属于内部实现（slot 注册通过闭包引用它们；测试直接导入 src 路径）。

packages/client/ui-sidebar/src/client/SidebarRoot.module.css

@@ -24,6 +24,19 @@
   padding: 18px 10px 6px;
 }
 
+/* Scrollbars in the column are a pointer affordance: the shell adds this
+   class whenever the pointer is not inside (SidebarRoot.tsx owns the linger),
+   and rebinding ui-theme's indirection pair to `transparent` takes the thumb
+   out of every scroll region nested under it — the workspace browser's
+   session list today. `transparent` rather than `display: none` on the bar:
+   the reservation (`scrollbar-gutter: stable` on the list) stays in force, so
+   revealing the thumb never reflows a row. Rebinding contract and the two
+   rendering paths it reaches: ui-theme's README. */
+.root.quietBars {
+  --dsh-scrollbar-thumb: transparent;
+  --dsh-scrollbar-thumb-hover: transparent;
+}
+
 /* Collapse phase 1: the whole frozen-width content fades out in place over
    150ms; at settle the children unmount/snap to the rail layout. */
 .fading > * {

packages/client/ui-sidebar/src/client/SidebarRoot.tsx

@@ -8,6 +8,11 @@
  * button and the foot is the `sidebar.workspaces` registrant's, and the foot
  * is the `sidebar.settings` registrant's; the shell hands them the wide flag
  * (plus an expand request callback for the browser).
+ *
+ * The column also owns whether the scroll regions nested in it draw a
+ * scrollbar at all: the shell tracks the pointer and rebinds ui-theme's
+ * scrollbar indirection away while it is elsewhere, so a list the user is not
+ * pointing at carries no bar.
  */
 import { useEffect, useRef, useState } from 'react'
 import clsx from 'clsx'
@@ -22,6 +27,14 @@ import css from './SidebarRoot.module.css'
 /** Wide-content unmount delay; matches the 150ms wide-content fade-out. */
 const COLLAPSE_SETTLE_MS = 150
 
+/**
+ * How long the column's scrollbars stay drawn after the pointer leaves it.
+ * The bar is a pointer affordance here, and hiding it on the leave event
+ * itself makes it blink out while the pointer is only crossing the column's
+ * edge — on the way to the conversation, or around a portalled menu.
+ */
+const SCROLLBAR_LINGER_MS = 2000
+
 /**
  * Render the sidebar column shell.
  * @param props - composed slot props (runtime share + injected callbacks, contract/slots.ts).
@@ -56,10 +69,29 @@ export function SidebarRoot({
   const everWide = useRef(!collapsed)
   if (!collapsed) everWide.current = true
 
+  // Scrollbars in the column follow the pointer (.quietBars rebinds them
+  // away): drawn while it is inside, and for SCROLLBAR_LINGER_MS after it
+  // leaves. A pointer that returns within that window cancels the pending
+  // hide rather than restarting from a hidden bar.
+  const [pointerInside, setPointerInside] = useState(false)
+  const lingerTimer = useRef<number | undefined>(undefined)
+  useEffect(() => () => { window.clearTimeout(lingerTimer.current) }, [])
+
   return (
     <div
-      className={clsx(css.root, !wide && css.collapsed, !wide && everWide.current && css.railIn, collapsed && wide && css.fading)}
+      className={clsx(
+        css.root, !wide && css.collapsed, !wide && everWide.current && css.railIn,
+        collapsed && wide && css.fading, !pointerInside && css.quietBars,
+      )}
       style={wide ? { width: collapsed ? lastWideWidth.current : width } : undefined}
+      onPointerEnter={() => {
+        window.clearTimeout(lingerTimer.current)
+        setPointerInside(true)
+      }}
+      onPointerLeave={() => {
+        window.clearTimeout(lingerTimer.current)
+        lingerTimer.current = window.setTimeout(() => { setPointerInside(false) }, SCROLLBAR_LINGER_MS)
+      }}
     >
       <div className={css.logoRow}>
         {/* Expanded, the wordmark doubles as a New Session shortcut; the

packages/client/ui-sidebar/tests/__snapshots__/sidebar-snapshot.spec.tsx.snap

@@ -5,7 +5,7 @@ exports[`sidebar shell snapshots > renders the collapsed rail after the crossfad
   data-slot="sidebar"
 >
   <div
-    class="root collapsed railIn"
+    class="root collapsed railIn quietBars"
     style=""
   >
     <div
@@ -65,7 +65,7 @@ exports[`sidebar shell snapshots > renders the expanded column (wordmark, capsul
   data-slot="sidebar"
 >
   <div
-    class="root"
+    class="root quietBars"
     style="width: 300px;"
   >
     <div
@@ -135,7 +135,7 @@ exports[`sidebar shell snapshots > renders the expanded column in the default lo
   data-slot="sidebar"
 >
   <div
-    class="root"
+    class="root quietBars"
     style="width: 300px;"
   >
     <div

packages/client/ui-sidebar/tests/pointer-scrollbars.spec.tsx

@@ -0,0 +1,96 @@
+// @vitest-environment jsdom
+/**
+ * Pointer-revealed scrollbars, the shell's half: which class state the column
+ * carries as the pointer crosses it. The stylesheet rule that state drives is
+ * asserted in scrollbar-quiet-styles.spec.ts (node environment — a jsdom spec
+ * has no file: module URL to read the sheet through).
+ */
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { act, cleanup, fireEvent, render } from '@testing-library/react'
+import type { SidebarRootComponentProps, SidebarSectionOwnerProps } from '../src/client/contract/slots.ts'
+import { SidebarRoot } from '../src/client/SidebarRoot.tsx'
+import { en } from '../src/client/locales.ts'
+
+const t: SidebarRootComponentProps['t'] = key => (en as Record<string, string>)[key] ?? key
+/** The shell never reads the global hooks; the props share carries them regardless. */
+const neverHook = (() => { throw new Error('shell must not read global hooks') }) as never
+
+afterEach(() => {
+  cleanup()
+  vi.useRealTimers()
+})
+
+/**
+ * Render the shell and expose its column element.
+ * @returns the column element and whether it currently carries the quiet state.
+ */
+function mountColumn(): { column: HTMLElement; quiet: () => boolean } {
+  const view = render(
+    <SidebarRoot
+      collapsed={false} width={300}
+      useSessions={neverHook} useWorkspaces={neverHook}
+      startSession={vi.fn()} toggleSidebar={vi.fn()} t={t}
+      renderSlot={((_key: string, owner: SidebarSectionOwnerProps) =>
+        <div data-testid="region" data-wide={owner.wide} />) as SidebarRootComponentProps['renderSlot']}
+    />,
+  )
+  const column = view.container.firstElementChild
+  if (!(column instanceof HTMLElement)) throw new Error('sidebar column not rendered')
+  // CSS-module locals are hashed in this bench, so the state is read as a
+  // substring of the class list rather than as an exact local name.
+  return { column, quiet: () => [...column.classList].some(name => name.includes('quietBars')) }
+}
+
+/**
+ * Cross the pointer into or out of the column. React synthesizes
+ * `pointerenter`/`pointerleave` from `pointerover`/`pointerout`, so the raw
+ * enter and leave events it does not listen to would assert nothing.
+ * @param column - the sidebar column element.
+ * @param direction - `in` to enter the column, `out` to leave it.
+ */
+function movePointer(column: HTMLElement, direction: 'in' | 'out'): void {
+  const outside = document.body
+  if (direction === 'in') fireEvent.pointerOver(column, { relatedTarget: outside })
+  else fireEvent.pointerOut(column, { relatedTarget: outside })
+}
+
+describe('SidebarRoot pointer-revealed scrollbars', () => {
+  it('draws them only while the pointer is inside, and lingers on the way out', () => {
+    vi.useFakeTimers()
+    const { column, quiet } = mountColumn()
+    // At rest — the pointer has never been over the column — the bars are off.
+    expect(quiet()).toBe(true)
+    movePointer(column, 'in')
+    expect(quiet()).toBe(false)
+    movePointer(column, 'out')
+    // The linger: still drawn just before the window closes, gone just after.
+    act(() => { vi.advanceTimersByTime(1999) })
+    expect(quiet()).toBe(false)
+    act(() => { vi.advanceTimersByTime(1) })
+    expect(quiet()).toBe(true)
+  })
+
+  it('cancels a pending hide when the pointer comes back', () => {
+    vi.useFakeTimers()
+    const { column, quiet } = mountColumn()
+    movePointer(column, 'in')
+    movePointer(column, 'out')
+    act(() => { vi.advanceTimersByTime(1000) })
+    movePointer(column, 'in')
+    // The first leave's timer would fire here; a cancelled one leaves the bars
+    // drawn, which is what keeps a pointer skirting the edge from blinking them.
+    act(() => { vi.advanceTimersByTime(5000) })
+    expect(quiet()).toBe(false)
+  })
+
+  it('drops the pending hide when the column unmounts', () => {
+    vi.useFakeTimers()
+    const { column } = mountColumn()
+    movePointer(column, 'in')
+    movePointer(column, 'out')
+    cleanup()
+    // A timer surviving the unmount would call setState on a dead component.
+    expect(() => { vi.advanceTimersByTime(5000) }).not.toThrow()
+    expect(vi.getTimerCount()).toBe(0)
+  })
+})

packages/client/ui-sidebar/tests/scrollbar-quiet-styles.spec.ts

@@ -0,0 +1,33 @@
+/**
+ * The quiet-column rule as CSS text: the state SidebarRoot toggles
+ * (pointer-scrollbars.spec.tsx) hides a scrollbar only through this rule, and
+ * ui-theme's gate checks the rebinding contract's shape without knowing which
+ * sheet states which half.
+ */
+import { readFileSync } from 'node:fs'
+import { fileURLToPath } from 'node:url'
+import { describe, expect, it } from 'vitest'
+
+const css = readFileSync(fileURLToPath(new URL('../src/client/SidebarRoot.module.css', import.meta.url)), 'utf8')
+/** Declarations only: the sheet's prose names the properties it explains. */
+const declarationText = css.replace(/\/\*[\s\S]*?\*\//g, ' ')
+
+describe('SidebarRoot.module.css quiet column', () => {
+  it('rebinds the ui-theme indirection pair to transparent', () => {
+    // The pair, not the resting thumb alone: rebinding one leaves the other
+    // painting its base-surface colour the moment the pointer reaches the bar.
+    const rule = /\.root\.quietBars\s*\{([^{}]*)\}/.exec(declarationText)
+    expect(rule).not.toBeNull()
+    const declarations = (rule![1] ?? '').split(';').map(part => part.trim()).filter(Boolean).sort()
+    expect(declarations).toEqual([
+      '--dsh-scrollbar-thumb-hover: transparent',
+      '--dsh-scrollbar-thumb: transparent',
+    ].sort())
+  })
+
+  it('leaves the gutter reservation to the scrolling region', () => {
+    // Hiding the thumb must not move a row: the reservation lives on the list
+    // (ui-workspace), so the column states colour only.
+    expect(declarationText).not.toMatch(/scrollbar-gutter/)
+  })
+})

packages/client/ui-theme/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/client/ui-theme/README.md
-README.md: a1ff7d840dae86f5da98de1208ecda3b8b62026b
-README.zh.md: 2e034f3173baabb3eea6b4ae670d5070c95480be
+README.md: 88e21fe214ec806b101050949690283d811be36d
+README.zh.md: 4ed45070234acb78a2e5edef52578b504ae53077

packages/client/ui-theme/README.md

@@ -6,7 +6,7 @@ Theme plugin: ThemeService over the --dsw-* token base stylesheets (static scale
 
 `src/styles/` holds five sheets, all imported by the web shell's `base.css`: `base.css`, `design-platform.css`, `scrollbar.css`, `gradient-shadow-text.css`, and `shiki.css`. `scrollbar.css` is the sole consumer of the `--dsw-alias-scrollbar-*` tokens and must follow `design-platform.css`, which declares them.
 
-Scrollbar rebinding contract: `scrollbar.css` binds `--dsh-scrollbar-thumb` and `--dsh-scrollbar-thumb-hover` on `body` to the l1 (base-surface) tokens, and both rendering paths read that pair. An elevated surface (menu, popover, dialog) sets `--dsh-scrollbar-thumb: var(--dsw-alias-scrollbar-bg-l2)` and `--dsh-scrollbar-thumb-hover: var(--dsw-alias-scrollbar-hover-l2)` on its own container; one rebind retints whichever path the engine took.
+Scrollbar rebinding contract: `scrollbar.css` binds `--dsh-scrollbar-thumb` and `--dsh-scrollbar-thumb-hover` on `body` to the l1 (base-surface) tokens, and both rendering paths read that pair. An elevated surface (menu, popover, dialog) sets `--dsh-scrollbar-thumb: var(--dsw-alias-scrollbar-bg-l2)` and `--dsh-scrollbar-thumb-hover: var(--dsw-alias-scrollbar-hover-l2)` on its own container; one rebind retints whichever path the engine took. The pair's other legal target is `transparent`, which draws no thumb at all — [ui-sidebar](../ui-sidebar/README.md) rebinds its column that way while the pointer is elsewhere. A rebind to the l1 pair is not a rebind; it restates the base-surface default.
 
 The two paths are mutually exclusive by construction. `scrollbar-width`/`scrollbar-color` sit inside `@supports not selector(::-webkit-scrollbar)` because a non-`auto` value of either makes Chromium and Safari discard every `::-webkit-scrollbar*` rule for that element, `::-webkit-scrollbar-thumb:hover` included — declaring both unconditionally leaves `--dsh-scrollbar-thumb-hover` with no rendering anywhere. Firefox therefore takes the standard properties and WebKit-based engines take the pseudo-elements, so the hover token only ever renders through the pseudo-element path. Reasoning and the measured computed values: [the scrollbar Agent Note](../../../.agents/notes/implemented/bug-fix/2026-07-28-themed-scrollbars-and-reserved-gutter.md).
 

packages/client/ui-theme/README.zh.md

@@ -6,7 +6,7 @@
 
 `src/styles/` 下有五张样式表，全部由 web 壳的 `base.css` 导入：`base.css`、`design-platform.css`、`scrollbar.css`、`gradient-shadow-text.css` 与 `shiki.css`。`scrollbar.css` 是 `--dsw-alias-scrollbar-*` token 的唯一消费方，必须排在声明这些 token 的 `design-platform.css` 之后。
 
-滚动条重新绑定契约：`scrollbar.css` 在 `body` 上把 `--dsh-scrollbar-thumb` 与 `--dsh-scrollbar-thumb-hover` 绑定到 l1（基础表面）token，两条渲染路径都读取这一组变量。高层级表面（菜单、浮层、对话框）在自己的容器上设置 `--dsh-scrollbar-thumb: var(--dsw-alias-scrollbar-bg-l2)` 与 `--dsh-scrollbar-thumb-hover: var(--dsw-alias-scrollbar-hover-l2)`；一次重新绑定即可为引擎实际走的那条路径换色。
+滚动条重新绑定契约：`scrollbar.css` 在 `body` 上把 `--dsh-scrollbar-thumb` 与 `--dsh-scrollbar-thumb-hover` 绑定到 l1（基础表面）token，两条渲染路径都读取这一组变量。高层级表面（菜单、浮层、对话框）在自己的容器上设置 `--dsh-scrollbar-thumb: var(--dsw-alias-scrollbar-bg-l2)` 与 `--dsh-scrollbar-thumb-hover: var(--dsw-alias-scrollbar-hover-l2)`；一次重新绑定即可为引擎实际走的那条路径换色。这组变量另一个合法的目标是 `transparent`，即完全不绘制滑块——[ui-sidebar](../ui-sidebar/README.md) 在指针不在栏内时就这样重新绑定自己的列。绑回 l1 那组不算重新绑定，它只是重述基础表面的默认值。
 
 两条路径在构造上互斥。`scrollbar-width`／`scrollbar-color` 写在 `@supports not selector(::-webkit-scrollbar)` 之内，因为这两个属性中的任一个只要取非 `auto` 值，Chromium 与 Safari 就会丢弃该元素上的全部 `::-webkit-scrollbar*` 规则，`::-webkit-scrollbar-thumb:hover` 也在其中——若无条件地同时声明，`--dsh-scrollbar-thumb-hover` 在任何引擎上都不会被渲染。因此 Firefox 走标准属性，WebKit 系引擎走伪元素，hover token 只经由伪元素这条路径渲染。推理过程与实测计算值见[滚动条 Agent Note（agent 决策记录）](../../../.agents/notes/implemented/bug-fix/2026-07-28-themed-scrollbars-and-reserved-gutter.md)。
 

packages/client/ui-theme/tests/scrollbar-styles.spec.ts

@@ -31,6 +31,8 @@ const DARK_ATTRIBUTE = '[data-ds-dark-theme]'
 const TOKEN_PREFIX = '--dsw-alias-scrollbar-'
 /** Prefix of the rebindable indirection scrollbar.css owns. */
 const INDIRECTION_PREFIX = '--dsh-scrollbar-'
+/** The one non-token rebind value: a surface that draws no thumb at all. */
+const HIDDEN_THUMB = 'transparent'
 
 /**
  * Flatten a stylesheet into rules. Whitespace, declaration order, and trailing
@@ -452,11 +454,19 @@ describe('elevated surface rebinds', () => {
     }
   })
 
-  it('every rebind targets the l2 elevation pair', () => {
+  it('every rebind targets the l2 elevation pair or hides the bar outright', () => {
+    // Two targets, and nothing else. An elevated surface moves the pair to l2;
+    // a surface that draws no bar at all states `transparent` (ui-sidebar's
+    // column, whose scrollbars follow the pointer). What this rejects is a
+    // rebind to l1, which restates the base-surface default under a name that
+    // reads as an elevation, and a literal colour, which leaves the palette.
     for (const { file, rule } of rebindRules) {
       for (const [property, value] of rule.declarations) {
         if (!property.startsWith(INDIRECTION_PREFIX)) continue
-        for (const token of varReferences(value)) {
+        if (value === HIDDEN_THUMB) continue
+        const tokens = varReferences(value)
+        expect(tokens, `${file}: ${property}: ${value}`).not.toEqual([])
+        for (const token of tokens) {
           expect(token, `${file}: ${property}`).toMatch(/-l2$/)
         }
       }