fix(chat): keep viewport pinned to bottom while assistant streams

[Nkburdick]

Summary

Fixes the chat scroller dropping behind the streaming text. While the assistant streams a long response token-by-token, the existing auto-scroll \$effect (length-based) doesn't fire — so lines drift below the viewport and the user has to manually scroll.

Why

Caught 2026-04-30 EOD when Nick asked "So you said the chat streaming scroller was already implemented, right?" — verifying revealed the original implementation only handled the new-message-append case. Streaming mutates the assistant message's content field within the same array entry; messages.length is unchanged, so the \$effect at line 134 silently skips the streaming case.

What changed

In the SSE token event branch of both chat thread files, after applying the content patch:

• If user is at bottom (isScrolledToBottom) → tick().then(() => scrollTop = scrollHeight)
• Else → hasNewMessages = true (lights up the existing pill, lets user tap to catch up)

Identical change in both files since they have identical streaming structure:

• src/routes/projects/[slug]/chats/[threadId]/+page.svelte
• src/routes/areas/[slug]/chats/[threadId]/+page.svelte

What's NOT changed

• The append-path \$effect still handles brand-new messages
• The done-event post-stream scroll still runs
• User-scrolled-up state continues to surface the pill (no auto-yank)

Test plan

• bun run check clean (0 errors, 3 pre-existing warnings unrelated)
• Send a long prompt to an assistant and watch it stream — viewport should track the trailing line
• Mid-stream, scroll up manually — verify auto-scroll stops, pill appears, taps to bottom
• Compare project + area chat — both behave identically

Notes

• After merge, run cd /opt/oracle-stack && docker compose pull oracle && docker compose up -d oracle on KVM2 (oracle-app deploy is manual until pai-p3-infra-007 ships).

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Enhanced scroll behavior during message streaming across chat interfaces. The viewport now automatically maintains position at the bottom when new content arrives, and displays a "new messages" indicator when users have scrolled up, ensuring better visibility of incoming messages.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 56bb03e9-594e-4879-b92e-6e655165adc7

📥 Commits

Reviewing files that changed from the base of the PR and between 14789cd and 2145b9d.

📒 Files selected for processing (2)

• src/routes/areas/[slug]/chats/[threadId]/+page.svelte
• src/routes/projects/[slug]/chats/[threadId]/+page.svelte

---

📝 Walkthrough

Walkthrough

Two chat thread pages—one in the areas route and one in the projects route—now explicitly manage scroll behavior during SSE token streaming events, keeping the viewport pinned when at the bottom or marking new messages when scrolled up.

Changes

Cohort / File(s)	Summary
Chat Thread Scroll Management src/routes/areas/[slug]/chats/[threadId]/+page.svelte, src/routes/projects/[slug]/chats/[threadId]/+page.svelte	Added scroll behavior handling during token streaming: if user is at the bottom, schedules a scrollTop update to keep viewport pinned to growing content; otherwise sets hasNewMessages flag to surface new message notifications.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• feat(forge): Three glazier tasks covering the surgical three-fix patch #13: Modifies streaming message handling and scroll behavior in the same chat thread pages, adding viewport pinning logic for token events.

Poem

🐰 Hopping through the tokens, keeping chat in view,
When the bottom calls, we scroll on through,
But if you drift up to see what's new,
A message appears—a little "ping" just for you! 💬✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'fix(chat): keep viewport pinned to bottom while assistant streams' directly and clearly summarizes the main change—fixing viewport scroll behavior during token streaming in chat.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/chat-streaming-scroll-keepup

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Review rate limit: 0/1 reviews remaining, refill in 60 minutes.}

_{Comment @coderabbitai help to get the list of available commands and usage tips.}