fix(file-viewer): prevent scroll jump to top during Mothership streaming

- Fix root cause: MarkdownCheckboxCtx.Provider was conditionally rendered,
  causing the scroll container to unmount/remount when isStreaming flipped,
  resetting scrollTop to 0 on every stream start
- Add useScrollAnchor hook with spacer element to preserve scroll position
  when streamed content temporarily shrinks the scroll container
- Linger active session ID on complete to prevent streamingContent→undefined
  flicker between consecutive tool calls on the same file
- Gate upsert activation on incoming session having renderable content
- Fix shouldShowStreamingFilePanel to keep panel mounted during linger
- Fix use-chat post-write navigation to work with lingered completed session
- Fix useAutoScroll to check proximity before pinning to bottom on stream start

Author: waleedlatif1

apps/sim/app/workspace/[workspaceId]/files/components/file-viewer/preview-panel.tsx

@@ -34,7 +34,7 @@ import 'prismjs/components/prism-python'
 import { cn } from '@/lib/core/utils/cn'
 import { extractTextContent } from '@/lib/core/utils/react-node-text'
 import { getFileExtension } from '@/lib/uploads/utils/file-utils'
-import { useAutoScroll } from '@/hooks/use-auto-scroll'
+import { useScrollAnchor } from '@/hooks/use-scroll-anchor'
 import { DataTable } from './data-table'
 import { ZoomablePreview } from './zoomable-preview'
 
@@ -866,7 +866,10 @@ const MarkdownPreview = memo(function MarkdownPreview({
   onCheckboxToggle?: (checkboxIndex: number, checked: boolean) => void
 }) {
   const { push: navigate } = useRouter()
-  const { ref: autoScrollRef } = useAutoScroll(isStreaming && !disableAutoScroll)
+  const { ref: autoScrollRef, spacerRef } = useScrollAnchor(
+    isStreaming && !disableAutoScroll,
+    content
+  )
 
   const contentRef = useRef(content)
   contentRef.current = content
@@ -921,16 +924,13 @@ const MarkdownPreview = memo(function MarkdownPreview({
       >
         {markdownContent}
       </Streamdown>
+      <div ref={spacerRef} aria-hidden />
     </div>
   )
 
   return (
     <NavigateCtx.Provider value={navigate}>
-      {onCheckboxToggle ? (
-        <MarkdownCheckboxCtx.Provider value={ctxValue}>{body}</MarkdownCheckboxCtx.Provider>
-      ) : (
-        body
-      )}
+      <MarkdownCheckboxCtx.Provider value={ctxValue}>{body}</MarkdownCheckboxCtx.Provider>
     </NavigateCtx.Provider>
   )
 })

apps/sim/app/workspace/[workspaceId]/files/components/file-viewer/text-editor-state.test.ts

@@ -337,3 +337,112 @@ describe('syncTextEditorContentState — streaming finalize shortcuts', () => {
     expect(next.content).toBe('v2')
   })
 })
+
+describe('syncTextEditorContentState — inter-session content shrink (replace mode)', () => {
+  // These tests cover the path that causes scroll jumps in the file viewer.
+  // During "session linger" (session 1 complete, session 2 not yet started),
+  // the state machine stays in 'streaming' phase with session 1's full content.
+  // When session 2's first chunk arrives it is often SHORTER than session 1's
+  // final content. The state machine must correctly replace the content rather
+  // than treating it as already-final or dropping it.
+
+  it('replaces long linger content with a short first chunk from a new session', () => {
+    // State during linger: streaming with session 1's full long content
+    const lingerState = streaming(
+      'a very long document with many paragraphs',
+      'a very long document with many paragraphs',
+      ''
+    )
+    // Session 2 first chunk arrives — much shorter than session 1's content
+    const next = syncTextEditorContentState(lingerState, {
+      canReconcileToFetchedContent: false,
+      fetchedContent: undefined,
+      streamingContent: 'short',
+      streamingMode: 'replace',
+    })
+    expect(next.phase).toBe('streaming')
+    expect(next.content).toBe('short')
+    expect(next.lastStreamedContent).toBe('short')
+  })
+
+  it('correctly transitions to the new chunk even when it is a single character', () => {
+    const lingerState = streaming(
+      'full document\nmany lines\nof content',
+      'full document\nmany lines\nof content',
+      ''
+    )
+    const next = syncTextEditorContentState(lingerState, {
+      canReconcileToFetchedContent: false,
+      fetchedContent: undefined,
+      streamingContent: '#',
+      streamingMode: 'replace',
+    })
+    expect(next.phase).toBe('streaming')
+    expect(next.content).toBe('#')
+  })
+
+  it('does not finalize early when the new short chunk happens to equal savedContent', () => {
+    // savedContent '' and streamingContent '' — but we are in streaming phase,
+    // not ready, so content should remain in streaming phase
+    const lingerState = streaming('long content', 'long content', 'old saved')
+    const next = syncTextEditorContentState(lingerState, {
+      canReconcileToFetchedContent: false,
+      fetchedContent: undefined,
+      streamingContent: '',
+      streamingMode: 'replace',
+    })
+    expect(next.phase).toBe('streaming')
+    expect(next.content).toBe('')
+  })
+
+  it('stays streaming across multiple growing chunks after the shrink', () => {
+    const lingerState = streaming('final long document', 'final long document', '')
+
+    const chunk1 = syncTextEditorContentState(lingerState, {
+      canReconcileToFetchedContent: false,
+      fetchedContent: undefined,
+      streamingContent: '# New',
+      streamingMode: 'replace',
+    })
+    expect(chunk1.phase).toBe('streaming')
+    expect(chunk1.content).toBe('# New')
+
+    const chunk2 = syncTextEditorContentState(chunk1, {
+      canReconcileToFetchedContent: false,
+      fetchedContent: undefined,
+      streamingContent: '# New Section\n\nSome text',
+      streamingMode: 'replace',
+    })
+    expect(chunk2.phase).toBe('streaming')
+    expect(chunk2.content).toBe('# New Section\n\nSome text')
+
+    const chunk3 = syncTextEditorContentState(chunk2, {
+      canReconcileToFetchedContent: false,
+      fetchedContent: undefined,
+      streamingContent: '# New Section\n\nSome text that is now longer than the original',
+      streamingMode: 'replace',
+    })
+    expect(chunk3.phase).toBe('streaming')
+    expect(chunk3.content).toBe('# New Section\n\nSome text that is now longer than the original')
+  })
+
+  it('synthetic file (canReconcile=false) finalizes with current content when streaming ends', () => {
+    // After the new session finishes streaming, since there is no fetchedContent
+    // to reconcile with (synthetic file has no db key), it must finalize immediately.
+    const finalChunk = streaming(
+      '# Complete Document\n\nAll done.',
+      '# Complete Document\n\nAll done.',
+      ''
+    )
+    const next = syncTextEditorContentState(finalChunk, {
+      canReconcileToFetchedContent: false,
+      fetchedContent: undefined,
+      streamingContent: undefined,
+      streamingMode: 'replace',
+    })
+    expect(next.phase).toBe('ready')
+    expect(next.content).toBe('# Complete Document\n\nAll done.')
+    expect(next.savedContent).toBe('# Complete Document\n\nAll done.')
+    expect(next.lastStreamedContent).toBeNull()
+  })
+})

apps/sim/app/workspace/[workspaceId]/home/components/mothership-view/mothership-view.tsx

@@ -30,11 +30,11 @@ function shouldShowStreamingFilePanel(
   previewSession: FilePreviewSession | null | undefined,
   active: MothershipResource | null
 ): boolean {
-  if (!previewSession || previewSession.status === 'complete' || !active) return false
+  if (!previewSession || !hasRenderableFilePreviewContent(previewSession) || !active) return false
   if (active.id === 'streaming-file') return true
   if (active.type !== 'file') return false
   if (active.id && previewSession.fileId === active.id) {
-    return hasRenderableFilePreviewContent(previewSession)
+    return true
   }
   return false
 }

apps/sim/app/workspace/[workspaceId]/home/hooks/use-chat.ts

@@ -3402,7 +3402,11 @@ export function useChat(
                 if (parentToolCallId) {
                   subagentByParentToolCallId.delete(parentToolCallId)
                 }
-                if (previewSessionRef.current && !activePreviewSessionIdRef.current) {
+                if (
+                  previewSessionRef.current &&
+                  (!activePreviewSessionIdRef.current ||
+                    previewSessionRef.current.status === 'complete')
+                ) {
                   const lastFileResource = resourcesRef.current.find(
                     (r) => r.type === 'file' && r.id !== 'streaming-file'
                   )