[juliusmarminge]

Problem

In a streamed Browser tab, every wheel message became its own CDP mouseWheel dispatch. Real Chromium takes about 16–33 ms per wheel dispatch, so a burst of scrolling queued seconds of work ahead of the next click. On main, 120 wheels at 50 Hz followed by a click delayed the click about 1.9 s. Ordinary scrolling through the web client, which already sums wheel deltas once per animation frame, still delayed it about 3.1 s.

Change

SessionControl.humanLatest now carries the queued input plus an optional merge function. The default merge keeps the newest input, which is what hover already did. Streamed wheel input now goes through humanLatest with a merge that adds up deltaX/deltaY. Its key is the point, modifiers and delta signs.

A wheel only merges into a wheel that is still waiting, next to it, with the same key. These keep wheels separate:

• the dispatch already running;
• any other input in between (click, key, hover, text, navigate, resize);
• releasing or retaking control;
• a change of position, modifiers or scroll direction. Keeping reversals separate leaves scrolling that is clamped at a page edge unchanged.

Total scroll distance and ordering relative to other input are preserved. No contract or client changes.

Scope and approval

Fixes #28217

Verification

Tests. vp test run apps/server/src/preview/ServerBrowser.test.ts apps/server/src/preview/SessionControl.test.ts apps/server/src/preview/ServerBrowserStream.test.ts: 80 passed.

• The new case "orders streamed input correctly for wheel burst before a click" fails on main with 123 dispatches instead of 4: the running move, 120 wheels, mouse down and mouse up. With the fix it sends the running move, one mouseWheel deltaY:1440, mouse down and mouse up.
• The existing "wheel and keyboard boundaries" case now expects one merged wheel (deltaY 50).
• The new "wheel position, modifier and direction boundaries" case covers the merge keys.

vp run check:changed passed: format, lint, knip, typecheck and tests.

Runtime transcript. This is a server-only fix with no visible render change, so the proof is the test above plus this transcript.

• Setup: an isolated dev server (T3CODE_DEV_SEED=0) with the repo's Chrome for Testing, on Linux x64. The streamed Browser tab is under the human's control and shows a synthetic local page that logs wheel and click events.
• Driver: the reporter's stream driver and fixture. It sends input over the authenticated viewer WebSocket.
• Client-path run: 360 DOM WheelEvents at 120 Hz on the streamed canvas, so the web client's per-frame batching applies.

Run	main 15fe283272: wheels delivered / click delay	fix 8369d8be75: wheels delivered / click delay	Scroll distance (both sides)
120 wheels at 50 Hz, then click	120 / 1,959 ms	60 / 61 ms	1,440 px
repeat	120 / 1,930 ms	68 / 74 ms	1,440 px
240 wheels at 100 Hz, then click	240 / 6,346 ms	65 / 65 ms	2,880 px
client path, 360 DOM wheels at 120 Hz	160 sent, 160 / 3,073 ms	181 sent, 75 / 19 ms	1,440 px

Every run delivered exactly one click, and the final scrollY equalled the summed delta.

Not checked:

• page wheel listeners, scroll snapping and nested-scroller chaining during a merged backlog;
• the mobile client against a pool device, which reaches the same server path;
• relay and tunnel connections.

Model and harness: Claude Opus 5.5 (lead, red test, implementation, review B) and GPT-6.1 Sol (review A), through Claude Code and Codex in T3 Code.

🤖 Generated with Claude Code

[macroscopeapp]

Approvability

Verdict: Approved at 8369d8b

Macroscope's review found this PR approvable — Compatible queued wheel events are combined so streamed browser clicks avoid waiting behind redundant scroll dispatches. Existing control checks and ordering across intervening actions remain intact, keeping the change narrowly scoped.

No code changes detected at 316b241. Prior analysis still applies.

^{You can add or adjust custom eligibility rules. Learn more.}

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	5.0 KiB	5.0 KiB	0 B (0.0%)	6.8 KiB	✅
Codex	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Codex	Live turn WebSocket wire	1.2 KiB	1.2 KiB	0 B (0.0%)	2.0 KiB	✅
Codex	Live turn WebSocket decoded	21.6 KiB	21.6 KiB	0 B (0.0%)	24.4 KiB	✅
Codex	Live turn messages	2	2	0 (0.0%)	8	✅
Claude	Total thread wire	5.0 KiB	5.0 KiB	−18 B (−0.3%)	6.8 KiB	✅
Claude	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Claude	Live turn WebSocket wire	1.2 KiB	1.2 KiB	−18 B (−1.4%)	2.0 KiB	✅
Claude	Live turn WebSocket decoded	21.9 KiB	21.9 KiB	−41 B (−0.2%)	24.4 KiB	✅
Claude	Live turn messages	2	1	−1 (−50.0%)	8	✅

Baseline: f6503f0 · PR result: 316b241 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 108.5 KiB
• Claude decoded thread snapshot: 108.8 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

[shivamhwp]

[GPT-6.1 Sol][Codex] on behalf of shivam.

Julius has taken over #28217 in this PR, so the issue-watch run is handing it over and starting no further tests.

Independent reproduction on main 15fe2832727893cbdd2214383f9f971474849695, physical corex, Linux x64, isolated dev server and streamed Chromium tab:

• A blocked running move followed by 120 wheels and a click produced 123 dispatches instead of the expected four in the service regression.
• Through the real authenticated viewer WebSocket, 120 wheels at 50 Hz with no artificial workload delayed the subsequent click by 1,749 ms. The page received 120 wheel events, a summed delta of 1,440 px, final scrollY of 1,440 px, and exactly one click.

The source confirms wheels use the FIFO human path while button-free hover uses humanLatest. These results corroborate the reported baseline; I have not tested this PR's head 8369d8be7573ac76f14436cab1ce7759c4e1d4e2, native clients, or relay/tunnel behavior.

Baseline recording, Linux web client, showing the wheel burst and delayed click:

before.mp4

[coderabbitai]

No actionable comments were generated in the recent review.

Walkthrough

The change adds input merging to SessionControl.humanLatest and uses it to coalesce compatible waiting wheel events in ServerBrowser. Wheel deltas are summed. Position, modifier, and direction changes remain aggregation boundaries.

Changes: Wheel input coalescing

Layer / File(s)	Summary
Typed queued input merging apps/server/src/preview/SessionControl.ts, apps/server/src/preview/SessionControl.test.ts	humanLatest now accepts typed input and an optional merge function. Matching waiting work merges its input and uses the updated callback. Existing hover tests use the updated method signature.
Wheel event coalescing apps/server/src/preview/ServerBrowser.ts, apps/server/src/preview/ServerBrowser.test.ts	Wheel events use humanLatest and sum deltas when position, modifiers, and delta directions match. Tests cover a 120-event burst and aggregation boundaries.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Medium

Sequence Diagram(s)

sequenceDiagram
  participant Viewer
  participant ServerBrowser
  participant SessionControl
  participant BrowserDispatch
  Viewer->>ServerBrowser: Send wheel input
  ServerBrowser->>SessionControl: Queue input with merge function
  SessionControl->>SessionControl: Merge compatible waiting wheel deltas
  SessionControl->>BrowserDispatch: Run callback with merged input

Loading

Possibly related issues

• pingdotgg/t3code#17538 — The change applies queued-input coalescing to streamed browser wheel bursts.

Possibly related PRs

• pingdotgg/t3code#22739 — This PR builds on the humanLatest method and mouse-move coalescing added there.

Suggested reviewers: rookmate, maria-rcks

Merge Risk: Moderate · up to 316b2

Streamed Browser clicks no longer wait behind queued wheel bursts, and total scroll distance is preserved. Pages that react to each wheel event separately, such as per-event carousels, may behave differently, and a click right after a merged scroll may land before the scroll settles. Check both in a real browser, or accept them explicitly, before merging.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly identifies the server fix for streamed Browser clicks delayed by queued wheel bursts. It is specific, concise, and uses the expected conventional format.
Description check		The description includes all required sections: Problem, Change, Scope and approval, and Verification. It explains the behavior, implementation, tests, runtime results, and known gaps in sufficient de…
Linked Issues check		[#28217] requires merging adjacent waiting streamed wheel inputs. It requires preserving scroll distance, input ordering, and control ownership. It treats running input, intervening actions, position,…
Out of Scope Changes check		The PR changes four files, according to the whole-PR summary: ServerBrowser.ts, SessionControl.ts, SessionControl.test.ts, and ServerBrowser.test.ts. The whole-PR diff of ServerBrowser.ts changes onl…

Full details: Description check

Explanation

The description includes all required sections: Problem, Change, Scope and approval, and Verification. It explains the behavior, implementation, tests, runtime results, and known gaps in sufficient detail.

Full details: Linked Issues check

Explanation

[#28217] requires merging adjacent waiting streamed wheel inputs. It requires preserving scroll distance, input ordering, and control ownership. It treats running input, intervening actions, position, modifier, and direction changes, and control release or retake as boundaries.

Whole-PR diff (merge base f6503f0 to head 316b241) of apps/server/src/preview/ServerBrowser.ts routes message.type === "wheel" through tab.control.humanLatest. The key includes x, y, modifiers, and the signs of deltaX and deltaY. The merge function sums deltaX and deltaY. Hover keeps its own key. All other input still uses human.

Head SessionControl.ts implements the merge. latestHuman stores the queued input and its run callback. A matching key merges only into the latest queued entry. latestHuman is cleared when the entry starts running, on any other enqueue, and on owner change. These clear points match the boundaries in the issue. When the tab is closed or the viewer does not own it, humanLatest falls back to human, so ownership is checked at dispatch time.

The author reports 80 passing tests. The reported tests include a deterministic burst regression (123 dispatches reduced to 4) and boundary cases. I did not rerun these tests.

Limits: the incremental diff from previous reviewed head 8369d8b was unavailable. The changes since that head were therefore not assessed separately. The current whole-PR code still supports the earlier PASS.

Page wheel listeners, scroll snapping, and nested-scroller behavior remain open in the issue. The issue does not set them as coding requirements, so they are not assessed here.

Full details: Out of Scope Changes check

Explanation

The PR changes four files, according to the whole-PR summary: ServerBrowser.ts, SessionControl.ts, SessionControl.test.ts, and ServerBrowser.test.ts.

The whole-PR diff of ServerBrowser.ts changes only the viewer input dispatch path. The dispatch helper now accepts an input argument. Hover and wheel use humanLatest. Wheel is the feature under [#28217]. Hover changes only to pass the input argument required by the new humanLatest signature. No resize batching, contract, or client change appears in that diff.

Head SessionControl.ts adds a generic input value and an optional merge function to humanLatest. The default merge keeps the newest input, so existing hover behavior is kept. This generic support is needed for the wheel merge.

SessionControl.test.ts only adds the new undefined argument to existing humanLatest calls. The ServerBrowser.test.ts changes add the wheel burst, aggregation, and boundary cases required by [#28217].

The whole-PR diff is based on the merge base f6503f0. It does not contain unrelated main commits. The summary lists no other changed files.

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• Fix comments: Fix CodeRabbit comments on this PR

Prompt to fix review comments

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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
Review comments at @apps/server/src/preview/ServerBrowser.ts:
- Around line 2652-2655: Update the wheel-event coalescing around the deltaX and
deltaY aggregation so page listeners that cancel wheel events still receive each
input event and can advance once per event. Do not merge these events unless the
implementation preserves that behavior; add a real-browser regression covering a
canceling wheel listener.
- Around line 2647-2650: Update the merged wheel-input path around
`tab.control.humanLatest` so it waits for scrolling to settle before allowing a
following click to be processed; do not treat completion of
`Input.dispatchMouseEvent` as completion of the scroll. Preserve merged wheel
dispatch rather than restoring every original wheel event.

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