feat(desktop): show when a message was sent on hover

Hovering a message reveals its time with the hover actions ("3:27 PM",
"Yesterday, 3:27 PM", "Sep 28, 3:27 PM"); the tip adds the age for same-day
messages and the full date. User bubbles show it beside Restore/Stop, replies
at the head of the action bar. It stands down while display.timestamps is on,
since the transcript already prints every stamp (#118405).

apps/desktop/src/components/assistant-ui/thread/assistant-message.tsx

@@ -23,6 +23,7 @@ import {
   messageContentText,
   pickPrimaryPreviewTarget
 } from '@/components/assistant-ui/thread/content'
+import { MessageHoverTime } from '@/components/assistant-ui/thread/message-hover-time'
 import { MESSAGE_PARTS_COMPONENTS } from '@/components/assistant-ui/thread/message-parts'
 import { ReactionPicker } from '@/components/assistant-ui/thread/message-reactions'
 import { ResponseMessageIds } from '@/components/assistant-ui/thread/response-group'
@@ -1043,6 +1044,7 @@ const AssistantActionBar: FC<MessageActionProps & { durationS?: number }> = ({
         }
         data-slot="aui_msg-actions"
       >
+        <MessageHoverTime className="mr-1 px-0.5" />
         {onBranchInNewChat && (
           <TooltipIconButton
             onClick={() => {

apps/desktop/src/components/assistant-ui/thread/message-hover-time.tsx

@@ -0,0 +1,69 @@
+import { useAuiState } from '@assistant-ui/react'
+import { useStore } from '@nanostores/react'
+import type { FC } from 'react'
+
+import { Tip } from '@/components/ui/tooltip'
+import { useI18n } from '@/i18n'
+import { DAY, fmtFullDateTime, relativeTime } from '@/lib/time'
+import { cn } from '@/lib/utils'
+import { $displayTimestamps } from '@/store/display-timestamps'
+
+import { formatMessageTimestamp } from './timestamp'
+
+/** When the message landed, in seconds: a reply counts from its completion.
+ *  `createdAt` already carries the row's own timestamp (see
+ *  `messageCreatedAt`), falling back to now only for a just-sent message. */
+const useMessageTime = (): number | undefined =>
+  useAuiState(s => {
+    const completedAt = (s.message.metadata?.custom as { timelineCompletedAt?: unknown } | undefined)
+      ?.timelineCompletedAt
+
+    const at = typeof completedAt === 'number' ? completedAt : (s.message.createdAt?.getTime() ?? NaN) / 1000
+
+    return Number.isFinite(at) && at > 0 ? at : undefined
+  })
+
+/** Rendered only while the tip is open, so "5 min ago" is as of the hover.
+ *  The age is only worth saying within the day; past that the label already
+ *  names the day, and an hour-rounded "2 days ago" would contradict a
+ *  calendar "Yesterday". */
+const HoverTimeDetail: FC<{ ms: number }> = ({ ms }) => {
+  const full = fmtFullDateTime.format(new Date(ms))
+
+  return <span className="tabular-nums">{Date.now() - ms < DAY ? `${relativeTime(ms)} · ${full}` : full}</span>
+}
+
+const clockOnly = (time: string) => time
+
+/**
+ * The time a message landed, revealed with the message's hover actions
+ * ("3:27 PM"; "Yesterday, 3:27 PM"; "Sep 28, 3:27 PM"), with the age and full
+ * date in its tip. Visibility belongs to the host's hover group: this paints
+ * the label only. Stands down while `display.timestamps` is on, because the
+ * transcript already prints every stamp.
+ */
+export const MessageHoverTime: FC<{ className?: string }> = ({ className }) => {
+  const { t } = useI18n()
+  const timelineStamps = useStore($displayTimestamps)
+  const seconds = useMessageTime()
+
+  if (timelineStamps || seconds === undefined) {
+    return null
+  }
+
+  const date = new Date(seconds * 1000)
+
+  return (
+    <Tip label={<HoverTimeDetail ms={date.getTime()} />} side="top">
+      <time
+        className={cn(
+          'pointer-events-auto cursor-default select-none whitespace-nowrap text-[0.6875rem] leading-5 tabular-nums text-(--ui-text-tertiary)',
+          className
+        )}
+        dateTime={date.toISOString()}
+      >
+        {formatMessageTimestamp(date, { today: clockOnly, yesterday: t.assistant.thread.yesterday })}
+      </time>
+    </Tip>
+  )
+}

apps/desktop/src/components/assistant-ui/thread/user-message.tsx

@@ -8,6 +8,7 @@ import {
   messageContentText,
   PROCESS_NOTIFICATION_RE
 } from '@/components/assistant-ui/thread/content'
+import { MessageHoverTime } from '@/components/assistant-ui/thread/message-hover-time'
 import { ReactionBadge, ReactionPicker } from '@/components/assistant-ui/thread/message-reactions'
 import { BackgroundResult } from '@/components/assistant-ui/thread/system-message'
 import { threadUserOrdinal } from '@/components/assistant-ui/thread/thread-message-index'
@@ -502,47 +503,48 @@ export const UserMessage: FC<{
                     </button>
                   </ActionBarPrimitive.Edit>
                 )}
-                {(showStop || showRestore) && (
-                  <div className="pointer-events-none absolute right-2 bottom-2 z-10 flex items-center justify-center opacity-0 transition-opacity group-hover/user-message:opacity-100 group-focus-within/user-message:opacity-100">
-                    {showStop ? (
+                {/* Hover cluster, bottom-right: when it was sent, then Stop or
+                    Restore. Its fill masks the last line's tail while shown. */}
+                <div className="pointer-events-none absolute right-2 bottom-2 z-10 flex items-center gap-1 rounded-md bg-(--dt-user-bubble) pl-1 opacity-0 transition-opacity group-hover/user-message:opacity-100 group-hover/user-message:transition-none group-focus-within/user-message:opacity-100">
+                  <MessageHoverTime className={cn(!showStop && !showRestore && 'pr-0.5')} />
+                  {showStop ? (
+                    <button
+                      aria-label={copy.stop}
+                      className={cn('pointer-events-auto size-5', USER_ACTION_ICON_BUTTON_CLASS)}
+                      onClick={event => {
+                        event.preventDefault()
+                        event.stopPropagation()
+                        void onCancel?.()
+                      }}
+                      type="button"
+                    >
+                      {StopGlyph}
+                    </button>
+                  ) : showRestore ? (
+                    <Tip label={copy.restoreFromHere}>
                       <button
-                        aria-label={copy.stop}
-                        className={cn('pointer-events-auto size-5', USER_ACTION_ICON_BUTTON_CLASS)}
+                        aria-label={copy.restoreCheckpoint}
+                        className={cn('pointer-events-auto size-6', USER_ACTION_ICON_BUTTON_CLASS)}
                         onClick={event => {
                           event.preventDefault()
                           event.stopPropagation()
-                          void onCancel?.()
+                          triggerHaptic('selection')
+                          onRequestRestoreConfirm?.(messageId, {
+                            text: messageText,
+                            userOrdinal: runtimeUserOrdinal
+                          })
+                        }}
+                        onPointerDown={event => {
+                          event.preventDefault()
+                          event.stopPropagation()
                         }}
                         type="button"
                       >
-                        {StopGlyph}
+                        <Codicon name="discard" size="0.875rem" />
                       </button>
-                    ) : (
-                      <Tip label={copy.restoreFromHere}>
-                        <button
-                          aria-label={copy.restoreCheckpoint}
-                          className={cn('pointer-events-auto size-6', USER_ACTION_ICON_BUTTON_CLASS)}
-                          onClick={event => {
-                            event.preventDefault()
-                            event.stopPropagation()
-                            triggerHaptic('selection')
-                            onRequestRestoreConfirm?.(messageId, {
-                              text: messageText,
-                              userOrdinal: runtimeUserOrdinal
-                            })
-                          }}
-                          onPointerDown={event => {
-                            event.preventDefault()
-                            event.stopPropagation()
-                          }}
-                          type="button"
-                        >
-                          <Codicon name="discard" size="0.875rem" />
-                        </button>
-                      </Tip>
-                    )}
-                  </div>
-                )}
+                    </Tip>
+                  ) : null}
+                </div>
               </div>
             </ReactionPicker>
             {/* Below the bubble, same register as the assistant action row:

apps/desktop/src/lib/time.ts

@@ -22,6 +22,9 @@ export const fmtDayTime = new Intl.DateTimeFormat(undefined, {
 // Medium date + short time (command center session detail).
 export const fmtDateTime = new Intl.DateTimeFormat(undefined, { dateStyle: 'medium', timeStyle: 'short' })
 
+// Weekday + full date + clock (message time tooltips).
+export const fmtFullDateTime = new Intl.DateTimeFormat(undefined, { dateStyle: 'full', timeStyle: 'short' })
+
 // Date only, "5 Jun 2026" (starmap tooltip).
 export const fmtDate = new Intl.DateTimeFormat(undefined, { day: 'numeric', month: 'short', year: 'numeric' })