Extract thread grouping logic and add stable wrapper identity

[adamleithp]

Problem

The chat thread's tool-run and agent-turn grouping logic was embedded in ChatThread.tsx, making it hard to test and reuse. Additionally, during streaming, the grouping wrappers (ToolGroupItem, AgentTurn) were recreated on every chunk even when their member items hadn't changed, causing unnecessary re-renders of memoized rows and defeating the performance benefit of memoization.

Changes

Extract grouping logic to new module:

• Created threadGrouping.ts with groupToolRuns(), groupIntoTurns(), and the new createStableTurnGrouper() function
• Moved helper functions (isToolCallItem, isInvisibleItem, INVISIBLE_UPDATES) to the new module
• Added comprehensive unit tests in threadGrouping.test.ts covering all grouping scenarios

Implement stable wrapper identity:

• createStableTurnGrouper() preserves wrapper object identity across calls when member items are reference-equal
• Reuses previous ToolGroupItem and AgentTurn wrappers if their member arrays haven't changed
• Enables memoized rows to skip re-renders during streaming (only active-turn wrappers change per chunk)

Update ChatThread to use stable grouper:

• Instantiate createStableTurnGrouper() once per mounted thread via useRef
• Call turnGrouper.update(items) in the rows memo instead of groupIntoTurns(groupToolRuns(items))
• Added effect to clear group overrides when collapse mode changes (matching legacy thread behavior)
• Increased diff worker pool size from default 8 to 2 (conversation diffs don't need parallelism)

Update ToolGroup component:

• Accept groupId prop (the run's first tool id) to key manual expand/collapse overrides
• Implement collapse-mode logic: "all" keeps groups collapsed, "partial" streams live turn expanded then collapses on completion, "none" keeps everything expanded
• Manual toggle overrides the mode until mode changes (thread wipes overrides then)
• Use controlled open state instead of defaultOpen so groups actually collapse on turn completion

Update VirtualThreadScrollBody:

• Add measureElementImmediately callback to resize rows synchronously on mount
• Fixes paint-then-jump issue where rows scrolled into view would render at 80px estimate then resize

Update sessionViewStore:

• Add useGroupOverride(id) hook for fine-grained subscription to a single group's override
• Prevents re-rendering all mounted groups when any group's override changes

How did you test this?

• Added 12 unit tests in threadGrouping.test.ts covering:
  • Tool run grouping with invisible items and visible breaks
  • Turn grouping with user-initiated boundaries
  • Stable wrapper identity preservation across streaming chunks
  • Wrapper rebuilding when members change or runs grow
• Updated existing ToolGroup.test.tsx to pass new groupId prop
• Verified existing tests pass with the refactored logic

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

https://claude.ai/code/session_01EAy2CqYiHQeDTgCEAvAZtG

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[github-actions]

React Doctor found 1 issue in 1 file · 1 warning.

1 warning

src/features/sessions/components/chat-thread/ChatThread.tsx

• ⚠️ src/features/sessions/components/chat-thread/ChatThread.tsx:970 Missing effect dependencies exhaustive-deps

_{Reviewed by React Doctor for commit a505535.}

[charlesvien]

Hey @adamleithp, we've migrated this repo into the PostHog/posthog monorepo, so this PR needs to be recreated there.

Easiest path: check out the monorepo and run the /porting-code-prs skill with your agent pointed at this PR. It has the migration guide and path remappings baked in so the agent doesn't have to figure them out. You don't strictly need it though, just asking your agent to port the PR works fine too.

Shout if you get stuck.