fix(ui): stabilize virtual list scroll compensation

packages/ui/src/components/virtual-follow-list.tsx

@@ -402,8 +402,13 @@ export default function VirtualFollowList<T>(props: VirtualFollowListProps<T>) {
     if (!anchor) return
     const containerRect = containerRef.getBoundingClientRect()
     const rect = anchor.getBoundingClientRect()
-    const isAboveViewport = rect.bottom < containerRect.top
-    if (!isAboveViewport) {
+    // Determine whether the item was fully above the viewport *before* the
+    // height delta applied. Items can expand downward into the viewport; in that
+    // case we still need to compensate to keep existing visible content stable.
+    const bottomAfter = rect.bottom
+    const bottomBefore = bottomAfter - delta
+    const wasAboveViewport = bottomBefore < containerRect.top
+    if (!wasAboveViewport) {
       return
     }
 
@@ -445,7 +450,6 @@ export default function VirtualFollowList<T>(props: VirtualFollowListProps<T>) {
         containerRef.scrollTop = nextTop
         lastKnownScrollTop = nextTop
       }
-
     })
   }
 
@@ -531,23 +535,23 @@ export default function VirtualFollowList<T>(props: VirtualFollowListProps<T>) {
   let lastActiveState = false
   createEffect(() => {
     const active = isActive()
-      if (active) {
-        resolvePendingActiveScroll()
-        if (!lastActiveState && autoScroll()) {
-          requestScrollToBottom(true)
+    if (active) {
+      resolvePendingActiveScroll()
+      if (!lastActiveState && autoScroll()) {
+        requestScrollToBottom(true)
 
-          // When switching back to a cached session pane, items can mount/measure
-          // after the initial scroll jump. Re-pin once layout settles so the
-          // viewport stays at the bottom.
+        // When switching back to a cached session pane, items can mount/measure
+        // after the initial scroll jump. Re-pin once layout settles so the
+        // viewport stays at the bottom.
+        requestAnimationFrame(() => {
           requestAnimationFrame(() => {
-            requestAnimationFrame(() => {
-              scheduleAutoPinToBottom()
-            })
+            scheduleAutoPinToBottom()
           })
-        }
-      } else if (autoScroll()) {
-        pendingActiveScroll = true
+        })
       }
+    } else if (autoScroll()) {
+      pendingActiveScroll = true
+    }
     lastActiveState = active
   })
 

packages/ui/src/components/virtual-item.tsx

@@ -174,11 +174,15 @@ interface VirtualItemProps {
 export default function VirtualItem(props: VirtualItemProps) {
   const resolveContent = () => (typeof props.children === "function" ? (props.children as () => JSX.Element)() : props.children)
   const cachedHeight = sizeCache.get(props.cacheKey)
+  const fallbackPlaceholderHeight = () => props.minPlaceholderHeight ?? MIN_PLACEHOLDER_HEIGHT
   // Default to hidden until we can determine visibility.
   // This avoids keeping heavy DOM alive when IntersectionObserver
   // doesn't fire (common for hidden/zero-sized scroll roots).
   const [isIntersecting, setIsIntersecting] = createSignal(false)
-  const [measuredHeight, setMeasuredHeight] = createSignal(cachedHeight ?? 0)
+  // Keep measuredHeight aligned with the *effective layout height* while hidden.
+  // When content first mounts, onHeightChange deltas should reflect the DOM's
+  // placeholder height (not 0), otherwise scroll compensation can overshoot.
+  const [measuredHeight, setMeasuredHeight] = createSignal(cachedHeight ?? fallbackPlaceholderHeight())
   const [hasMeasured, setHasMeasured] = createSignal(cachedHeight !== undefined)
   let hasReportedMeasurement = Boolean(cachedHeight && cachedHeight > 0)
   let pendingVisibility: boolean | null = null
@@ -395,7 +399,7 @@ export default function VirtualItem(props: VirtualItemProps) {
       setMeasuredHeight(cached)
       setHasMeasured(true)
     } else {
-      setMeasuredHeight(0)
+      setMeasuredHeight(fallbackPlaceholderHeight())
       setHasMeasured(false)
     }
   })

packages/ui/src/styles/messaging/virtual-follow-list.css

@@ -2,6 +2,8 @@
   @apply flex-1 min-h-0 overflow-y-auto flex flex-col gap-0.5;
   background-color: var(--surface-base);
   color: inherit;
+  /* Prevent browser scroll anchoring fighting our virtualization compensation. */
+  overflow-anchor: none;
 }
 
 .message-stream-block {