fix(vscode): speed up subagent transcript loading

[marius-kilocode]

Subagent tabs waited for session metadata and the entire transcript before navigating, so long-running child sessions felt slow to open. The initial snapshot also discarded buffered SSE updates received during the fetch, which could leave a reasoning part empty when reasoning completed in that window.

Track child sessions before panel initialization, navigate as soon as the webview is ready, and load metadata concurrently. Subagent transcripts now use the same paginated loading path and row virtualizer as normal sessions, while buffered live updates are applied after the initial snapshot so completed reasoning remains visible.

Benchmark

Cold editor-tab opens of the same real 253-message Explore subagent, comparing the pre-fix commit with this branch using matched debug bundles and a warmed backend:

Metric	Before	After	Change
Transcript ready	1,365 ms	1,298 ms	4.9% faster
Trace duration	1,541 ms	1,478 ms	4.1% faster
Long tasks	4	2	50% fewer
Long-task time	596 ms	482 ms	19.1% lower
Script time	2,713 ms	2,570 ms	5.2% lower
Layout time	72.7 ms	61.5 ms	15.5% lower
Paint time	63.3 ms	54.0 ms	14.8% lower

The previous path loaded all 253 messages immediately. The new path loaded the bounded latest page and exposed older history through normal pagination; both settled with one mounted transcript row at the measured viewport.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

The changes are well-structured and the logic is sound. Key design decisions reviewed:

• preserveStream flag (KiloProvider.ts:1538): Correctly skips streams.drop() for the sub-agent viewer so buffered SSE updates (reasoning completions arriving during the snapshot fetch) are not discarded.
• streams.flush() ordering (KiloProvider.ts:1549): Flushing buffered updates after messagesLoaded preserves correct delivery order — snapshot first, then live deltas. This is exactly right.
• trackSession before resolveWebviewPanel (SubAgentViewerProvider.ts:47): Starting SSE event accumulation before the panel subscribes closes the race where reasoning deltas arrived before the viewer was ready.
• try/catch in SubAgentViewerProvider.ts:59-69: The outer try legitimately guards the synchronous getClient() throw (which occurs when not connected); async rejections from the .get() promise are covered by the .catch() chain. Both legs log the same message, which is intentional.
• registerSession side-effects: stopCurrentSessionProcesses inside registerSession is a no-op for a freshly created KiloProvider instance (no prior contextSessionID/currentSession), so the async metadata fetch racing with the initial load is safe.
• Pagination trade-off: Switching from limit: 0 to paginated loading for sub-agent transcripts is an intentional change well-justified by the benchmark. Long histories now use normal pagination rather than a single unbounded fetch.

Files Reviewed (5 files)

• .changeset/quick-subagent-transcripts.md
• packages/kilo-vscode/src/KiloProvider.ts
• packages/kilo-vscode/src/SubAgentViewerProvider.ts
• packages/kilo-vscode/src/kilo-provider/message-page.ts
• packages/kilo-vscode/tests/unit/kilo-provider-load-messages.test.ts

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_{Reviewed by claude-sonnet-4.6 · 1,112,151 tokens}

_{Review guidance: REVIEW.md from base branch main}