refactor(desktop): give the sidebar row geometry and a bounded cache one owner

Row geometry lived inside chrome.tsx as private consts, so anything that
wanted to line up with a session row copied the literals instead — the lead
cell alone appeared verbatim in six files. Its own comment warns that owning
height anywhere else makes rows float 1-2px off sessions, which is exactly
what a copy invites. Split the measurements into row-geometry.ts, keep the
public names re-exported from chrome.tsx, and put the lead cell and
ConnectionGlyph on the plugin SDK so a plugin composes the same box rather
than re-deriving it. ConnectionGlyph takes a className now so a caller can
tint it without forking the component.

LruCache is the same move for a different leak: katex-memo.ts had a private
one with a hardcoded ceiling, and the bot rail had two Maps that never
evicted. One shared class, exported, katex's twin deleted.

apps/desktop/src/app/chat/sidebar/chrome.tsx

@@ -9,6 +9,14 @@ import { compactNumber } from '@/lib/format'
 import { cn } from '@/lib/utils'
 import { $sidebarRowMeta } from '@/store/layout'
 
+import {
+  SIDEBAR_ROW_INSET,
+  SIDEBAR_ROW_LABEL,
+  SIDEBAR_ROW_LEAD,
+  SIDEBAR_ROW_MIN_H,
+  SIDEBAR_ROW_PAD_TRAIL
+} from './row-geometry'
+
 // Shared, content-agnostic sidebar chrome — used by both the flat session
 // sections and the project/workspace tree, so it lives outside either to keep
 // imports one-directional (no index <-> projects cycle).
@@ -18,39 +26,10 @@ export function SidebarSectionMeta({ children }: { children: React.ReactNode })
   return <span className="shrink-0 text-[0.6875rem] font-medium text-(--ui-text-quaternary)">{children}</span>
 }
 
-// ── Row geometry (session row is canonical — everything composes these) ─────
-//
-// Height lives ONLY on SidebarRowShell (min-h-[1.625rem]). Inset children
-// stretch to fill the cell and center content internally — never items-center
-// on the shell grid, or short clusters (projects) float 1–2px off sessions.
-//
-// `rowPadX` is the BODY's padding: the lead's inset, plus the gap the label
-// keeps from the actions column, both inside the row's click target.
-// `rowPadTrail` is the row's own trailing inset and belongs to the SHELL — the
-// only box containing both the actions column AND the card's in-body cluster,
-// so one class insets every trailing thing a row can render. Owned anywhere
-// else, the age / chips / kebab sit flush on the border box, which is exactly
-// where a working row paints its arc (`.arc-row` has zero standoff) — the ring
-// ran through the text.
-
-const rowMinH = 'min-h-[1.625rem]'
-const rowPadX = 'pl-2 pr-2'
-const rowPadTrail = 'pr-2'
-const rowGap = 'gap-1.5'
-const rowLead = 'grid size-3.5 shrink-0 place-items-center'
-const rowInset = cn(rowPadX, rowGap, 'flex h-full min-w-0 items-center self-stretch py-0.5')
-// `truncate` is overflow:hidden. `leading-none` (line-height: 1) makes the
-// line box equal the em-square, so glyph ink that sticks out — Segoe UI on
-// Windows is ~1.33em — gets shaved. 1.35 leaves room; the shell still owns
-// row height, so the extra leading just centers.
-export const SIDEBAR_TRUNCATED_LEADING = 'leading-[1.35]' as const
-const rowLabel = cn('min-w-0 truncate text-[0.8125rem] text-(--ui-text-secondary)', SIDEBAR_TRUNCATED_LEADING)
-
-/** Inbox-style card (workspace + age, title + preview, model + size). */
-export const SIDEBAR_ROW_CARD_MIN_H = 'min-h-[3.375rem]' as const
-
-/** Codicon size in sidebar row leads — matches the file tree (`tree.tsx`). */
-export const SIDEBAR_LEAD_ICON_SIZE = '0.875rem' as const
+// Row geometry lives in `row-geometry.ts` — see that file for why each class
+// belongs to the box it belongs to. Re-exported here because this module is
+// where callers already look for row chrome.
+export { SIDEBAR_LEAD_ICON_SIZE, SIDEBAR_ROW_CARD_MIN_H, SIDEBAR_TRUNCATED_LEADING } from './row-geometry'
 
 /** Vertical stack of rows (gap-px, single column). */
 export function SidebarRowStack({ className, ...props }: React.ComponentProps<'div'>) {
@@ -100,7 +79,12 @@ export function SidebarRowShell({
 }: React.ComponentProps<'div'> & { actions?: React.ReactNode; actionsClassName?: string }) {
   return (
     <div
-      className={cn(rowMinH, rowPadTrail, 'grid grid-cols-[minmax(0,1fr)_auto] items-stretch rounded-md', className)}
+      className={cn(
+        SIDEBAR_ROW_MIN_H,
+        SIDEBAR_ROW_PAD_TRAIL,
+        'grid grid-cols-[minmax(0,1fr)_auto] items-stretch rounded-md',
+        className
+      )}
       {...props}
     >
       {children}
@@ -115,12 +99,12 @@ export function SidebarRowShell({
 
 /** Multi-control left cluster (project rows). */
 export function SidebarRowCluster({ className, ...props }: React.ComponentProps<'div'>) {
-  return <div className={cn(rowInset, className)} {...props} />
+  return <div className={cn(SIDEBAR_ROW_INSET, className)} {...props} />
 }
 
 /** Session row main tap target. */
 export function SidebarRowBody({ className, ...props }: React.ComponentProps<'button'>) {
-  return <RowButton className={cn(rowInset, 'bg-transparent text-left', className)} {...props} />
+  return <RowButton className={cn(SIDEBAR_ROW_INSET, 'bg-transparent text-left', className)} {...props} />
 }
 
 /** Tappable label — underline/truncate live on the inner span, not the button. */
@@ -132,19 +116,19 @@ export function SidebarRowLink({
 }: React.ComponentProps<'button'> & { labelClassName?: string }) {
   return (
     <RowButton className={cn('min-w-0 shrink bg-transparent p-0 text-left', className)} {...props}>
-      <span className={cn(rowLabel, labelClassName)}>{children}</span>
+      <span className={cn(SIDEBAR_ROW_LABEL, labelClassName)}>{children}</span>
     </RowButton>
   )
 }
 
 /** Fixed leading column (dot, icon, drag handle). */
 export function SidebarRowLead({ className, ...props }: React.ComponentProps<'span'>) {
-  return <span className={cn(rowLead, className)} {...props} />
+  return <span className={cn(SIDEBAR_ROW_LEAD, className)} {...props} />
 }
 
 /** Standard row label typography. */
 export function SidebarRowLabel({ className, ...props }: React.ComponentProps<'span'>) {
-  return <span className={cn(rowLabel, className)} {...props} />
+  return <span className={cn(SIDEBAR_ROW_LABEL, className)} {...props} />
 }
 
 /** What a group's sessions add up to, for the Show options that count something. */

apps/desktop/src/app/chat/sidebar/connection-glyph.tsx

@@ -1,11 +1,21 @@
 import type { DesktopRegistryConnection } from '@/global'
 import { Cloud, Monitor, Network, Terminal } from '@/lib/icons'
+import { cn } from '@/lib/utils'
+
+import { SIDEBAR_ROW_LEAD } from './row-geometry'
 
 // One glyph per connection kind — device, cloud, network, terminal — shared by
-// the statusbar switcher, its menu, and the fleet profile rail so a gateway
-// looks the same wherever it is named. Dependency-free on purpose (icons and
-// a type only) so light components can use it without pulling in stores.
-export function ConnectionGlyph({ connection }: { connection: Pick<DesktopRegistryConnection, 'kind'> }) {
+// the statusbar switcher, its menu, the fleet profile rail and the Bots rail so
+// a gateway looks the same wherever it is named. Dependency-free on purpose
+// (icons, a type and class strings) so light components can use it without
+// pulling in stores.
+export function ConnectionGlyph({
+  className,
+  connection
+}: {
+  className?: string
+  connection: Pick<DesktopRegistryConnection, 'kind'>
+}) {
   const Icon =
     connection.kind === 'local'
       ? Monitor
@@ -18,7 +28,7 @@ export function ConnectionGlyph({ connection }: { connection: Pick<DesktopRegist
   return (
     <span
       aria-hidden="true"
-      className="grid size-3.5 shrink-0 place-items-center text-(--ui-text-quaternary)"
+      className={cn(SIDEBAR_ROW_LEAD, 'text-(--ui-text-quaternary)', className)}
       data-connection-kind={connection.kind}
       data-slot="connection-glyph"
     >

apps/desktop/src/app/chat/sidebar/row-geometry.ts

@@ -0,0 +1,55 @@
+import { cn } from '@/lib/utils'
+
+// The sidebar row's measurements, in one place. The session row is canonical
+// and everything else composes these — a row that spells its own geometry
+// floats 1–2px off the sessions above it, which is how the drift starts.
+//
+// Split out of `chrome.tsx` so the literals are reachable without the
+// components: light, store-free callers (a connection glyph) and the plugin
+// SDK both need the lead cell, and neither should have to pull nanostores to
+// line up with a session row.
+//
+// Height lives ONLY on the shell (`SIDEBAR_ROW_MIN_H`). Inset children stretch
+// to fill the cell and center content internally — never `items-center` on the
+// shell grid, or short clusters (projects) float off sessions.
+//
+// `SIDEBAR_ROW_PAD_X` is the BODY's padding: the lead's inset, plus the gap the
+// label keeps from the actions column, both inside the row's click target.
+// `SIDEBAR_ROW_PAD_TRAIL` is the row's own trailing inset and belongs to the
+// SHELL — the only box containing both the actions column AND the card's
+// in-body cluster, so one class insets every trailing thing a row can render.
+// Owned anywhere else, the age / chips / kebab sit flush on the border box,
+// which is exactly where a working row paints its arc (`.arc-row` has zero
+// standoff) — the ring ran through the text.
+
+export const SIDEBAR_ROW_MIN_H = 'min-h-[1.625rem]' as const
+export const SIDEBAR_ROW_PAD_X = 'pl-2 pr-2' as const
+export const SIDEBAR_ROW_PAD_TRAIL = 'pr-2' as const
+export const SIDEBAR_ROW_GAP = 'gap-1.5' as const
+
+/** Fixed leading cell — dot, icon, drag handle. Every row's label starts at the
+ *  same left edge because they all reserve this exact box. */
+export const SIDEBAR_ROW_LEAD = 'grid size-3.5 shrink-0 place-items-center' as const
+
+export const SIDEBAR_ROW_INSET = cn(
+  SIDEBAR_ROW_PAD_X,
+  SIDEBAR_ROW_GAP,
+  'flex h-full min-w-0 items-center self-stretch py-0.5'
+)
+
+// `truncate` is overflow:hidden. `leading-none` (line-height: 1) makes the line
+// box equal the em-square, so glyph ink that sticks out — Segoe UI on Windows
+// is ~1.33em — gets shaved. 1.35 leaves room; the shell still owns row height,
+// so the extra leading just centers.
+export const SIDEBAR_TRUNCATED_LEADING = 'leading-[1.35]' as const
+
+export const SIDEBAR_ROW_LABEL = cn(
+  'min-w-0 truncate text-[0.8125rem] text-(--ui-text-secondary)',
+  SIDEBAR_TRUNCATED_LEADING
+)
+
+/** Inbox-style card (workspace + age, title + preview, model + size). */
+export const SIDEBAR_ROW_CARD_MIN_H = 'min-h-[3.375rem]' as const
+
+/** Codicon size in sidebar row leads — matches the file tree (`tree.tsx`). */
+export const SIDEBAR_LEAD_ICON_SIZE = '0.875rem' as const

apps/desktop/src/lib/katex-memo.ts

@@ -45,6 +45,8 @@ import type { Pluggable } from 'unified'
 import { SKIP, visitParents } from 'unist-util-visit-parents'
 import type { VFile } from 'vfile'
 
+import { LruCache } from '@/lib/lru-cache'
+
 interface KatexMemoOptions {
   /**
    * Color used for KaTeX errors when we fall back to the lenient parser.
@@ -69,40 +71,7 @@ type CachedRender = ElementContent[]
 
 const CACHE_LIMIT = 512
 
-class LruCache<K, V> {
-  private readonly map = new Map<K, V>()
-
-  get(key: K): undefined | V {
-    const value = this.map.get(key)
-
-    if (value === undefined) {
-      return undefined
-    }
-
-    // Refresh recency by re-inserting at the tail. Map iteration order is
-    // insertion order, so the oldest entry is at the head.
-    this.map.delete(key)
-    this.map.set(key, value)
-
-    return value
-  }
-
-  set(key: K, value: V): void {
-    if (this.map.has(key)) {
-      this.map.delete(key)
-    } else if (this.map.size >= CACHE_LIMIT) {
-      const oldest = this.map.keys().next().value
-
-      if (oldest !== undefined) {
-        this.map.delete(oldest)
-      }
-    }
-
-    this.map.set(key, value)
-  }
-}
-
-const cache = new LruCache<string, CachedRender>()
+const cache = new LruCache<string, CachedRender>(CACHE_LIMIT)
 
 function cacheKey(displayMode: boolean, value: string): string {
   // `\u0001` is a control character that (a) won't appear in normal

apps/desktop/src/lib/lru-cache.test.ts

@@ -0,0 +1,68 @@
+import { describe, expect, it } from 'vitest'
+
+import { LruCache } from './lru-cache'
+
+describe('LruCache', () => {
+  it('never grows past its ceiling', () => {
+    const cache = new LruCache<number, string>(3)
+
+    for (let key = 0; key < 100; key += 1) {
+      cache.set(key, `v${key}`)
+    }
+
+    expect(cache.size).toBe(3)
+    expect([...cache.keys()]).toEqual([97, 98, 99])
+  })
+
+  it('evicts the least recently READ entry, not the least recently written', () => {
+    const cache = new LruCache<string, number>(2)
+
+    cache.set('a', 1)
+    cache.set('b', 2)
+    cache.get('a')
+    cache.set('c', 3)
+
+    expect(cache.get('a')).toBe(1)
+    expect(cache.get('b')).toBeUndefined()
+    expect(cache.get('c')).toBe(3)
+  })
+
+  it('overwrites in place rather than evicting a live neighbour', () => {
+    const cache = new LruCache<string, number>(2)
+
+    cache.set('a', 1)
+    cache.set('b', 2)
+    cache.set('a', 9)
+
+    expect(cache.size).toBe(2)
+    expect(cache.get('a')).toBe(9)
+    expect(cache.get('b')).toBe(2)
+  })
+
+  it('answers membership without promoting the entry', () => {
+    const cache = new LruCache<string, number>(2)
+
+    cache.set('a', 1)
+    cache.set('b', 2)
+    expect(cache.has('a')).toBe(true)
+    cache.set('c', 3)
+
+    expect(cache.has('a')).toBe(false)
+    expect(cache.has('b')).toBe(true)
+  })
+
+  it('frees the slot on delete', () => {
+    const cache = new LruCache<string, number>(2)
+
+    cache.set('a', 1)
+
+    expect(cache.delete('a')).toBe(true)
+    expect(cache.delete('a')).toBe(false)
+    expect(cache.size).toBe(0)
+
+    cache.set('b', 2)
+    cache.set('c', 3)
+
+    expect([...cache.keys()]).toEqual(['b', 'c'])
+  })
+})

apps/desktop/src/lib/lru-cache.ts

@@ -0,0 +1,67 @@
+/**
+ * A `Map` with a ceiling. The renderer stays open for days, so a module-level
+ * cache keyed by anything user-driven (a URL, a connection id, an equation)
+ * grows for the life of the window unless something evicts.
+ *
+ * Only for values that can be REGENERATED: eviction costs a recompute or a
+ * refetch, never correctness. State whose loss changes behaviour — an unsent
+ * draft, an unread watermark, a freshness fence — is a record, not a cache,
+ * and belongs in a plain `Map`.
+ */
+export class LruCache<K, V> {
+  private readonly entries = new Map<K, V>()
+  private readonly max: number
+
+  constructor(max: number) {
+    this.max = max
+  }
+
+  get size(): number {
+    return this.entries.size
+  }
+
+  /** Read and mark most-recently-used. */
+  get(key: K): undefined | V {
+    const value = this.entries.get(key)
+
+    if (value === undefined) {
+      return undefined
+    }
+
+    // Map iterates in insertion order, so re-inserting moves the entry to the
+    // tail and leaves the least recently used one at the head.
+    this.entries.delete(key)
+    this.entries.set(key, value)
+
+    return value
+  }
+
+  /** Membership WITHOUT touching recency — for the `has` / `set` / `get`
+   *  populate-on-miss shape, where the `get` does the touching. */
+  has(key: K): boolean {
+    return this.entries.has(key)
+  }
+
+  /** Write, evicting the least recently used entry once the cache is full. */
+  set(key: K, value: V): void {
+    if (this.entries.has(key)) {
+      this.entries.delete(key)
+    } else if (this.entries.size >= this.max) {
+      const oldest = this.entries.keys().next().value
+
+      if (oldest !== undefined) {
+        this.entries.delete(oldest)
+      }
+    }
+
+    this.entries.set(key, value)
+  }
+
+  delete(key: K): boolean {
+    return this.entries.delete(key)
+  }
+
+  keys(): IterableIterator<K> {
+    return this.entries.keys()
+  }
+}

apps/desktop/src/sdk/index.ts

@@ -1351,6 +1351,16 @@ export {
  *  circle beside it — a plugin's own dot inverts core's color vocabulary the
  *  moment either side moves. */
 export { SessionStatusDot, type SessionStatusDotProps } from '@/app/chat/session-status-dot'
+/** The sidebar row's leading cell — the fixed box a dot, icon or handle sits in.
+ *  Reserve it and your label starts on the same left edge as every session row
+ *  above you; spell the classes yourself and the row drifts. The session row is
+ *  canonical; `row-geometry.ts` explains what each measurement belongs to. */
+export { SidebarRowLead } from '@/app/chat/sidebar/chrome'
+/** One glyph per gateway kind — device, cloud, terminal, network. The statusbar
+ *  switcher, the fleet profile rail and any plugin rail listing gateways share
+ *  it, so a connection looks the same wherever it is named. */
+export { ConnectionGlyph } from '@/app/chat/sidebar/connection-glyph'
+export { SIDEBAR_ROW_LEAD, SIDEBAR_TRUNCATED_LEADING } from '@/app/chat/sidebar/row-geometry'
 export { PALETTE_AREA, type PaletteContribution } from '@/app/command-palette/contrib'
 /** THE master-detail toolkit core uses for list+inspector surfaces (Scheduled
  *  jobs, Kanban, …): a dense left `PanelList` of `PanelListRow`s beside a
@@ -1379,6 +1389,7 @@ export {
   PanelSectionLabel
 } from '@/app/overlays/panel'
 export { type RouteContribution, ROUTES_AREA, SIDEBAR_NAV_AREA, type SidebarNavContribution } from '@/app/routes'
+
 /** THE full per-toolset config panel core Settings renders — provider picker,
  *  env vars / API keys, model catalog picker, and post-setup runners. Route-
  *  decoupled (the "manage keys" deep link is a no-op outside the router); pass
@@ -1397,7 +1408,6 @@ export {
   type ModelMenuController
 } from '@/app/shell/model-catalog-menu'
 export type { StatusbarItem } from '@/app/shell/statusbar-controls'
-
 export type { TitlebarTool } from '@/app/shell/titlebar-controls'
 /** THE whole Capabilities surface (Skills / Tools / MCP tabs, installed
  *  lists, full-skill detail pane, embedded hub picker with one-click
@@ -1560,6 +1570,10 @@ export type { HermesOpenTarget } from '@/lib/hermes-open-target'
 export * as icons from '@/lib/icons'
 export { type KeybindContribution, KEYBINDS_AREA } from '@/lib/keybinds/actions'
 export { formatModifierToken } from '@/lib/keybinds/combo'
+/** A `Map` with a ceiling, for the module-level caches a plugin keeps across
+ *  a renderer that stays open for days. Only for values that can be
+ *  regenerated — eviction costs a recompute or a refetch, never correctness. */
+export { LruCache } from '@/lib/lru-cache'
 /** The app's deterministic identity color for a name (profiles, assignees,
  *  authors), its translucent tag fill, and the curated picker swatches — so
  *  plugin-rendered identities read the same hue as everywhere else. The