fix(file-viewer): preserve session linger in hydrate/upsert and clear spacer after stream

Author: waleedlatif1

apps/sim/app/workspace/[workspaceId]/home/hooks/use-file-preview-sessions.ts

@@ -90,9 +90,12 @@ export function reduceFilePreviewSessions(
         }
       }
 
+      const successor = pickActiveSessionId(nextSessions, state.activeSessionId)
       return {
         sessions: nextSessions,
-        activeSessionId: pickActiveSessionId(nextSessions, state.activeSessionId),
+        // Preserve linger: if the active session is a completed (lingered) one
+        // and no non-complete successor exists, keep it rather than going null.
+        activeSessionId: successor ?? state.activeSessionId,
       }
     }
 
@@ -108,7 +111,8 @@ export function reduceFilePreviewSessions(
 
       let nextActiveSessionId: string | null
       if (action.activate === false || action.session.status === 'complete') {
-        nextActiveSessionId = pickActiveSessionId(nextSessions, state.activeSessionId)
+        const successor = pickActiveSessionId(nextSessions, state.activeSessionId)
+        nextActiveSessionId = successor ?? state.activeSessionId
       } else {
         // Don't steal active from a session that already has renderable content
         // just because a new empty/pending session arrived. Wait until the new

apps/sim/hooks/use-scroll-anchor.ts

@@ -174,7 +174,9 @@ export function useScrollAnchor(isStreaming: boolean, content?: string) {
     const spacer = spacerRef.current
     if (!el) return
 
-    if (!hasUserScrolledRef.current) {
+    // Clear the spacer when the user hasn't scrolled (auto-follow mode) or when
+    // streaming has ended (content is stable; no more clip-prevention needed).
+    if (!hasUserScrolledRef.current || !isStreaming) {
       if (spacer) spacer.style.minHeight = '0'
       return
     }
@@ -203,7 +205,7 @@ export function useScrollAnchor(isStreaming: boolean, content?: string) {
     if (el.scrollTop < targetScrollTop) {
       el.scrollTop = targetScrollTop
     }
-  }, [content])
+  }, [content, isStreaming])
 
   return {
     ref: callbackRef,