fix(file-viewer): prevent scroll jump to top during Mothership streaming by waleedlatif1 · Pull Request #4559 · simstudioai/sim

waleedlatif1 · 2026-05-11T23:04:01Z

Summary

Fix root cause of scroll jump: MarkdownCheckboxCtx.Provider was conditionally rendered around the scroll container, causing it to unmount/remount (and reset scrollTop to 0) every time streaming started
Add useScrollAnchor hook with a spacer element that inflates scrollHeight to prevent the browser from clamping scrollTop when streamed content temporarily shrinks
Linger activeSessionId on complete so streamingContent never becomes undefined between two consecutive Mothership tool calls on the same file
Gate session activation in upsert on the incoming session having renderable content, preventing a blank flash when a new session starts before its first chunk arrives
Fix shouldShowStreamingFilePanel to keep the panel mounted while the lingered session is active
Fix use-chat post-write navigation condition to work with lingered completed sessions
Fix useAutoScroll to check proximity before pinning to bottom on stream start (prevents unwanted jump-to-bottom in the chat panel)

Type of Change

Bug fix

Testing

Tested manually — scrolled to middle of a file, asked Mothership to make multi-step edits. Scroll position preserved across tool call boundaries. Also verified single patch, fresh write, and cross-file edits all behave correctly.

Unit tests added:

use-scroll-anchor.test.ts: 15 tests for computeSpacerShortage pure function
use-file-preview-sessions.test.tsx: 7 new tests for linger, content-gate, and edge cases
text-editor-state.test.ts: 5 new tests for inter-session content shrink scenario

Checklist

Code follows project style guidelines
Self-reviewed my changes
Tests added/updated and passing
No new warnings introduced
I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

- 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

vercel · 2026-05-11T23:04:06Z

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Project	Deployment	Actions	Updated (UTC)
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cursor · 2026-05-11T23:04:10Z

PR Summary

Medium Risk
Medium risk because it changes streaming preview session activation/linger behavior and introduces new scroll anchoring logic that could affect viewer/chat scrolling and resource navigation edge cases.

Overview
Prevents the file viewer from jumping to the top during Mothership streaming by keeping the markdown scroll container mounted (always rendering MarkdownCheckboxCtx.Provider) and replacing useAutoScroll with a new useScrollAnchor + spacer element to preserve scrollTop when replace-mode chunks temporarily shrink content.

Updates file preview session state so activeSessionId lingers on a completed session until a successor with renderable content arrives (including updated hydrate/upsert selection rules), and adjusts Mothership view/chat navigation conditions to match this lingered-complete behavior.

Refines useAutoScroll to only pin to bottom on stream start when already near the bottom, and adds focused unit tests covering inter-session shrink scenarios, scroll-anchor calculations, and preview-session reducer edge cases.

^{Reviewed by Cursor Bugbot for commit 791f76a. Configure here.}

greptile-apps · 2026-05-11T23:15:53Z

Greptile Summary

This PR fixes a scroll-jump bug in the Mothership file-viewer that occurs between consecutive streaming tool calls. The root causes were a conditionally-rendered context provider unmounting the scroll container on every stream start, and activeSessionId resetting to null between tool calls (collapsing streamingContent to undefined).

MarkdownCheckboxCtx.Provider is now always rendered (removing the conditional that caused the scroll container to unmount/remount and reset scrollTop to 0 on each stream start).
Session linger: activeSessionId is held on the most-recently-completed session until a successor with renderable content upserts, keeping streamingContent non-undefined across tool-call boundaries; hydrate and upsert(activate:false) also preserve the linger via successor ?? state.activeSessionId.
useScrollAnchor replaces useAutoScroll in MarkdownPreview; a spacer div inflates scrollHeight to prevent the browser clamping scrollTop when replace-mode streaming temporarily produces shorter content, and useAutoScroll is patched to not jump-to-bottom in the chat panel unless the user is already near the bottom.

Confidence Score: 5/5

This PR is safe to merge — all changes are scoped to the file-viewer and chat scroll paths, and each fix is independently tested.

The root causes are addressed at both the React tree level (unconditional MarkdownCheckboxCtx.Provider) and the state-machine level (linger preserves activeSessionId across tool-call gaps). The useScrollAnchor spacer logic is pure-function-testable and well-covered. Previously-flagged issues with linger clearance in hydrate and upsert(activate:false) were resolved before this review. No new auth, data, or API surface is touched.

No files require special attention. The most complex new file is use-scroll-anchor.ts, and its pure helpers are thoroughly unit-tested.

Important Files Changed

Filename	Overview
apps/sim/hooks/use-scroll-anchor.ts	New hook that anchors scroll position during streaming via a spacer element; exposes pure helpers `computeSpacerShortage` and `shouldReengage` that are well-covered by unit tests. Logic is sound.
apps/sim/app/workspace/[workspaceId]/home/hooks/use-file-preview-sessions.ts	Session-linger logic added to `complete`, `hydrate`, and `upsert` cases; previously-flagged issues with linger clearance now resolved with `successor ?? state.activeSessionId` pattern throughout.
apps/sim/app/workspace/[workspaceId]/files/components/file-viewer/preview-panel.tsx	Switches from `useAutoScroll` to `useScrollAnchor`, adds the spacer `<div>`, and unconditionally wraps body in `MarkdownCheckboxCtx.Provider` to fix the unmount/remount root cause.
apps/sim/hooks/use-auto-scroll.ts	Stream-start now checks proximity before pinning to bottom, preventing unwanted jump-to-bottom in the chat panel when the user has scrolled up.
apps/sim/app/workspace/[workspaceId]/home/components/mothership-view/mothership-view.tsx	`shouldShowStreamingFilePanel` now keeps the panel mounted when a lingered completed session has renderable content, aligning with the new linger semantics.
apps/sim/app/workspace/[workspaceId]/home/hooks/use-chat.ts	Post-write navigation condition extended to fire when the preview session is in a lingered completed state, not only when `activePreviewSessionIdRef` is null.

_{Reviews (4): Last reviewed commit: "chore(file-viewer): trim verbose inline ..." | Re-trigger Greptile}

… spacer after stream

waleedlatif1 · 2026-05-11T23:34:54Z

@greptile

waleedlatif1 · 2026-05-11T23:34:59Z

@cursor review

cursor

✅ Bugbot reviewed your changes and found no new issues!

Comment @cursor review or bugbot run to trigger another review on this PR

^{Reviewed by Cursor Bugbot for commit 77f6364. Configure here.}

…s, add hydrate linger tests

waleedlatif1 · 2026-05-12T00:01:11Z

@greptile

waleedlatif1 · 2026-05-12T00:01:15Z

@cursor review

…ggers onScroll

Pulls the spacer-guard re-engage condition out of onScroll into an exported pure function so the false-re-engage invariant (spacer active → no re-engage) is covered by automated tests rather than relying on manual QA. Adds 8 unit tests for shouldReengage alongside the existing 15 for computeSpacerShortage.

waleedlatif1 · 2026-05-12T00:30:37Z

@greptile

waleedlatif1 · 2026-05-12T00:30:40Z

@cursor review

cursor

✅ Bugbot reviewed your changes and found no new issues!

Comment @cursor review or bugbot run to trigger another review on this PR

^{Reviewed by Cursor Bugbot for commit 791f76a. Configure here.}

greptile-apps Bot reviewed May 11, 2026

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

Comment thread apps/sim/hooks/use-scroll-anchor.ts Outdated

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

f021927

… spacer after stream

vercel Bot temporarily deployed to Preview May 11, 2026 23:19 Inactive

chore(file-viewer): trim verbose comments to match codebase style

77f6364

vercel Bot temporarily deployed to Preview May 11, 2026 23:27 Inactive

cursor Bot reviewed May 11, 2026

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fix(file-viewer): re-engage auto-scroll when user scrolls back to bottom

3f6c813

vercel Bot temporarily deployed to Preview May 11, 2026 23:51 Inactive

chore(file-viewer): update scroll-anchor tsdoc, remove test separator…

62ecc74

…s, add hydrate linger tests

vercel Bot temporarily deployed to Preview May 11, 2026 23:58 Inactive

cursor Bot reviewed May 12, 2026

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

fix(file-viewer): prevent false re-engage when spacer restoration tri…

3b99234

…ggers onScroll

vercel Bot temporarily deployed to Preview May 12, 2026 00:21 Inactive

vercel Bot temporarily deployed to Preview May 12, 2026 00:28 Inactive

chore(file-viewer): trim verbose inline comments in use-scroll-anchor

791f76a

vercel Bot temporarily deployed to Preview May 12, 2026 00:29 Inactive

chore(file-viewer): final comment cleanup before merge

6b350d0

vercel Bot temporarily deployed to Preview May 12, 2026 00:38 Inactive

cursor Bot reviewed May 12, 2026

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waleedlatif1 merged commit b7c34d4 into staging May 12, 2026
13 checks passed

waleedlatif1 deleted the fix/fil branch May 12, 2026 00:49

waleedlatif1 mentioned this pull request May 12, 2026

v0.6.74: security hardening, workers recycling, next-mdx-remote and opentelemetry updates, data drains to snowflake, blob, datadog, bigquery #4561

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waleedlatif1 commented May 11, 2026

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Confidence Score: 5/5

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