Bump to v2026.10.9

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

USER_GUIDE.md

@@ -68,9 +68,20 @@ new message, or if you message them again yourself.
 Type in the message box at the bottom of a room and press **Enter** to
 send. Use **Shift+Enter** to add a line break without sending.
 
+Each message shows when it was sent, in your local time. Messages from today
+show just the time; older ones add the day — "Yesterday 4:04 PM", a weekday
+like "Mon 4:04 PM" for the past week, then the date ("Oct 2, 4:04 PM", with
+the year if it's not this one). Hover a time to see the full date.
+
 ## Formatting
 
-Messages support Markdown:
+Messages support Markdown. Prefer buttons to typing the syntax? Click the
+**Aa** button next to the emoji button in the composer to open a formatting
+toolbar — it inserts the markdown below into your message for you (select some
+text first to wrap it, or click a button again to take the formatting back
+off). It stays collapsed until you open it, and remembers your choice.
+
+Or type it yourself:
 
 - `**bold**`, `*italic*`, `~~strikethrough~~`
 - `` `inline code` `` and fenced code blocks (three backticks)

backend/pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "ds-chat"
-version = "2026.9.26"
+version = "2026.10.9"
 description = "DS Chat backend service"
 license = { text = "AGPL-3.0-or-later" }
 requires-python = ">=3.11"

frontend/package.json

@@ -1,14 +1,15 @@
 {
   "name": "frontend",
   "private": true,
-  "version": "2026.9.26",
+  "version": "2026.10.9",
   "license": "AGPL-3.0-or-later",
   "type": "module",
   "scripts": {
     "dev": "vite",
     "build": "tsc -b && vite build",
     "lint": "oxlint",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "test": "vitest run"
   },
   "dependencies": {
     "markdown-to-jsx": "^9.10.2",
@@ -18,13 +19,16 @@
     "vite-plugin-pwa": "^1.3.0"
   },
   "devDependencies": {
+    "@testing-library/react": "^16.3.3",
     "@types/node": "^24.13.3",
     "@types/react": "^19.2.17",
     "@types/react-dom": "^19.2.3",
     "@vitejs/plugin-react": "^6.0.4",
+    "jsdom": "^30.1.1",
     "oxlint": "^1.75.0",
     "typescript": "~6.0.2",
     "vite": "^8.2.0",
+    "vitest": "^5.0.2",
     "workbox-cacheable-response": "^7.4.1",
     "workbox-expiration": "^7.4.1",
     "workbox-precaching": "^7.4.1",

frontend/src/components/Composer.css

@@ -105,6 +105,34 @@
   cursor: not-allowed;
 }
 
+.composer-format-toggle {
+  width: 36px;
+  height: 36px;
+  flex: none;
+  border-radius: var(--radius);
+  background: transparent;
+  color: var(--ds-muted);
+  border: 1px solid var(--ds-border);
+  font-size: 0.82rem;
+  font-weight: 700;
+  line-height: 1;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  cursor: pointer;
+}
+
+.composer-format-toggle:hover:not(:disabled),
+.composer-format-toggle-open {
+  color: var(--ds-text);
+  border-color: var(--ds-accent);
+}
+
+.composer-format-toggle:disabled {
+  opacity: 0.5;
+  cursor: not-allowed;
+}
+
 .composer-attach-wrap {
   position: relative;
   flex: none;

frontend/src/components/Composer.tsx

@@ -15,10 +15,13 @@ import { getUploadLimit } from '../api/uploads'
 import { useCustomEmoji } from '../context/CustomEmojiContext'
 import { EMOJI_SHORTCODES, SHORTCODE_BY_GLYPH } from '../lib/emojiShortcodes'
 import { formatFileSize } from '../lib/fileSize'
+import { getFormattingToolbarOpen, setFormattingToolbarOpen } from '../lib/formattingToolbarPref'
+import { applyFormat, type FormatAction } from '../lib/markdownFormat'
 import { getRecentEmoji, recordEmojiUsed } from '../lib/recentEmoji'
 import type { MyRoomItem, RoomMember } from '../types'
 import { EmojiPicker } from './EmojiPicker'
 import { EmojiShortcodeAutocomplete, type EmojiShortcodeMatch } from './EmojiShortcodeAutocomplete'
+import { FormattingToolbar } from './FormattingToolbar'
 import { MentionAutocomplete } from './MentionAutocomplete'
 import { RoomReferenceAutocomplete } from './RoomReferenceAutocomplete'
 import './Composer.css'
@@ -136,6 +139,7 @@ export function Composer({ roomId, roomName, isDm, members, rooms, disabled, arc
   const [emojiActiveIndex, setEmojiActiveIndex] = useState(0)
   const [dragActive, setDragActive] = useState(false)
   const [attachMenuOpen, setAttachMenuOpen] = useState(false)
+  const [toolbarOpen, setToolbarOpen] = useState(getFormattingToolbarOpen)
   const { byShortcode: customEmojiByShortcode } = useCustomEmoji()
   const textareaRef = useRef<HTMLTextAreaElement>(null)
   const fileInputRef = useRef<HTMLInputElement>(null)
@@ -449,6 +453,29 @@ export function Composer({ roomId, roomName, isDm, members, rooms, disabled, arc
     })
   }
 
+  function toggleToolbar() {
+    const next = !toolbarOpen
+    setToolbarOpen(next)
+    setFormattingToolbarOpen(next)
+  }
+
+  // #75: the toolbar only ever edits the textarea's plain markdown text --
+  // nothing about what gets sent changes.
+  function applyFormatAction(action: FormatAction) {
+    const el = textareaRef.current
+    if (!el) return
+    const result = applyFormat(
+      { value, start: el.selectionStart ?? value.length, end: el.selectionEnd ?? value.length },
+      action,
+    )
+    setValue(result.value)
+    requestAnimationFrame(() => {
+      el.focus()
+      el.setSelectionRange(result.start, result.end)
+      autoGrow()
+    })
+  }
+
   function insertEmoji(emoji: string) {
     const el = textareaRef.current
     setEmojiPickerOpen(false)
@@ -517,6 +544,9 @@ export function Composer({ roomId, roomName, isDm, members, rooms, disabled, arc
         </div>
       )}
       {uploadError && <div className="composer-status composer-error">{uploadError}</div>}
+      {toolbarOpen && (
+        <FormattingToolbar id="composer-formatting-toolbar" disabled={isDisabled} onAction={applyFormatAction} />
+      )}
       <div className="composer-box">
         <input
           ref={photoInputRef}
@@ -589,6 +619,18 @@ export function Composer({ roomId, roomName, isDm, members, rooms, disabled, arc
             />
           )}
         </div>
+        <button
+          type="button"
+          className={`composer-format-toggle${toolbarOpen ? ' composer-format-toggle-open' : ''}`}
+          onClick={toggleToolbar}
+          disabled={isDisabled}
+          aria-label="Formatting options"
+          aria-expanded={toolbarOpen}
+          aria-controls="composer-formatting-toolbar"
+          title="Formatting options"
+        >
+          Aa
+        </button>
         <div className="composer-textarea-wrap">
           <textarea
             ref={textareaRef}

frontend/src/components/FormattingToolbar.css

@@ -0,0 +1,41 @@
+.formatting-toolbar {
+  display: flex;
+  flex-wrap: wrap;
+  align-items: center;
+  /* Spacing, not divider lines, between groups -- a border on every group
+     after the first leaves a dangling line at the start of the second row
+     whenever the toolbar wraps on a narrow screen. */
+  gap: 2px 14px;
+}
+
+.formatting-toolbar-group {
+  display: flex;
+  gap: 2px;
+}
+
+.formatting-toolbar-btn {
+  min-width: 30px;
+  height: 30px;
+  padding: 0 6px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  background: transparent;
+  border: none;
+  border-radius: 6px;
+  color: var(--ds-muted);
+  font-size: 0.82rem;
+  line-height: 1;
+  text-decoration: none;
+  cursor: pointer;
+}
+
+.formatting-toolbar-btn:hover:not(:disabled) {
+  background: var(--ds-surface-2);
+  color: var(--ds-text);
+}
+
+.formatting-toolbar-btn:disabled {
+  opacity: 0.5;
+  cursor: not-allowed;
+}

frontend/src/components/FormattingToolbar.test.tsx

@@ -0,0 +1,55 @@
+import { cleanup, fireEvent, render, screen } from '@testing-library/react'
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import type { FormatAction } from '../lib/markdownFormat'
+import { FormattingToolbar } from './FormattingToolbar'
+
+afterEach(cleanup)
+
+const EXPECTED: [string, FormatAction][] = [
+  ['Bold', { kind: 'inline', marker: '**' }],
+  ['Italic', { kind: 'inline', marker: '*' }],
+  ['Strikethrough', { kind: 'inline', marker: '~~' }],
+  ['Inline code', { kind: 'inline', marker: '`' }],
+  ['Code block', { kind: 'codeBlock' }],
+  ['Link', { kind: 'link' }],
+  ['Quote', { kind: 'prefix', prefix: 'quote' }],
+  ['Bulleted list', { kind: 'prefix', prefix: 'bullet' }],
+  ['Numbered list', { kind: 'prefix', prefix: 'ordered' }],
+  ['Heading 1', { kind: 'heading', level: 1 }],
+  ['Heading 2', { kind: 'heading', level: 2 }],
+  ['Heading 3', { kind: 'heading', level: 3 }],
+  ['Subscript', { kind: 'inline', marker: '~' }],
+  ['Superscript', { kind: 'inline', marker: '^' }],
+]
+
+describe('FormattingToolbar', () => {
+  it.each(EXPECTED)('%s button calls onAction with its action', (label, action) => {
+    const onAction = vi.fn()
+    render(<FormattingToolbar id="t" onAction={onAction} />)
+    fireEvent.click(screen.getByRole('button', { name: label }))
+    expect(onAction).toHaveBeenCalledTimes(1)
+    expect(onAction).toHaveBeenCalledWith(action)
+  })
+
+  it('disables every button when disabled, but leaves the help link usable', () => {
+    render(<FormattingToolbar id="t" disabled onAction={vi.fn()} />)
+    const buttons = screen.getAllByRole('button') as HTMLButtonElement[]
+    expect(buttons).toHaveLength(EXPECTED.length)
+    expect(buttons.every((b) => b.disabled)).toBe(true)
+    expect(screen.getByRole('link', { name: 'Formatting help' })).toBeTruthy()
+  })
+
+  it('opens help in a new tab so the draft in the composer is not lost', () => {
+    render(<FormattingToolbar id="t" onAction={vi.fn()} />)
+    const help = screen.getByRole('link', { name: 'Formatting help' })
+    expect(help.getAttribute('href')).toBe('/help#formatting')
+    expect(help.getAttribute('target')).toBe('_blank')
+    expect(help.getAttribute('rel')).toContain('noopener')
+  })
+
+  it('keeps focus in the textarea by cancelling mousedown on buttons', () => {
+    render(<FormattingToolbar id="t" onAction={vi.fn()} />)
+    // fireEvent returns false when the event's default was prevented.
+    expect(fireEvent.mouseDown(screen.getByRole('button', { name: 'Bold' }))).toBe(false)
+  })
+})

frontend/src/components/FormattingToolbar.tsx

@@ -0,0 +1,104 @@
+import type { ReactNode } from 'react'
+import type { FormatAction, InlineMarker } from '../lib/markdownFormat'
+import './FormattingToolbar.css'
+
+interface FormattingToolbarProps {
+  id: string
+  disabled?: boolean
+  onAction: (action: FormatAction) => void
+}
+
+interface ToolbarButton {
+  label: string
+  // Shows the syntax the button inserts -- the toolbar doubles as the
+  // "quick guide" for people who'd rather learn to type it themselves.
+  title: string
+  content: ReactNode
+  action: FormatAction
+}
+
+const inline = (marker: InlineMarker): FormatAction => ({ kind: 'inline', marker })
+
+const GROUPS: ToolbarButton[][] = [
+  [
+    { label: 'Bold', title: 'Bold: **text**', content: <b>B</b>, action: inline('**') },
+    { label: 'Italic', title: 'Italic: *text*', content: <i>I</i>, action: inline('*') },
+    { label: 'Strikethrough', title: 'Strikethrough: ~~text~~', content: <s>S</s>, action: inline('~~') },
+  ],
+  [
+    { label: 'Inline code', title: 'Inline code: `code`', content: '</>', action: inline('`') },
+    { label: 'Code block', title: 'Code block: ``` on its own lines', content: '{ }', action: { kind: 'codeBlock' } },
+  ],
+  [
+    {
+      label: 'Link',
+      title: 'Link: [text](url)',
+      content: (
+        <svg width="15" height="15" viewBox="0 0 20 20" fill="none" aria-hidden="true">
+          <path
+            d="M8.5 11.5a3 3 0 0 0 4.2 0l2.6-2.6a3 3 0 0 0-4.2-4.2l-.9.9M11.5 8.5a3 3 0 0 0-4.2 0l-2.6 2.6a3 3 0 0 0 4.2 4.2l.9-.9"
+            stroke="currentColor"
+            strokeWidth="1.6"
+            strokeLinecap="round"
+            strokeLinejoin="round"
+          />
+        </svg>
+      ),
+      action: { kind: 'link' },
+    },
+    { label: 'Quote', title: 'Quote: > text', content: '“', action: { kind: 'prefix', prefix: 'quote' } },
+  ],
+  [
+    { label: 'Bulleted list', title: 'Bulleted list: - item', content: '•', action: { kind: 'prefix', prefix: 'bullet' } },
+    { label: 'Numbered list', title: 'Numbered list: 1. item', content: '1.', action: { kind: 'prefix', prefix: 'ordered' } },
+  ],
+  [
+    { label: 'Heading 1', title: 'Heading 1: # text', content: 'H1', action: { kind: 'heading', level: 1 } },
+    { label: 'Heading 2', title: 'Heading 2: ## text', content: 'H2', action: { kind: 'heading', level: 2 } },
+    { label: 'Heading 3', title: 'Heading 3: ### text', content: 'H3', action: { kind: 'heading', level: 3 } },
+  ],
+  [
+    { label: 'Subscript', title: 'Subscript: ~text~ (no spaces)', content: 'x₂', action: inline('~') },
+    { label: 'Superscript', title: 'Superscript: ^text^ (no spaces)', content: 'x²', action: inline('^') },
+  ],
+]
+
+export function FormattingToolbar({ id, disabled, onAction }: FormattingToolbarProps) {
+  return (
+    <div id={id} className="formatting-toolbar" role="toolbar" aria-label="Formatting">
+      {GROUPS.map((group) => (
+        <div key={group[0].label} className="formatting-toolbar-group">
+          {group.map((button) => (
+            <button
+              key={button.label}
+              type="button"
+              className="formatting-toolbar-btn"
+              title={button.title}
+              aria-label={button.label}
+              disabled={disabled}
+              // Keeps focus (and the selection) in the textarea -- a click
+              // on a button would otherwise blur it, and on mobile dismiss
+              // the keyboard.
+              onMouseDown={(e) => e.preventDefault()}
+              onClick={() => onAction(button.action)}
+            >
+              {button.content}
+            </button>
+          ))}
+        </div>
+      ))}
+      {/* A new tab on purpose: the draft lives in Composer's own state, so
+          navigating this one away would lose it. */}
+      <a
+        className="formatting-toolbar-btn formatting-toolbar-help"
+        href="/help#formatting"
+        target="_blank"
+        rel="noopener noreferrer"
+        title="Formatting help (opens in a new tab)"
+        aria-label="Formatting help"
+      >
+        ?
+      </a>
+    </div>
+  )
+}

frontend/src/components/MessageList.css

@@ -40,12 +40,17 @@
 }
 
 .message-author {
+  /* Lets a very long display name shrink/wrap instead of pushing the
+     (now longer, e.g. "Yesterday 4:04 PM") timestamp out of the row. */
+  min-width: 0;
   font-size: 0.86rem;
   font-weight: 700;
   color: var(--ds-text);
 }
 
 .message-time {
+  flex: none;
+  white-space: nowrap;
   font-size: 0.68rem;
   color: var(--ds-muted);
   font-family: var(--mono);

frontend/src/components/MessageList.tsx

@@ -2,8 +2,10 @@ import type { CSSProperties } from 'react'
 import { useEffect, useMemo, useRef, useState } from 'react'
 import { getRoomFileUrl, getRoomImageUrl } from '../api/rooms'
 import { useAuth } from '../context/AuthContext'
+import { useLocalDay } from '../hooks/useLocalDay'
 import { formatFileSize } from '../lib/fileSize'
-import { avatarUrlFor, displayNameFor, senderColorIndex, statusFor } from '../lib/messageGrouping'
+import { formatMessageTime, formatMessageTimeTitle } from '../lib/formatMessageTime'
+import { avatarUrlFor, displayNameFor, isGroupStart, senderColorIndex, statusFor } from '../lib/messageGrouping'
 import type { ChatMessageEnvelope, Message, MessageFileInfo, RoomMember } from '../types'
 import { EMOJI_PICKER_MAX_HEIGHT, EmojiPicker } from './EmojiPicker'
 import { FilePreviewModal, getPreviewKind } from './FilePreviewModal'
@@ -69,13 +71,6 @@ function FileAttachmentCard({ file, roomId, onPreview }: FileAttachmentCardProps
 // trigger a download the moment the browser tries to fetch it).
 const PLAYABLE_VIDEO_CONTENT_TYPES = new Set(['video/mp4', 'video/webm', 'video/ogg'])
 
-// #73: Slack's own threshold for the same "still grouped, but it's been a
-// while" call -- past this gap a same-sender message starts a new group
-// (its own avatar/name/timestamp) even with nobody else posting in
-// between, so a message sent minutes later doesn't hide under a stale
-// timestamp from the start of the run.
-const GROUP_BREAK_MS = 5 * 60 * 1000
-
 interface VideoAttachmentProps {
   file: MessageFileInfo
   roomId: string
@@ -134,6 +129,7 @@ export function MessageList({
   onDelete,
 }: MessageListProps) {
   const { user } = useAuth()
+  const now = useLocalDay()
   // #71: same viewer preference MessageContent applies to in-text emoji,
   // looked up separately here since a reaction pill isn't a descendant of
   // that component's wrapper div (see EmojiGlyph's own comment).
@@ -218,19 +214,15 @@ export function MessageList({
         // consecutive messages from the same sender only repeat the
         // avatar/name/timestamp header on the first one in the run --
         // applies uniformly, including to your own messages. Also breaks on
-        // a long gap (see GROUP_BREAK_MS) so a message sent well after the
-        // rest of the run still gets its own visible timestamp.
-        const isGroupStart =
-          !prev ||
-          prev.user_id !== msg.user_id ||
-          new Date(msg.created_at).getTime() - new Date(prev.created_at).getTime() > GROUP_BREAK_MS
+        // a long gap or a new calendar day -- see isGroupStart.
+        const groupStart = isGroupStart(prev, msg)
         const editing = editingId === msg.id
         const deleted = !!msg.deleted_at
 
         return (
-          <div key={msg.id} className={`message-row${isGroupStart ? ' message-row-start' : ''}`}>
+          <div key={msg.id} className={`message-row${groupStart ? ' message-row-start' : ''}`}>
             <div className="message-avatar-slot">
-              {isGroupStart && (
+              {groupStart && (
                 <UserAvatar
                   username={msg.username}
                   colorIndex={senderColorIndex(msg.username, members)}
@@ -240,11 +232,11 @@ export function MessageList({
               )}
             </div>
             <div className="message-content">
-              {isGroupStart && (
+              {groupStart && (
                 <div className="message-header">
                   <span className="message-author">{displayNameFor(msg.username, members)}</span>
-                  <span className="message-time">
-                    {new Date(msg.created_at).toLocaleTimeString([], { hour: 'numeric', minute: '2-digit' })}
+                  <span className="message-time" title={formatMessageTimeTitle(msg.created_at)}>
+                    {formatMessageTime(msg.created_at, now)}
                   </span>
                 </div>
               )}

frontend/src/hooks/useDesktopRoomBridge.test.ts

@@ -0,0 +1,68 @@
+import { renderHook } from '@testing-library/react'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import * as desktopBridge from '../lib/desktopBridge'
+import { useDesktopRoomBridge } from './useDesktopRoomBridge'
+
+describe('useDesktopRoomBridge', () => {
+  beforeEach(() => {
+    vi.spyOn(desktopBridge, 'notifyRoomOpened')
+    vi.spyOn(desktopBridge, 'notifyUnreadCount')
+  })
+
+  afterEach(() => {
+    vi.restoreAllMocks()
+  })
+
+  it('calls roomOpened once when entering a room', () => {
+    renderHook(({ roomId, count }) => useDesktopRoomBridge(roomId, count), {
+      initialProps: { roomId: 'room-1', count: 0 },
+    })
+    expect(desktopBridge.notifyRoomOpened).toHaveBeenCalledTimes(1)
+  })
+
+  it('does not call roomOpened again on a rerender with the same roomId', () => {
+    const { rerender } = renderHook(({ roomId, count }) => useDesktopRoomBridge(roomId, count), {
+      initialProps: { roomId: 'room-1', count: 0 },
+    })
+    expect(desktopBridge.notifyRoomOpened).toHaveBeenCalledTimes(1)
+
+    // Unrelated rerender -- unreadRoomCount changed, roomId did not.
+    rerender({ roomId: 'room-1', count: 3 })
+    rerender({ roomId: 'room-1', count: 3 })
+    expect(desktopBridge.notifyRoomOpened).toHaveBeenCalledTimes(1)
+  })
+
+  it('calls roomOpened again on a genuine transition to a different room', () => {
+    const { rerender } = renderHook(({ roomId, count }) => useDesktopRoomBridge(roomId, count), {
+      initialProps: { roomId: 'room-1', count: 0 },
+    })
+    rerender({ roomId: 'room-2', count: 0 })
+    expect(desktopBridge.notifyRoomOpened).toHaveBeenCalledTimes(2)
+  })
+
+  it('does not call roomOpened when there is no room id', () => {
+    renderHook(({ roomId, count }) => useDesktopRoomBridge(roomId, count), {
+      initialProps: { roomId: undefined as string | undefined, count: 0 },
+    })
+    expect(desktopBridge.notifyRoomOpened).not.toHaveBeenCalled()
+  })
+
+  it('sends the unread count whenever it changes, independent of roomOpened', () => {
+    const { rerender } = renderHook(({ roomId, count }) => useDesktopRoomBridge(roomId, count), {
+      initialProps: { roomId: undefined as string | undefined, count: 0 },
+    })
+    expect(desktopBridge.notifyUnreadCount).toHaveBeenCalledWith(0)
+
+    rerender({ roomId: undefined, count: 2 })
+    expect(desktopBridge.notifyUnreadCount).toHaveBeenCalledWith(2)
+    expect(desktopBridge.notifyRoomOpened).not.toHaveBeenCalled()
+  })
+})
+
+describe('desktop bridge functions with no window.dsDesktop', () => {
+  it('remain optional -- calling them does not throw when the bridge is absent', () => {
+    expect(window.dsDesktop).toBeUndefined()
+    expect(() => desktopBridge.notifyRoomOpened()).not.toThrow()
+    expect(() => desktopBridge.notifyUnreadCount(5)).not.toThrow()
+  })
+})

frontend/src/hooks/useDesktopRoomBridge.ts

@@ -0,0 +1,26 @@
+import { useEffect } from 'react'
+import { notifyRoomOpened, notifyUnreadCount } from '../lib/desktopBridge'
+
+// Tells the Electron desktop wrapper (a) when the user has genuinely
+// entered a room, so it can clear that room's tray indicator, and (b) the
+// current unread-room count, so the tray badge stays accurate. Both are
+// no-ops in every browser build (see desktopBridge.ts).
+//
+// roomOpened must fire only on a genuine transition to a real room id --
+// never merely because the window was shown, focused, restored, or opened
+// from the tray, and never when no room is open at all. Depending on just
+// the id (a primitive, not the room object) is what makes this safe to
+// call from a component that rerenders often: an unrelated rerender that
+// leaves activeRoomId unchanged doesn't rerun the effect, so there's no
+// separate de-duplication to get wrong here -- it falls out of React's own
+// effect-dependency comparison.
+export function useDesktopRoomBridge(activeRoomId: string | undefined, unreadRoomCount: number): void {
+  useEffect(() => {
+    if (!activeRoomId) return
+    notifyRoomOpened()
+  }, [activeRoomId])
+
+  useEffect(() => {
+    notifyUnreadCount(unreadRoomCount)
+  }, [unreadRoomCount])
+}

frontend/src/hooks/useLocalDay.test.ts

@@ -0,0 +1,67 @@
+import { act, renderHook } from '@testing-library/react'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { useLocalDay } from './useLocalDay'
+
+function setVisibility(state: 'visible' | 'hidden') {
+  Object.defineProperty(document, 'visibilityState', { configurable: true, get: () => state })
+}
+
+describe('useLocalDay', () => {
+  beforeEach(() => {
+    vi.useFakeTimers()
+    setVisibility('visible')
+  })
+
+  afterEach(() => {
+    vi.useRealTimers()
+    // Remove the own-property override so jsdom's real getter shows through again.
+    delete (document as unknown as Record<string, unknown>).visibilityState
+  })
+
+  it('rolls over to the next day when local midnight passes', () => {
+    vi.setSystemTime(new Date(2026, 9, 9, 23, 59, 30))
+    const { result } = renderHook(() => useLocalDay())
+    expect(result.current.getDate()).toBe(9)
+
+    act(() => {
+      vi.advanceTimersByTime(60_000)
+    })
+    expect(result.current.getDate()).toBe(10)
+  })
+
+  it('refreshes when the tab becomes visible after the clock moved a day (e.g. a sleeping laptop)', () => {
+    vi.setSystemTime(new Date(2026, 9, 9, 12, 0))
+    const { result } = renderHook(() => useLocalDay())
+
+    // The clock jumps without any timer firing in between.
+    vi.setSystemTime(new Date(2026, 9, 10, 12, 0))
+    act(() => {
+      document.dispatchEvent(new Event('visibilitychange'))
+    })
+    expect(result.current.getDate()).toBe(10)
+  })
+
+  it('keeps the same value (no rerender) when visibility changes on the same day', () => {
+    vi.setSystemTime(new Date(2026, 9, 9, 12, 0))
+    const { result } = renderHook(() => useLocalDay())
+    const first = result.current
+
+    vi.setSystemTime(new Date(2026, 9, 9, 18, 0))
+    act(() => {
+      document.dispatchEvent(new Event('visibilitychange'))
+    })
+    expect(result.current).toBe(first)
+  })
+
+  it('ignores visibility changes while the tab is hidden', () => {
+    vi.setSystemTime(new Date(2026, 9, 9, 12, 0))
+    const { result } = renderHook(() => useLocalDay())
+
+    setVisibility('hidden')
+    vi.setSystemTime(new Date(2026, 9, 10, 12, 0))
+    act(() => {
+      document.dispatchEvent(new Event('visibilitychange'))
+    })
+    expect(result.current.getDate()).toBe(9)
+  })
+})

frontend/src/hooks/useLocalDay.ts

@@ -0,0 +1,46 @@
+import { useEffect, useState } from 'react'
+import { isSameLocalDay } from '../lib/formatMessageTime'
+
+function msUntilNextLocalMidnight(now: Date): number {
+  // One second past, so the timer never lands a hair before midnight.
+  const next = new Date(now.getFullYear(), now.getMonth(), now.getDate() + 1, 0, 0, 1)
+  return next.getTime() - now.getTime()
+}
+
+// A `now` that only changes when the local calendar day does -- at midnight,
+// and when a tab becomes visible again (a laptop that slept through
+// midnight). Without it, a tab left open overnight keeps showing yesterday's
+// messages as a bare "4:04 PM" until something unrelated rerenders the list.
+export function useLocalDay(): Date {
+  const [now, setNow] = useState(() => new Date())
+
+  useEffect(() => {
+    let timer: ReturnType<typeof setTimeout>
+
+    function refresh() {
+      const current = new Date()
+      // Same state object when the day hasn't changed, so no rerender.
+      setNow((prev) => (isSameLocalDay(prev, current) ? prev : current))
+    }
+
+    function schedule() {
+      timer = setTimeout(() => {
+        refresh()
+        schedule()
+      }, msUntilNextLocalMidnight(new Date()))
+    }
+
+    function handleVisibilityChange() {
+      if (document.visibilityState === 'visible') refresh()
+    }
+
+    schedule()
+    document.addEventListener('visibilitychange', handleVisibilityChange)
+    return () => {
+      clearTimeout(timer)
+      document.removeEventListener('visibilitychange', handleVisibilityChange)
+    }
+  }, [])
+
+  return now
+}

frontend/src/lib/desktopBridge.ts

@@ -15,6 +15,12 @@ declare global {
   interface Window {
     dsDesktop?: {
       setUnreadCount?(unreadRoomCount: number): void
+      // Tells the tray/dock icon the user has genuinely entered a room --
+      // the desktop wrapper's own signal to clear that room's tray
+      // indicator. Deliberately not fired just because the window was
+      // shown/focused/restored, or opened from the tray: none of those mean
+      // the user actually looked at the room the unread message is in.
+      roomOpened?(): void
       showNotification?(notification: DesktopNotificationRequest): void
       onNotificationClick?(callback: (roomId: string) => void): () => void
     }
@@ -38,6 +44,16 @@ export function isDesktopClickListenerSupported(): boolean {
   return typeof window.dsDesktop?.onNotificationClick === 'function'
 }
 
+// Both no-op silently if the bridge or this specific method isn't present,
+// same as showDesktopNotification below -- callers never need to guard.
+export function notifyRoomOpened(): void {
+  window.dsDesktop?.roomOpened?.()
+}
+
+export function notifyUnreadCount(unreadRoomCount: number): void {
+  window.dsDesktop?.setUnreadCount?.(unreadRoomCount)
+}
+
 // No-ops silently if the bridge or this specific method isn't present --
 // callers don't need to guard, matching the rest of this module's
 // capability-detect-per-method philosophy (see #49's "feature-test each

frontend/src/lib/formatMessageTime.test.ts

@@ -0,0 +1,82 @@
+import { describe, expect, it } from 'vitest'
+import { formatMessageTime, formatMessageTimeTitle, isSameLocalDay } from './formatMessageTime'
+
+// Local-time constructors throughout, and expected text built with the same
+// locale calls the code uses -- so these hold in any timezone and any ICU
+// locale instead of hardcoding "3:00 PM".
+const NOW = new Date(2026, 9, 9, 15, 0) // Fri Oct 9 2026, 3:00 PM local
+
+const iso = (d: Date) => d.toISOString()
+const timeText = (d: Date) => d.toLocaleTimeString([], { hour: 'numeric', minute: '2-digit' })
+const weekday = (d: Date) => d.toLocaleDateString([], { weekday: 'short' })
+
+describe('formatMessageTime', () => {
+  it('shows just the time for a message sent today', () => {
+    const morning = new Date(2026, 9, 9, 8, 5)
+    expect(formatMessageTime(iso(morning), NOW)).toBe(timeText(morning))
+  })
+
+  it('prefixes "Yesterday" for yesterday', () => {
+    const d = new Date(2026, 9, 8, 23, 59)
+    expect(formatMessageTime(iso(d), NOW)).toBe(`Yesterday ${timeText(d)}`)
+  })
+
+  it('treats 11:59 PM vs 12:03 AM as different days despite being minutes apart', () => {
+    const justBeforeMidnight = new Date(2026, 9, 8, 23, 59)
+    const justAfterMidnight = new Date(2026, 9, 9, 0, 3)
+    expect(formatMessageTime(iso(justBeforeMidnight), justAfterMidnight)).toBe(
+      `Yesterday ${timeText(justBeforeMidnight)}`,
+    )
+    expect(formatMessageTime(iso(justAfterMidnight), justAfterMidnight)).toBe(timeText(justAfterMidnight))
+  })
+
+  it('uses the short weekday for 2 to 6 days ago', () => {
+    const twoDays = new Date(2026, 9, 7, 12, 0)
+    const sixDays = new Date(2026, 9, 3, 12, 0)
+    expect(formatMessageTime(iso(twoDays), NOW)).toBe(`${weekday(twoDays)} ${timeText(twoDays)}`)
+    expect(formatMessageTime(iso(sixDays), NOW)).toBe(`${weekday(sixDays)} ${timeText(sixDays)}`)
+  })
+
+  it('switches to a month and day at 7 days, without the year when it is this year', () => {
+    const d = new Date(2026, 9, 2, 12, 0)
+    const label = formatMessageTime(iso(d), NOW)
+    expect(label).toBe(`${d.toLocaleDateString([], { month: 'short', day: 'numeric' })}, ${timeText(d)}`)
+    expect(label).not.toContain('2026')
+  })
+
+  it('adds the year for a message from a different year', () => {
+    const d = new Date(2025, 11, 25, 9, 30)
+    const label = formatMessageTime(iso(d), NOW)
+    expect(label).toBe(
+      `${d.toLocaleDateString([], { month: 'short', day: 'numeric', year: 'numeric' })}, ${timeText(d)}`,
+    )
+    expect(label).toContain('2025')
+  })
+
+  it('counts Dec 31 -> Jan 1 as yesterday', () => {
+    const d = new Date(2026, 11, 31, 22, 0)
+    expect(formatMessageTime(iso(d), new Date(2027, 0, 1, 10, 0))).toBe(`Yesterday ${timeText(d)}`)
+  })
+
+  it('treats a slightly-future timestamp (clock skew) as today', () => {
+    const tomorrow = new Date(2026, 9, 10, 9, 0)
+    expect(formatMessageTime(iso(tomorrow), NOW)).toBe(timeText(tomorrow))
+  })
+})
+
+describe('formatMessageTimeTitle', () => {
+  it('includes the full date, with the year', () => {
+    expect(formatMessageTimeTitle(iso(new Date(2026, 9, 2, 16, 4)))).toContain('2026')
+  })
+})
+
+describe('isSameLocalDay', () => {
+  it('is false across midnight and true within a day', () => {
+    expect(isSameLocalDay(new Date(2026, 9, 8, 23, 59), new Date(2026, 9, 9, 0, 1))).toBe(false)
+    expect(isSameLocalDay(new Date(2026, 9, 9, 0, 1), new Date(2026, 9, 9, 23, 59))).toBe(true)
+  })
+
+  it('accepts ISO strings as well as Dates', () => {
+    expect(isSameLocalDay(iso(new Date(2026, 9, 9, 8, 0)), new Date(2026, 9, 9, 20, 0))).toBe(true)
+  })
+})

frontend/src/lib/formatMessageTime.ts

@@ -0,0 +1,44 @@
+// "Today" is the viewer's local calendar day, so everything here goes through
+// local-time getters, never UTC.
+type DateLike = Date | string
+
+function toDate(d: DateLike): Date {
+  return d instanceof Date ? d : new Date(d)
+}
+
+// A calendar-day number, not elapsed milliseconds: built from the local
+// y/m/d via Date.UTC so a 23- or 25-hour DST day still counts as one day,
+// and 11:58 PM vs 12:01 AM (3 minutes apart) correctly counts as 1 day apart.
+function dayNumber(d: Date): number {
+  return Math.floor(Date.UTC(d.getFullYear(), d.getMonth(), d.getDate()) / 86_400_000)
+}
+
+export function isSameLocalDay(a: DateLike, b: DateLike): boolean {
+  return dayNumber(toDate(a)) === dayNumber(toDate(b))
+}
+
+// A bare "4:04 PM" gives no way to tell today's message from one sent last
+// week. Today stays time-only; anything older gains just enough date to
+// place it.
+export function formatMessageTime(iso: string, now: Date = new Date()): string {
+  const sent = new Date(iso)
+  const time = sent.toLocaleTimeString([], { hour: 'numeric', minute: '2-digit' })
+  // <= 0 also covers a timestamp slightly in the future (clock skew between
+  // server and this device) -- better shown as today than as a negative age.
+  const daysAgo = dayNumber(now) - dayNumber(sent)
+  if (daysAgo <= 0) return time
+  if (daysAgo === 1) return `Yesterday ${time}`
+  // Under 7 so the weekday is never the same as today's (7 days ago would be).
+  if (daysAgo < 7) return `${sent.toLocaleDateString([], { weekday: 'short' })} ${time}`
+  const date = sent.toLocaleDateString([], {
+    month: 'short',
+    day: 'numeric',
+    year: sent.getFullYear() === now.getFullYear() ? undefined : 'numeric',
+  })
+  return `${date}, ${time}`
+}
+
+// Hover text: the one place the full, unambiguous date is always available.
+export function formatMessageTimeTitle(iso: string): string {
+  return new Date(iso).toLocaleString([], { dateStyle: 'full', timeStyle: 'short' })
+}

frontend/src/lib/formattingToolbarPref.test.ts

@@ -0,0 +1,31 @@
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { getFormattingToolbarOpen, setFormattingToolbarOpen } from './formattingToolbarPref'
+
+afterEach(() => {
+  localStorage.clear()
+  vi.restoreAllMocks()
+})
+
+describe('formatting toolbar preference', () => {
+  it('is collapsed by default', () => {
+    expect(getFormattingToolbarOpen()).toBe(false)
+  })
+
+  it('remembers being opened and closed', () => {
+    setFormattingToolbarOpen(true)
+    expect(getFormattingToolbarOpen()).toBe(true)
+    setFormattingToolbarOpen(false)
+    expect(getFormattingToolbarOpen()).toBe(false)
+  })
+
+  it('falls back to collapsed when storage is unavailable', () => {
+    vi.spyOn(Storage.prototype, 'getItem').mockImplementation(() => {
+      throw new Error('blocked')
+    })
+    vi.spyOn(Storage.prototype, 'setItem').mockImplementation(() => {
+      throw new Error('blocked')
+    })
+    expect(getFormattingToolbarOpen()).toBe(false)
+    expect(() => setFormattingToolbarOpen(true)).not.toThrow()
+  })
+})

frontend/src/lib/formattingToolbarPref.ts

@@ -0,0 +1,21 @@
+const KEY = 'ds-chat-formatting-toolbar-open'
+
+// Collapsed unless the user has opened it before -- and remembered, because
+// Composer remounts on every room switch and would otherwise snap shut each
+// time.
+export function getFormattingToolbarOpen(): boolean {
+  try {
+    return localStorage.getItem(KEY) === 'true'
+  } catch {
+    return false
+  }
+}
+
+export function setFormattingToolbarOpen(open: boolean): void {
+  try {
+    localStorage.setItem(KEY, String(open))
+  } catch {
+    // storage unavailable (private browsing, quota) -- the toolbar just
+    // won't remember its state, not fatal.
+  }
+}

frontend/src/lib/markdownFormat.test.ts

@@ -0,0 +1,139 @@
+import { describe, expect, it } from 'vitest'
+import { applyFormat, type FormatAction, type InlineMarker, type TextState } from './markdownFormat'
+
+// ⟦ and ⟧ mark the selection, so cases read as plain strings -- an empty
+// pair (⟦⟧) is a bare cursor.
+const L = '⟦'
+const R = '⟧'
+
+function parse(s: string): TextState {
+  const start = s.indexOf(L)
+  const end = s.indexOf(R) - 1
+  return { value: s.replace(L, '').replace(R, ''), start, end }
+}
+
+function render({ value, start, end }: TextState): string {
+  return value.slice(0, start) + L + value.slice(start, end) + R + value.slice(end)
+}
+
+function fmt(s: string, action: FormatAction): string {
+  return render(applyFormat(parse(s), action))
+}
+
+const inline = (marker: InlineMarker): FormatAction => ({ kind: 'inline', marker })
+
+describe('inline wrap', () => {
+  it('wraps the selection and keeps the inner text selected', () => {
+    expect(fmt('say ⟦hi⟧ now', inline('**'))).toBe('say **⟦hi⟧** now')
+  })
+
+  it('inserts an empty pair with the cursor between when nothing is selected', () => {
+    expect(fmt('say ⟦⟧now', inline('**'))).toBe('say **⟦⟧**now')
+  })
+
+  it('unwraps when the markers sit just outside the selection', () => {
+    expect(fmt('say **⟦hi⟧** now', inline('**'))).toBe('say ⟦hi⟧ now')
+  })
+
+  it('unwraps when the selection includes the markers', () => {
+    expect(fmt('say ⟦**hi**⟧ now', inline('**'))).toBe('say ⟦hi⟧ now')
+  })
+
+  it('keeps markers outside leading/trailing whitespace in the selection', () => {
+    expect(fmt('a⟦ hi ⟧b', inline('**'))).toBe('a **⟦hi⟧** b')
+  })
+
+  it('italic on a bold selection adds italic rather than stripping bold', () => {
+    expect(fmt('x **⟦hi⟧** y', inline('*'))).toBe('x ***⟦hi⟧*** y')
+  })
+
+  it('removes just bold from bold+italic', () => {
+    expect(fmt('x ***⟦hi⟧*** y', inline('**'))).toBe('x *⟦hi⟧* y')
+  })
+
+  it('removes just italic from bold+italic', () => {
+    expect(fmt('x ***⟦hi⟧*** y', inline('*'))).toBe('x **⟦hi⟧** y')
+  })
+
+  it('strikethrough toggles off without being confused by subscript', () => {
+    expect(fmt('x ~~⟦hi⟧~~ y', inline('~~'))).toBe('x ⟦hi⟧ y')
+    expect(fmt('a ⟦hi⟧ b', inline('~'))).toBe('a ~⟦hi⟧~ b')
+  })
+
+  it('handles inline code and superscript', () => {
+    expect(fmt('run ⟦ls⟧', inline('`'))).toBe('run `⟦ls⟧`')
+    expect(fmt('x⟦2⟧', inline('^'))).toBe('x^⟦2⟧^')
+  })
+
+  it.each(['**', '*', '~~', '~', '`', '^'] as InlineMarker[])('%s: applying twice restores the original', (marker) => {
+    const original = parse('a ⟦b⟧ c')
+    const once = applyFormat(original, inline(marker))
+    expect(once.value).not.toBe(original.value)
+    expect(applyFormat(once, inline(marker))).toEqual(original)
+  })
+})
+
+describe('line prefixes', () => {
+  it('prefixes the current line and moves the cursor with it', () => {
+    expect(fmt('hello⟦⟧', { kind: 'prefix', prefix: 'quote' })).toBe('> hello⟦⟧')
+    expect(fmt('one\ntw⟦⟧o', { kind: 'prefix', prefix: 'quote' })).toBe('one\n> tw⟦⟧o')
+  })
+
+  it('starts a list on a blank line', () => {
+    expect(fmt('⟦⟧', { kind: 'prefix', prefix: 'bullet' })).toBe('- ⟦⟧')
+  })
+
+  it('prefixes every selected line, then toggles them back off', () => {
+    const bullet: FormatAction = { kind: 'prefix', prefix: 'bullet' }
+    expect(fmt('⟦a\nb⟧', bullet)).toBe('⟦- a\n- b⟧')
+    expect(fmt('⟦- a\n- b⟧', bullet)).toBe('⟦a\nb⟧')
+  })
+
+  it('numbers lines sequentially, skipping blank lines, and renumbers existing numbers', () => {
+    const ordered: FormatAction = { kind: 'prefix', prefix: 'ordered' }
+    expect(fmt('⟦a\n\nb⟧', ordered)).toBe('⟦1. a\n\n2. b⟧')
+    expect(fmt('⟦1. a\nb⟧', ordered)).toBe('⟦1. a\n2. b⟧')
+    expect(fmt('⟦1. a\n2. b⟧', ordered)).toBe('⟦a\nb⟧')
+  })
+
+  it('does not include a line the selection only touches at its very start', () => {
+    expect(fmt('a\n⟦b\n⟧c', { kind: 'prefix', prefix: 'bullet' })).toBe('a\n⟦- b⟧\nc')
+  })
+})
+
+describe('headings', () => {
+  it('adds, replaces, and toggles off the heading level', () => {
+    expect(fmt('title⟦⟧', { kind: 'heading', level: 2 })).toBe('## title⟦⟧')
+    expect(fmt('⟦# hi⟧', { kind: 'heading', level: 2 })).toBe('⟦## hi⟧')
+    expect(fmt('⟦## hi⟧', { kind: 'heading', level: 1 })).toBe('⟦# hi⟧')
+    expect(fmt('⟦## hi⟧', { kind: 'heading', level: 2 })).toBe('⟦hi⟧')
+  })
+})
+
+describe('code block', () => {
+  it('inserts an empty fenced block with the cursor on the blank line', () => {
+    expect(fmt('⟦⟧', { kind: 'codeBlock' })).toBe('```\n⟦⟧\n```')
+  })
+
+  it('puts the fences on their own lines when the selection is mid-line', () => {
+    expect(fmt('see ⟦x = 1⟧ here', { kind: 'codeBlock' })).toBe('see \n```\n⟦x = 1⟧\n```\n here')
+  })
+
+  it('does not double the newline when the selection already ends with one', () => {
+    expect(fmt('⟦a\n⟧b', { kind: 'codeBlock' })).toBe('```\n⟦a⟧\n```\nb')
+  })
+})
+
+describe('link', () => {
+  it('wraps the selection and selects the placeholder url', () => {
+    expect(fmt('see ⟦docs⟧', { kind: 'link' })).toBe('see [docs](⟦url⟧)')
+  })
+
+  it('treats a selected url as the target and leaves the cursor in the text brackets', () => {
+    expect(fmt('⟦https://example.com⟧', { kind: 'link' })).toBe('[⟦⟧](https://example.com)')
+  })
+
+  it('inserts a placeholder link with its text selected when nothing is selected', () => {
+    expect(fmt('a ⟦⟧', { kind: 'link' })).toBe('a [⟦link text⟧](url)')
+  })
+})

frontend/src/lib/markdownFormat.ts

@@ -0,0 +1,204 @@
+// #75: pure text transforms behind the composer's formatting toolbar. Each
+// takes the textarea's value plus its selection and returns the same shape,
+// so Composer just applies the result -- keeping this free of React/DOM is
+// what makes the fiddly selection math directly unit-testable.
+export interface TextState {
+  value: string
+  start: number
+  end: number
+}
+
+export type InlineMarker = '**' | '*' | '~~' | '`' | '~' | '^'
+
+export type FormatAction =
+  | { kind: 'inline'; marker: InlineMarker }
+  | { kind: 'prefix'; prefix: 'quote' | 'bullet' | 'ordered' }
+  | { kind: 'heading'; level: 1 | 2 | 3 }
+  | { kind: 'codeBlock' }
+  | { kind: 'link' }
+
+// Markers whose doubled form is a *different* format (`**` bold vs `*`
+// italic, `~~` strikethrough vs `~` subscript), so "is this already
+// wrapped?" has to look at how long the run of marker characters is rather
+// than just whether one is present.
+const DOUBLED_IS_DIFFERENT = new Set(['*', '~'])
+
+// A run of 3 `*` is bold+italic: bold (2) and italic (1) are both "present".
+function hasMarker(marker: string, run: number): boolean {
+  if (marker.length === 2) return run >= 2
+  return DOUBLED_IS_DIFFERENT.has(marker[0]) ? run % 2 === 1 : run >= 1
+}
+
+function applyInline(state: TextState, marker: InlineMarker): TextState {
+  const { value, start, end } = state
+  const ch = marker[0]
+  const m = marker.length
+  const sel = value.slice(start, end)
+
+  // Selection includes the markers themselves (e.g. the user selected the
+  // whole `**word**`) -- unwrap inside the selection.
+  if (sel.length > 0) {
+    let lead = 0
+    while (lead < sel.length && sel[lead] === ch) lead++
+    let trail = 0
+    while (trail < sel.length - lead && sel[sel.length - 1 - trail] === ch) trail++
+    const run = Math.min(lead, trail)
+    if (run > 0 && hasMarker(marker, run) && sel.length > 2 * m) {
+      const inner = sel.slice(m, sel.length - m)
+      return { value: value.slice(0, start) + inner + value.slice(end), start, end: start + inner.length }
+    }
+  }
+
+  // Markers sit just outside the selection. Skipped when the selection has
+  // edge whitespace: markers adjacent to it belong to neighbouring spans
+  // (`*a* and *b*` with " and " selected), not to this selection.
+  const edgeWhitespace = sel.length > 0 && (/^\s/.test(sel) || /\s$/.test(sel))
+  if (!edgeWhitespace) {
+    let before = 0
+    while (start - before - 1 >= 0 && value[start - before - 1] === ch) before++
+    let after = 0
+    while (end + after < value.length && value[end + after] === ch) after++
+    const run = Math.min(before, after)
+    if (run > 0 && hasMarker(marker, run)) {
+      return {
+        value: value.slice(0, start - m) + sel + value.slice(end + m),
+        start: start - m,
+        end: end - m,
+      }
+    }
+  }
+
+  // Nothing selected (or only whitespace): drop in an empty pair, cursor between.
+  if (sel.trim() === '') {
+    return {
+      value: value.slice(0, start) + marker + marker + value.slice(start),
+      start: start + m,
+      end: start + m,
+    }
+  }
+
+  // Markdown doesn't treat `** word**` as bold -- keep the markers hugging
+  // the text, outside any whitespace the selection happens to include.
+  const innerStart = start + (sel.length - sel.trimStart().length)
+  const innerEnd = end - (sel.length - sel.trimEnd().length)
+  return {
+    value:
+      value.slice(0, innerStart) + marker + value.slice(innerStart, innerEnd) + marker + value.slice(innerEnd),
+    start: innerStart + m,
+    end: innerEnd + m,
+  }
+}
+
+// The whole lines a selection touches. A selection that ends right after a
+// newline (i.e. at the very start of the next line) doesn't count that line.
+function lineBounds(value: string, start: number, end: number) {
+  const lineStart = start === 0 ? 0 : value.lastIndexOf('\n', start - 1) + 1
+  const effectiveEnd = end > start && value[end - 1] === '\n' ? end - 1 : end
+  const nextNewline = value.indexOf('\n', effectiveEnd)
+  return { lineStart, lineEnd: nextNewline === -1 ? value.length : nextNewline }
+}
+
+function transformLines(state: TextState, transform: (lines: string[]) => string[]): TextState {
+  const { value, start, end } = state
+  const { lineStart, lineEnd } = lineBounds(value, start, end)
+  const oldBlock = value.slice(lineStart, lineEnd)
+  const newBlock = transform(oldBlock.split('\n')).join('\n')
+  const next = value.slice(0, lineStart) + newBlock + value.slice(lineEnd)
+  if (start === end) {
+    const cursor = Math.max(lineStart, start + (newBlock.length - oldBlock.length))
+    return { value: next, start: cursor, end: cursor }
+  }
+  // Keep the affected lines selected so the same button toggles it back off.
+  return { value: next, start: lineStart, end: lineStart + newBlock.length }
+}
+
+const ORDERED_RE = /^\d+\. /
+
+function applyPrefix(state: TextState, kind: 'quote' | 'bullet' | 'ordered'): TextState {
+  const marker = kind === 'quote' ? '> ' : '- '
+  const has = (line: string) => (kind === 'ordered' ? ORDERED_RE.test(line) : line.startsWith(marker))
+  const strip = (line: string) => (kind === 'ordered' ? line.replace(ORDERED_RE, '') : line.slice(marker.length))
+
+  return transformLines(state, (lines) => {
+    // Blank lines inside a multi-line selection are paragraph breaks, not
+    // list items -- only a lone blank line (cursor on an empty line) gets one.
+    const multi = lines.length > 1
+    const skip = (line: string) => multi && line === ''
+    const targets = lines.filter((line) => !skip(line))
+    if (targets.length > 0 && targets.every(has)) {
+      return lines.map((line) => (has(line) ? strip(line) : line))
+    }
+    let n = 0
+    return lines.map((line) => {
+      if (skip(line)) return line
+      n++
+      if (kind === 'ordered') return `${n}. ${strip(line)}`
+      return has(line) ? line : marker + line
+    })
+  })
+}
+
+const HEADING_RE = /^#{1,6} /
+
+function applyHeading(state: TextState, level: 1 | 2 | 3): TextState {
+  const prefix = '#'.repeat(level) + ' '
+  return transformLines(state, (lines) => {
+    const multi = lines.length > 1
+    const skip = (line: string) => multi && line === ''
+    const targets = lines.filter((line) => !skip(line))
+    if (targets.length > 0 && targets.every((line) => line.startsWith(prefix))) {
+      return lines.map((line) => (line.startsWith(prefix) ? line.slice(prefix.length) : line))
+    }
+    return lines.map((line) => (skip(line) ? line : prefix + line.replace(HEADING_RE, '')))
+  })
+}
+
+// A fence only counts at the start of a line, so mid-line cursors get a
+// newline inserted first.
+function applyCodeBlock(state: TextState): TextState {
+  const { value, start, end } = state
+  const before = value.slice(0, start)
+  const after = value.slice(end)
+  const sel = value.slice(start, end).replace(/\n$/, '')
+  const lead = before.length > 0 && !before.endsWith('\n') ? '\n' : ''
+  const trail = after.length > 0 && !after.startsWith('\n') ? '\n' : ''
+  const next = `${before}${lead}\`\`\`\n${sel}\n\`\`\`${trail}${after}`
+  const innerStart = before.length + lead.length + 4
+  return { value: next, start: innerStart, end: innerStart + sel.length }
+}
+
+function applyLink(state: TextState): TextState {
+  const { value, start, end } = state
+  const sel = value.slice(start, end)
+  let text: string
+  let selStart: number
+  let selEnd: number
+  if (/^https?:\/\/\S+$/.test(sel)) {
+    text = `[](${sel})`
+    selStart = selEnd = start + 1
+  } else if (sel) {
+    text = `[${sel}](url)`
+    selStart = start + sel.length + 3
+    selEnd = selStart + 3
+  } else {
+    text = '[link text](url)'
+    selStart = start + 1
+    selEnd = selStart + 9
+  }
+  return { value: value.slice(0, start) + text + value.slice(end), start: selStart, end: selEnd }
+}
+
+export function applyFormat(state: TextState, action: FormatAction): TextState {
+  switch (action.kind) {
+    case 'inline':
+      return applyInline(state, action.marker)
+    case 'prefix':
+      return applyPrefix(state, action.prefix)
+    case 'heading':
+      return applyHeading(state, action.level)
+    case 'codeBlock':
+      return applyCodeBlock(state)
+    case 'link':
+      return applyLink(state)
+  }
+}

frontend/src/lib/messageGrouping.test.ts

@@ -0,0 +1,39 @@
+import { describe, expect, it } from 'vitest'
+import { GROUP_BREAK_MS, isGroupStart } from './messageGrouping'
+
+const msg = (user: string, d: Date) => ({ user_id: user, created_at: d.toISOString() })
+
+describe('isGroupStart', () => {
+  const t = new Date(2026, 9, 9, 10, 0, 0)
+  const later = (ms: number) => new Date(t.getTime() + ms)
+
+  it('starts a group for the first message', () => {
+    expect(isGroupStart(undefined, msg('a', t))).toBe(true)
+  })
+
+  it('starts a group when the sender changes', () => {
+    expect(isGroupStart(msg('a', t), msg('b', later(1000)))).toBe(true)
+  })
+
+  it('keeps the same sender grouped within the break window', () => {
+    expect(isGroupStart(msg('a', t), msg('a', later(60_000)))).toBe(false)
+  })
+
+  it('groups at exactly the break threshold and splits just past it', () => {
+    expect(isGroupStart(msg('a', t), msg('a', later(GROUP_BREAK_MS)))).toBe(false)
+    expect(isGroupStart(msg('a', t), msg('a', later(GROUP_BREAK_MS + 1000)))).toBe(true)
+  })
+
+  it('measures the gap from the previous message, not the first in the run', () => {
+    const fourMin = 4 * 60_000
+    expect(isGroupStart(msg('a', later(fourMin)), msg('a', later(2 * fourMin)))).toBe(false)
+    expect(isGroupStart(msg('a', later(2 * fourMin)), msg('a', later(3 * fourMin)))).toBe(false)
+  })
+
+  it('starts a new group when the same sender crosses midnight, even a few minutes apart', () => {
+    const before = new Date(2026, 9, 8, 23, 58)
+    const after = new Date(2026, 9, 9, 0, 1)
+    expect(after.getTime() - before.getTime()).toBeLessThan(GROUP_BREAK_MS)
+    expect(isGroupStart(msg('a', before), msg('a', after))).toBe(true)
+  })
+})

frontend/src/lib/messageGrouping.ts

@@ -1,7 +1,31 @@
 import { getUserAvatarUrl } from '../api/users'
 import { hashIndex } from './avatar'
+import { isSameLocalDay } from './formatMessageTime'
 import type { RoomMember } from '../types'
 
+// #73: Slack's own threshold for the same "still grouped, but it's been a
+// while" call -- past this gap a same-sender message starts a new group
+// (its own avatar/name/timestamp) even with nobody else posting in
+// between, so a message sent minutes later doesn't hide under a stale
+// timestamp from the start of the run.
+export const GROUP_BREAK_MS = 5 * 60 * 1000
+
+interface Groupable {
+  user_id: string
+  created_at: string
+}
+
+// Whether `msg` gets its own avatar/name/timestamp header, vs. continuing
+// `prev`'s group. A new calendar day always breaks the group: now that
+// timestamps can carry a date, a same-sender run that crosses midnight
+// (11:58 PM, then 12:01 AM) would otherwise leave the later message under a
+// header showing the wrong day.
+export function isGroupStart(prev: Groupable | undefined, msg: Groupable): boolean {
+  if (!prev || prev.user_id !== msg.user_id) return true
+  if (new Date(msg.created_at).getTime() - new Date(prev.created_at).getTime() > GROUP_BREAK_MS) return true
+  return !isSameLocalDay(prev.created_at, msg.created_at)
+}
+
 export function senderColorIndex(username: string, members: RoomMember[]): number {
   const idx = members.findIndex((m) => m.username === username)
   if (idx >= 0) return idx

frontend/src/lib/roomIndicators.test.ts

@@ -0,0 +1,56 @@
+import { describe, expect, it } from 'vitest'
+import type { MyRoomItem } from '../types'
+import { clearRoomIndicators } from './roomIndicators'
+
+function makeRoom(overrides: Partial<MyRoomItem> & { id: string }): MyRoomItem {
+  return {
+    name: overrides.id,
+    description: null,
+    is_private: false,
+    is_dm: false,
+    is_archived: false,
+    owner_id: 'owner',
+    created_at: '2026-01-01T00:00:00Z',
+    role: 'member',
+    has_unread: false,
+    has_mention: false,
+    dm_partner: null,
+    email_notifications: false,
+    ...overrides,
+  }
+}
+
+describe('clearRoomIndicators', () => {
+  it('clears has_unread and has_mention on the matching room', () => {
+    const rooms = [makeRoom({ id: 'room-1', has_unread: true, has_mention: true })]
+    const result = clearRoomIndicators(rooms, 'room-1')
+    expect(result[0].has_unread).toBe(false)
+    expect(result[0].has_mention).toBe(false)
+  })
+
+  it('preserves unread state on every other room', () => {
+    const rooms = [
+      makeRoom({ id: 'room-1', has_unread: true, has_mention: true }),
+      makeRoom({ id: 'room-2', has_unread: true, has_mention: false }),
+      makeRoom({ id: 'room-3', has_unread: false, has_mention: false }),
+    ]
+    const result = clearRoomIndicators(rooms, 'room-1')
+    const room2 = result.find((r) => r.id === 'room-2')
+    const room3 = result.find((r) => r.id === 'room-3')
+    expect(room2?.has_unread).toBe(true)
+    expect(room3?.has_unread).toBe(false)
+  })
+
+  it('leaves untouched rooms as the same object reference', () => {
+    const room2 = makeRoom({ id: 'room-2', has_unread: true })
+    const rooms = [makeRoom({ id: 'room-1', has_unread: true }), room2]
+    const result = clearRoomIndicators(rooms, 'room-1')
+    expect(result[1]).toBe(room2)
+  })
+
+  it('is a no-op if the room id is not found', () => {
+    const rooms = [makeRoom({ id: 'room-1', has_unread: true })]
+    const result = clearRoomIndicators(rooms, 'does-not-exist')
+    expect(result[0].has_unread).toBe(true)
+  })
+})

frontend/src/lib/roomIndicators.ts

@@ -0,0 +1,9 @@
+import type { MyRoomItem } from '../types'
+
+// Pulled out of ChatShellPage as a pure function purely so it's directly
+// unit-testable without rendering the whole page -- behavior is unchanged.
+// Only touches the matching room; every other room's object reference (and
+// therefore its unread state) is left untouched.
+export function clearRoomIndicators(rooms: MyRoomItem[], roomId: string): MyRoomItem[] {
+  return rooms.map((r) => (r.id === roomId ? { ...r, has_unread: false, has_mention: false } : r))
+}

frontend/src/pages/ChatShellPage.tsx

@@ -1,4 +1,4 @@
-import { useCallback, useEffect, useState } from 'react'
+import { useCallback, useEffect, useMemo, useState } from 'react'
 import { useNavigate, useParams } from 'react-router-dom'
 import { NetworkError } from '../api/client'
 import { listMyRooms, listRoomMembers } from '../api/rooms'
@@ -12,7 +12,9 @@ import { Sidebar } from '../components/Sidebar'
 import { TopBar } from '../components/TopBar'
 import { useAuth } from '../context/AuthContext'
 import { useChatSocketContext } from '../context/ChatSocketContext'
+import { useDesktopRoomBridge } from '../hooks/useDesktopRoomBridge'
 import { MOBILE_BREAKPOINT, useWindowWidth } from '../hooks/useWindowWidth'
+import { clearRoomIndicators } from '../lib/roomIndicators'
 import type { MyRoomItem, RoomMember } from '../types'
 import './ChatShellPage.css'
 
@@ -60,10 +62,13 @@ export function ChatShellPage() {
 
   const socket = useChatSocketContext()
 
-  const clearRoomIndicators = useCallback((id: string) => {
-    setRooms((prev) => prev.map((r) => (r.id === id ? { ...r, has_unread: false, has_mention: false } : r)))
+  const handleRoomRead = useCallback((id: string) => {
+    setRooms((prev) => clearRoomIndicators(prev, id))
   }, [])
 
+  const unreadRoomCount = useMemo(() => rooms.filter((r) => r.has_unread).length, [rooms])
+  useDesktopRoomBridge(activeRoom?.id, unreadRoomCount)
+
   useEffect(
     () =>
       socket.subscribe((envelope) => {
@@ -134,7 +139,7 @@ export function ChatShellPage() {
               onToggleInfo={() => setInfoOpen((v) => !v)}
               infoOpen={infoOpen}
               socket={socket}
-              onRoomRead={clearRoomIndicators}
+              onRoomRead={handleRoomRead}
             />
           ) : (
             !isMobile && (

frontend/src/pages/HelpPage.tsx

@@ -1,6 +1,6 @@
 import { useEffect, useMemo, useState } from 'react'
 import Markdown from 'markdown-to-jsx'
-import { Link } from 'react-router-dom'
+import { Link, useLocation } from 'react-router-dom'
 import { createMarkdownOptions, preprocessMarkdown } from '../components/MessageContent'
 import { TopBar } from '../components/TopBar'
 import './HelpPage.css'
@@ -20,6 +20,15 @@ export function HelpPage() {
     [content],
   )
 
+  // #75: the composer's "?" links straight to /help#formatting, but the guide
+  // is fetched after the page loads -- the browser's own scroll-to-hash has
+  // already run (and found nothing) by the time the headings exist.
+  const { hash } = useLocation()
+  useEffect(() => {
+    if (!markdownPreview || !hash) return
+    document.getElementById(hash.slice(1))?.scrollIntoView()
+  }, [markdownPreview, hash])
+
   useEffect(() => {
     let cancelled = false
     fetch(GUIDE_URL)

frontend/vitest.config.ts

@@ -0,0 +1,13 @@
+import { defineConfig } from 'vitest/config'
+import react from '@vitejs/plugin-react'
+
+// Deliberately separate from vite.config.ts, not an extension of it -- that
+// file's VitePWA plugin does real build-time work (service worker
+// injection, manifest generation) that has nothing to do with running unit
+// tests and would just slow every test run down for no benefit.
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    environment: 'jsdom',
+  },
+})