perf(sidebar): skip full session-list rebuild when the payload is unchanged (#5455)

[ai-ag2026]

Problem

The 30s streaming poll and the gateway poll re-applied /api/sessions and ran a full sidebar DOM rebuild (list.innerHTML='' + one node tree per row) on every tick — even when the payload was byte-identical to what was already on screen, the common case on an idle sidebar. Part of the #5455 latency work.

Change

Add a signature over the applied rows + projects + coarse display state in _applySessionListPayload, and skip renderSessionListFromCache() when it matches the last rendered signature and no entrance animation is pending. A streaming row's fields advance each poll, so its signature changes and it still repaints. Client-only display toggles (search, active, density, select, lineage expansion) render through their own paths. The per-refresh INFLIGHT cleanup still runs on the skip path, and the 60s relative-time refresh plus every other render trigger call renderSessionListFromCache directly and are unaffected.

Behavior

Output-neutral: a skip only happens when the render would produce the same DOM.

Verification

In-browser: over an idle window, applied-payloads > DOM-rebuilds (redundant rebuilds skipped); sidebar still updates immediately on real changes (new session, title change, session switch, streaming indicator). node --check + ESLint runtime guard pass.

Refs #5455.

🤖 Generated with Claude Code

[greptile-apps]

Greptile Summary

This PR introduces a signature-based fast path inside _applySessionListPayload that short-circuits the full list.innerHTML = '' + node-tree rebuild when the serialized representation of all render-relevant state is unchanged from the last render — the common case during idle 30s/gateway polls.

• _sessionListRenderSignature() serializes a curated slim projection of _allSessions (key display fields only), _allProjects, active session, search text, density, select mode, profile counts, and source filter into a JSON string; a serialization failure returns null, which always forces a rebuild.
• Skip gate in _applySessionListPayload returns early (after running _purgeStaleInflightEntries) when _canRenderNow is true, no entrance animation is pending, the sig is non-null, and the sig matches the last cached value; streaming rows advance their fields on every poll so their sig changes and they still repaint.
• Test coverage adds a structural assertion that the slimmed sessionKeys list is used (not raw _allSessions) and backfills new state globals into the pushdown test harness.

Confidence Score: 4/5

The optimisation is output-neutral in the common case, but the profile-switch path (flagged in a prior review) can leave a post-skeleton list unrendered when two profiles share an identical sig; that window exists until the 60s time-refresh fires.

The sig-skip logic is well-guarded against animation, rename, and action-menu state. The one outstanding concern — _lastSessionListRenderSig not being reset by _invalidateSessionListRenders on profile switch — was identified in a previous review pass and is not addressed by this PR, meaning the stale-skeleton scenario is still reachable in production for users with identically-shaped profiles.

static/sessions.js — specifically the interaction between _invalidateSessionListRenders and _lastSessionListRenderSig around profile switches.

Important Files Changed

Filename	Overview
static/sessions.js	Adds _sessionListRenderSignature() and a sig-based skip gate inside _applySessionListPayload to avoid full DOM rebuilds on idle polls; the skip is correctly guarded by animation/rename/action-menu state but _lastSessionListRenderSig is never reset by _invalidateSessionListRenders(), leaving a window where a profile-switch to an identically-shaped list (e.g. two empty profiles) skips the post-skeleton render.
tests/test_webui_external_refresh_frontend.py	Adds a structural test verifying that _sessionListRenderSignature uses a slimmed session-key list rather than the raw _allSessions object, protecting against future schema additions silently breaking the skip.
tests/test_issue4766_sidebar_source_pushdown.py	Backfills the new globals (_sessionListRefreshAnimationPending, _lastSessionListRenderSig, _renamingSid, _sessionActionMenu, mocked _sessionListRenderSignature / _purgeStaleInflightEntries) into the test harness so existing pushdown tests continue to pass with the new skip gate in _applySessionListPayload.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[_applySessionListPayload called\npoll / WS push] --> B[Update _allSessions / _allProjects\ncounts, filters]
    B --> C[_sessionListSkeletonActive = false]
    C --> D{_canRenderNow?\n!renamingSid && !actionMenu}
    D -- No --> G[renderSessionListFromCache\nno-ops per own guards]
    D -- Yes --> E[Compute _renderSig\n_sessionListRenderSignature]
    E --> F{animationPending?\nor sig null?\nor sig != last?}
    F -- Skip conditions NOT met\nidentical idle payload --> H[_purgeStaleInflightEntries\nreturn early - DOM untouched]
    F -- Sig changed or\nanimation pending --> I[_lastSessionListRenderSig = _renderSig]
    I --> J[renderSessionListFromCache\nfull DOM rebuild]
    J --> K[_sessionListRefreshAnimationPending = false\nanimate FLIP if pending]

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%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
flowchart TD
    A[_applySessionListPayload called\npoll / WS push] --> B[Update _allSessions / _allProjects\ncounts, filters]
    B --> C[_sessionListSkeletonActive = false]
    C --> D{_canRenderNow?\n!renamingSid && !actionMenu}
    D -- No --> G[renderSessionListFromCache\nno-ops per own guards]
    D -- Yes --> E[Compute _renderSig\n_sessionListRenderSignature]
    E --> F{animationPending?\nor sig null?\nor sig != last?}
    F -- Skip conditions NOT met\nidentical idle payload --> H[_purgeStaleInflightEntries\nreturn early - DOM untouched]
    F -- Sig changed or\nanimation pending --> I[_lastSessionListRenderSig = _renderSig]
    I --> J[renderSessionListFromCache\nfull DOM rebuild]
    J --> K[_sessionListRefreshAnimationPending = false\nanimate FLIP if pending]

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_{Reviews (3): Last reviewed commit: "fix: narrow sidebar render-skip signatur..." | Re-trigger Greptile}

[nesquena-hermes]

Shipped in v0.51.844 — thanks @ai-ag2026! 🎉

The sidebar render-skip on idle polls is live (for #5455): the session list now skips its full DOM rebuild when a signature of the render inputs is unchanged, and rebuilds whenever anything visible changes.

This is a correctness-sensitive change (a false-skip = a stale sidebar), so it went through a deep dual gate and I fixed every finding on the release branch before shipping (all credited to you):

• Round 1 (3 fixes): the signature omitted rendered fields (pending/running streaming state, attention dots, source/read-only/worktree/lineage/model/profile + reference rows). Fixed by serializing the FULL applied row objects + _sidebarReferenceSessions instead of a hand-picked allowlist — inherently complete, can't rot when a new rendered field is added.
• Round 2 (2 fixes): recovering from the loading skeleton or the "couldn't load conversations" banner (both rendered outside the signature path) could false-skip and leave stale DOM. Fixed by capturing _hadSessionListSkeleton / _hadSessionListLoadError before clearing and forcing a real render on recovery.
• Round 3: Codex SAFE TO SHIP + Opus safe to ship (no stale-sidebar path remains). Full suite green (2 unrelated pre-existing env flakes). Regression tests added for all of the above.