feat(desktop): follow streaming output at bottom + jump-to-bottom button (#45263)

Strict sticky-bottom autoscroll for the chat thread: while the viewport is
parked at the bottom, the tail follows content growth (streaming tokens, late
measurement, Shiki re-highlight) via a useLayoutEffect keyed on the
virtualizer's own size signal, pinned in the same pre-paint pass as its
scrollToFn so the two never rubber-band. The gate is a single boolean — one
upward pixel (scroll/wheel/touch) disarms follow until the user returns to the
bottom.

Adds a floating jump-to-bottom control that appears once scrolled ~10px away
(above the dim threshold so a sub-pixel settle never flashes it), positioned
above the composer with respect to the status stack, with a subtle
scale + slide in/out animation that honours prefers-reduced-motion. The button
bridges to the virtualizer's re-arm + pin path through a small nanostore
emitter.

Supersedes #43624.

apps/desktop/src/app/chat/index.tsx

@@ -53,6 +53,7 @@ import { droppedFileInlineRefs, type SessionDragPayload, sessionInlineRef } from
 import type { ChatBarState } from './composer/types'
 import { type DroppedFile, partitionDroppedFiles } from './hooks/use-composer-actions'
 import { useFileDropZone } from './hooks/use-file-drop-zone'
+import { ScrollToBottomButton } from './scroll-to-bottom-button'
 import { SessionActionsMenu } from './sidebar/session-actions-menu'
 import { lastVisibleMessageIsUser, threadLoadingState } from './thread-loading'
 
@@ -391,6 +392,7 @@ export function ChatView({
             </Suspense>
           )}
         </AssistantRuntimeProvider>
+        {showChatBar && <ScrollToBottomButton />}
         <ChatDropOverlay kind={dragKind} />
         <ChatSwapOverlay profile={gatewaySwapTarget} />
       </div>

apps/desktop/src/app/chat/scroll-to-bottom-button.tsx

@@ -0,0 +1,58 @@
+import { useStore } from '@nanostores/react'
+import { useRef } from 'react'
+
+import { Codicon } from '@/components/ui/codicon'
+import { useI18n } from '@/i18n'
+import { triggerHaptic } from '@/lib/haptics'
+import { cn } from '@/lib/utils'
+import { $threadJumpButtonVisible, requestScrollToBottom } from '@/store/thread-scroll'
+
+/**
+ * Floating "jump to bottom" control. Sits centered just above the composer,
+ * clearing the out-of-flow status stack via the same measured-height CSS vars
+ * the thread's bottom clearance uses (`--composer-measured-height` +
+ * `--status-stack-measured-height`), so it never overlaps the queue / subagent
+ * / background cards. Visible only while the user has scrolled meaningfully
+ * away from the bottom; clicking re-arms sticky-bottom and pins the viewport.
+ *
+ * Enter/exit motion lives in styles.css under `.thread-jump-button` — a
+ * directional scale (contract in from 1.1, contract out to 0.9) keyed off
+ * `data-state`. `idle` (never-shown) stays silent so it can't flash on mount;
+ * `in`/`out` only swap once it has actually appeared.
+ */
+export function ScrollToBottomButton() {
+  const { t } = useI18n()
+  const visible = useStore($threadJumpButtonVisible)
+  const hasShownRef = useRef(false)
+
+  if (visible) {
+    hasShownRef.current = true
+  }
+
+  const state = visible ? 'in' : hasShownRef.current ? 'out' : 'idle'
+
+  return (
+    <button
+      aria-hidden={!visible}
+      aria-label={t.assistant.thread.scrollToBottom}
+      className={cn(
+        'thread-jump-button absolute left-1/2 z-20 grid size-8 place-items-center rounded-full',
+        'border border-border/65 bg-(--composer-fill) text-muted-foreground hover:text-foreground',
+        'backdrop-blur-[0.75rem] [-webkit-backdrop-filter:blur(0.75rem)]',
+        !visible && 'pointer-events-none'
+      )}
+      data-state={state}
+      onClick={() => {
+        triggerHaptic('selection')
+        requestScrollToBottom()
+      }}
+      style={{
+        bottom: 'calc(var(--composer-measured-height) + var(--status-stack-measured-height) + 0.625rem)'
+      }}
+      tabIndex={visible ? 0 : -1}
+      type="button"
+    >
+      <Codicon name="arrow-down" size="1rem" />
+    </button>
+  )
+}