fix(platform): keep cursor visible during typewriter buffer drain

[Israeltheminer]

Closes #550

Summary

• showCursor in typewriter-text.tsx was tied to isStreaming, which flips to false the moment the server marks the message complete — even though the buffer still has seconds of text left to drain at 50 CPS. Fixed by tying showCursor to isTyping from useStreamBuffer instead, so the cursor stays visible until the last character is revealed.
• Added isCurrentlyTyping check to IncrementalMarkdown's isLastElement logic to handle the mid-block case where endOffset > revealedLen (parser hasn't closed the element yet), which previously caused no cursor to be placed.
• Added a "cursor during partial reveal (mid-stream)" test suite covering mid-paragraph, post-anchor, mid-list, mid-heading, and multi-reveal cases — all previously untested.

Test plan

• Send a long prompt (e.g. "Write a 500-word essay on the Roman Empire") and confirm the cursor stays visible for the entire duration of the typewriter animation, including after the server finishes generating
• Confirm cursor disappears cleanly once the last character is revealed
• Run npm run test:ui --workspace=@tale/platform — all 16 tests pass

Summary by CodeRabbit

Release Notes

• Bug Fixes

  • Improved cursor visibility and placement during mid-block streaming reveals.
  • Fixed cursor visibility timing to persist until the final character is displayed.

• Tests

  • Added comprehensive test coverage for mid-stream cursor rendering scenarios.

[greptile-apps]

Greptile Summary

Fixed cursor visibility during typewriter buffer drain by switching from isStreaming to isTyping state. The cursor now stays visible until the last character is revealed, even after the server marks the message complete.

• Changed showCursor in typewriter-text.tsx to use isTyping from useStreamBuffer hook, which tracks the actual animation state rather than server streaming state
• Added isCurrentlyTyping logic in incremental-markdown.tsx to correctly identify when the reveal position is mid-block (startOffset < revealedLen && endOffset > revealedLen), fixing cursor placement before the parser closes the element
• Added comprehensive test coverage for mid-stream cursor scenarios (mid-paragraph, post-anchor, mid-list, mid-heading, and multi-reveal cases)

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The fix correctly addresses the root cause of the cursor disappearing prematurely. The logic change is straightforward: using isTyping (which tracks buffer drain state) instead of isStreaming (which tracks server state) ensures cursor visibility matches the actual animation. The isCurrentlyTyping addition properly handles the mid-block edge case with sound logic. Test coverage is comprehensive and validates all key scenarios.
• No files require special attention

Important Files Changed

Filename	Overview
services/platform/app/features/chat/components/typewriter-text.tsx	Changed showCursor to use isTyping from useStreamBuffer instead of isStreaming, ensuring cursor stays visible during buffer drain
services/platform/app/features/chat/components/incremental-markdown.tsx	Added isCurrentlyTyping check to handle mid-block cursor placement when parser hasn't closed the element yet
services/platform/app/features/chat/components/tests/incremental-markdown.test.tsx	Added comprehensive test suite for cursor behavior during partial reveal (mid-paragraph, post-anchor, mid-list, mid-heading scenarios)

_{Last reviewed commit: 1c89fea}

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR addresses cursor visibility issues during the buffer-drain phase of streaming responses. It modifies IncrementalMarkdown to detect when text is being actively typed mid-block by considering both start and end offsets of node positions, introducing startOffset in the cursor wrapper and an isCurrentlyTyping condition. It also updates TypewriterText to tie cursor visibility to the isTyping flag from the stream buffer rather than the message streaming status. Test coverage for mid-stream cursor scenarios is added.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~22 minutes

Possibly related PRs

• feat(platform): refine chat streaming UX and simplify marker parser #511 — Modifies IncrementalMarkdown's cursor-placement logic and its test suite with overlapping changes to offset detection.
• feat(platform): refactor chat stream buffer and typewriter rendering #510 — Updates streaming cursor and typing-flag behavior in incremental-markdown.tsx and typewriter-text.tsx with related cursor placement logic.
• feat: chat ux improvements and mobile responsiveness fixes #150 — Changes streaming cursor behavior and visibility logic in incremental-markdown.tsx and typewriter-text.tsx.

🚥 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 clearly and specifically describes the main fix: keeping the cursor visible during typewriter buffer drain, which is the primary objective of the PR.
Linked Issues check	✅ Passed	All code changes directly address the requirements from issue #550: cursor visibility now depends on isTyping instead of isStreaming, isCurrentlyTyping logic is added to IncrementalMarkdown, and test coverage for mid-stream partial reveals is provided.
Out of Scope Changes check	✅ Passed	All changes in the PR are directly scoped to fixing the streaming cursor bug: test coverage expansion, cursor visibility logic update, and mid-block detection support. No unrelated changes detected.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/550-streaming-cursor-mid-block

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[coderabbitai]

Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In
`@services/platform/app/features/chat/components/__tests__/incremental-markdown.test.tsx`:
- Around line 153-247: Tests never exercise the new isCurrentlyTyping branch
because StreamingMarkdown slices content before parsing (revealedContent =
content.slice(0, revealedLength)), so parser AST node endOffset can never be >
revealedLen; to fix add a targeted test that triggers isCurrentlyTyping either
by (A) testing the lower-level helper that checks isCurrentlyTyping/endOffset
directly (call the function that uses endOffset and revealedLen) or (B) mocking
the markdown parser used by StreamingMarkdown to return an AST node with
endOffset > revealedLen (so isCurrentlyTyping becomes true), referencing
StreamingMarkdown, revealedContent/revealedLen, endOffset and isCurrentlyTyping
to locate the code under test.

In `@services/platform/app/features/chat/components/incremental-markdown.tsx`:
- Around line 221-241: Remove the unreachable mid-block guard and its use:
delete the isCurrentlyTyping computation (the const isCurrentlyTyping = ...
block) and update the isLastElement expression in incremental-markdown.tsx to no
longer reference isCurrentlyTyping, relying instead on the existing endOffset
=== revealedLen and the trim-based check; ensure revealedLenRef/current and
hasCursorEligibleChild logic remain unchanged and run the existing cursor tests
to confirm behavior.

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ℹ️ Review info

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 1afa693 and 1c89fea.

📒 Files selected for processing (3)

• services/platform/app/features/chat/components/__tests__/incremental-markdown.test.tsx
• services/platform/app/features/chat/components/incremental-markdown.tsx
• services/platform/app/features/chat/components/typewriter-text.tsx