perf: reduce iterations in the fast path for the keyer composer updates

[OEvgeny]

Fixes #

Changelog Entry

• Improved ActivityKeyerComposer performance for append scenarios by adding an incremental fast path that only processes newly-appended activities, in PR #5790, in PR #5797, by @OEvgeny
  • Added frozen window optimization to limit reference comparisons to the last 1,000 activities with deferred verification of the frozen portion, in PR #5797, by @OEvgeny

Description

Building on the incremental keying fast path from #5790, this PR introduces a "frozen window" optimization that skips reference comparisons for older activities. Instead of comparing every activity reference on each render, only the last 1,000 activities are checked for the frozen prefix. A deferred verification runs after 10 seconds of inactivity to warn if any frozen activity was unexpectedly mutated.

Design

• A MUTABLE_ACTIVITY_WINDOW constant (1,000) defines how many trailing activities are compared reference-by-reference each render. Activities before this boundary are assumed stable.
• A FROZEN_CHECK_TIMEOUT (10,000 ms) triggers a deferred check using requestIdleCallback (via ponyfill) to validate the frozen assumption and emit console warnings per position if violated.
• The slow path (full recalculation) clears pending checks and warning state, since it re-processes everything.
• The fast-path append branch was refactored to prefer early-return for the "no new keys" case, reducing nesting.

Specific Changes

• Added FROZEN_CHECK_TIMEOUT and MUTABLE_ACTIVITY_WINDOW constants to ActivityKeyerComposer.tsx
• Fast-path prefix comparison now starts at frozenBoundary (max(0, min(prev, current) - 1000)) instead of 0
• Added useEffect with setTimeout + requestIdleCallback to verify frozen portion after quiet period
• Added pendingFrozenCheckRef and warnedPositionsRef to track deferred check state
• Slow path resets pending frozen check and warned positions
• Refactored fast-path append branch to use early return for no-new-keys case

---

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)

[Copilot]

Pull request overview

This PR improves ActivityKeyerComposer’s incremental (append-only) keying path by reducing per-render reference comparisons via a “frozen window” approach, and adds a deferred idle-time verification to detect unexpected mutations outside the window.

Changes:

• Introduces a frozen-window optimization that only compares the last 1,000 activities by reference on each render.
• Adds a deferred (10s quiet period + requestIdleCallback) verification step that warns if older (“frozen”) activities changed unexpectedly.
• Refactors the fast-path append branch to early-return when no new keys are created.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
packages/api/src/providers/ActivityKeyer/ActivityKeyerComposer.tsx	Implements frozen-window fast path, deferred verification, and fast-path refactor.
CHANGELOG.md	Documents the performance improvement and frozen-window optimization.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[compulim]

This is good. Will be even better if we have tests to show what happen after mutating beyond the last 1,000 activities.