fix(agent-browser): route oversized tabs snapshots to the tab list

[dormouse-bot]

Problem

Closes #296. In agent-browser-connection.ts, handleMessage routed stream messages purely by size: any payload longer than FRAME_PULSE_THRESHOLD (16384) was treated as a frame. A tabs snapshot carries url + title per tab, so a user with many open tabs (long URLs/titles) can push a tabs message past 16KB. When that happened the snapshot was silently dropped — hashed and emitted as a bare frame-pulse — and the tab list / active-tab selection never updated. Because the daemon re-broadcasts tabs on its heartbeat, a consistently-oversized tab list would appear frozen until a smaller snapshot happened to arrive.

The idle path (wantFrameData() false) is the dominant real-world case: wantsProvisionalFrame() returns false at steady state on hosts with crisp screenshots, so the fix had to cover it — not just the hover path.

Solution

Discriminate by content, not just size. A frame's bulk is a base64 JPEG body, and the base64 alphabet contains no " or :, so a compact "type":"tabs" / "type":"status" substring can never occur inside a real frame. It's a zero-false-positive marker for an oversized control message:

• If a large payload contains that marker → route it through the normal control dispatch (shared dispatchControl helper).
• Otherwise → unchanged frame fast path (hash + pulse, parse only when wantFrameData()).

This preserves the threshold's performance intent — frames never pay an extra parse, only rare oversized control messages do — and the change is strictly monotonic: it can only recover a previously-dropped snapshot, never misroute a frame.

One assumption: control messages are serialized as compact JSON (no space after the colon), which every in-repo fixture and JSON.stringify's default output confirm. If a daemon ever emitted spaced JSON, this fix would be a no-op for that case rather than a regression.

Testing

Added a regression test in the existing agent-browser-connection.test.ts that emits an ~24KB tabs snapshot (80 tabs with long URLs) on the default idle path and asserts the 80 tabs reach the snapshot. It fails on main ([] — dropped) and passes with the fix. Full file suite (8 tests) green; tsc --noEmit on the lib clean.

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Closes #296 — automated triage

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