fix(#552): release stick-to-bottom on any upward user scroll

The chat viewport previously only released stick-to-bottom when the
user scrolled up AND was already more than 200px from the bottom.
While reading near the end of a streaming response, any upward scroll
inside the bottom 200px did nothing — the ResizeObserver then yanked
the viewport back to the bottom on the next streaming chunk, producing
the 'can't scroll up' tug-of-war reported in #552.

Fix: any user-initiated upward scroll releases stick-to-bottom
immediately. Re-stick only when the user has stopped scrolling up
AND is at the bottom (<=NEAR_BOTTOM_THRESHOLD). Applied symmetrically
in ChatContainerRoot (handleScroll) and in chat-message-list's
handleUserScroll mirror.

src/components/prompt-kit/chat-container.tsx

@@ -48,13 +48,21 @@ function ChatContainerRoot({
     if (!element) return
 
     const handleScroll = () => {
-      // Track stick-to-bottom internally based on actual scroll position
+      // Track stick-to-bottom internally based on actual scroll position.
+      // Bug #552: previously we only released stick-to-bottom when the user
+      // both scrolled up AND was already >200px from bottom. That meant any
+      // upward scroll within the bottom 200px did nothing — and during heavy
+      // streaming the ResizeObserver immediately yanked the viewport back to
+      // the bottom on the next content growth, producing the "can't scroll up"
+      // tug-of-war. Fix: ANY user-initiated upward scroll releases stick. Only
+      // re-stick when the user has stopped scrolling up AND is right at the
+      // bottom (≤NEAR_BOTTOM_THRESHOLD).
       const distFromBottom =
         element.scrollHeight - element.scrollTop - element.clientHeight
       const wasScrollingUp = element.scrollTop < lastScrollTopRef.current - 5
       lastScrollTopRef.current = element.scrollTop
 
-      if (wasScrollingUp && distFromBottom > NEAR_BOTTOM_THRESHOLD) {
+      if (wasScrollingUp) {
         stickToBottomRef.current = false
       } else if (distFromBottom <= NEAR_BOTTOM_THRESHOLD) {
         stickToBottomRef.current = true

src/screens/chat/components/chat-message-list.tsx

@@ -722,7 +722,11 @@ function ChatMessageListComponent({
     const wasScrollingUp = metrics.scrollTop < lastScrollTopRef.current - 5
     lastScrollTopRef.current = metrics.scrollTop
 
-    if (wasScrollingUp && !nearBottom) {
+    // Bug #552: any user-initiated upward scroll releases stick-to-bottom
+    // (previously required >200px from bottom, which let streaming yank the
+    // viewport back down during near-bottom reading). Re-stick only when the
+    // user lands back at the bottom.
+    if (wasScrollingUp) {
       stickToBottomRef.current = false
       isNearBottomRef.current = false
     } else if (nearBottom) {