fix(desktop): stop chat scroll backward-jump from content-growth interim scrolls (#37997)

The thread scroll-anchor hook in apps/desktop/src/components/assistant-ui/
thread-virtualizer.tsx was disarming sticky-bottom whenever scrollTop
decreased by >1px between scroll events. That check was too eager: when
content height grows mid-frame (virtualizer measurement of a newly visible
turn, streaming token, Streamdown/Shiki re-tokenization, composer chip
toggle), the browser emits an interim 'scroll' event whose scrollTop is
smaller than the previous frame's because scrollHeight just jumped. The
rAF-scheduled pinToBottom hasn't run yet, so programmaticScrollPendingRef
is 0 and the disarm fired. With sticky-bottom disarmed the scroller stuck
~50px above bottom — the visible at-rest backward jump that #37997
describes (and the same root cause as the wheel-up variant in #37527).

Fix:
- Track scrollHeight per frame (lastHeightRef). Disarm on scrollTop
  decrease ONLY when scrollHeight did not grow this frame. Real upward
  user intent (scrollbar drag, keyboard PgUp, programmatic scrollIntoView)
  still disarms because it moves scrollTop without growing the content.
  Wheel-up and touchmove continue to disarm via their own listeners.
- Stop observing the scroller element itself in the ResizeObserver; only
  observe its content child. Viewport-only resizes (window resize,
  devtools panel toggle) no longer trigger spurious pins, matching the
  intent of the auto-stick-to-bottom behavior.

Verified:
- apps/desktop `tsc -b` clean.
- apps/desktop `vitest run src/components/assistant-ui/streaming.test.tsx`
  passes (9/9), including the existing wheel-up disarm regression test
  that asserts scrollTop stays at 420 after a wheel-up + content growth.

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

@@ -182,6 +182,7 @@ function useThreadScrollAnchor({ enabled, groupCount, scrollerRef, sessionKey, v
   // user-driven upward scroll; re-armed when they reach bottom again.
   const armedRef = useRef(true)
   const lastTopRef = useRef(0)
+  const lastHeightRef = useRef(0)
   // Counter that tracks how many scroll events we expect to be ours rather
   // than the user's. `pinToBottom` writes `el.scrollTop`, which fires an
   // async `scroll` event; without this guard the on-scroll handler can race
@@ -206,6 +207,7 @@ function useThreadScrollAnchor({ enabled, groupCount, scrollerRef, sessionKey, v
     programmaticScrollPendingRef.current += 1
     el.scrollTop = el.scrollHeight
     lastTopRef.current = el.scrollTop
+    lastHeightRef.current = el.scrollHeight
   }, [scrollerRef])
 
   const jumpToBottom = useCallback(() => {
@@ -250,6 +252,7 @@ function useThreadScrollAnchor({ enabled, groupCount, scrollerRef, sessionKey, v
       if (programmaticScrollPendingRef.current > 0) {
         programmaticScrollPendingRef.current -= 1
         lastTopRef.current = top
+        lastHeightRef.current = el.scrollHeight
         // Always re-arm — sticky-bottom should hold through clamp races.
         armedRef.current = true
         const atBottom = el.scrollHeight - (top + el.clientHeight) <= AT_BOTTOM_THRESHOLD
@@ -258,11 +261,26 @@ function useThreadScrollAnchor({ enabled, groupCount, scrollerRef, sessionKey, v
         return
       }
 
-      if (top + 1 < lastTopRef.current) {
+      // Disarm only when `scrollTop` decreases AND `scrollHeight` did NOT
+      // grow this frame. A bare `top < lastTopRef.current` check is unsafe:
+      // when content grows (virtualizer item measurement, streaming token,
+      // code highlight re-tokenization, composer chip), the browser emits
+      // an interim `scroll` event whose `scrollTop` is smaller than the
+      // previous frame's because `scrollHeight` jumped — this fires before
+      // the rAF-scheduled `pinToBottom` runs, so `programmaticScrollPendingRef`
+      // is 0. Treating that as a user scroll permanently disarmed sticky-bottom
+      // and produced the visible at-rest backward jump (#37997). Gating on a
+      // stable `scrollHeight` keeps real user-driven upward intent — scrollbar
+      // drag, keyboard PgUp, programmatic scrollIntoView — covered without
+      // the false positive. Wheel-up and touchmove still disarm via their
+      // own listeners below.
+      const heightGrew = el.scrollHeight > lastHeightRef.current
+      if (!heightGrew && top + 1 < lastTopRef.current) {
         armedRef.current = false
       }
 
       lastTopRef.current = top
+      lastHeightRef.current = el.scrollHeight
 
       const atBottom = el.scrollHeight - (top + el.clientHeight) <= AT_BOTTOM_THRESHOLD
 
@@ -323,8 +341,9 @@ function useThreadScrollAnchor({ enabled, groupCount, scrollerRef, sessionKey, v
 
     const observer = new ResizeObserver(schedulePin)
 
-    observer.observe(el)
-
+    // Observe ONLY the content (firstElementChild), not the scroller `el`
+    // itself. Resizes of the viewport/scroller (window resize, devtools
+    // panel toggle) shouldn't trigger a pin — only content growth should.
     if (el.firstElementChild) {
       observer.observe(el.firstElementChild)
     }