fix(web): prevent chat scroll yank while browsing history

[wbxl2000]

Related Issue

None — this is a reproducible bug in conversation history scrolling; the problem is explained below.

Problem

While a user browses older messages, two independent timing problems can move the chat viewport against their input:

1. Images or other content above the viewport can finish laying out after the user has scrolled up. The conversation pane disabled browser scroll anchoring globally, so those height changes shifted the visible content.
2. A wheel, touch, or scrollbar drag can signal an upward user action before the resulting scroll event. A previously queued bottom-follow frame could run in that gap and pull the viewport back down.

Loading another page of history has a related requirement: prepended turns must not move the current reading position, but that correction must be scoped to the session that started the request and must preserve scrolling that happens while the request is in flight.

What changed

• Keep native scroll anchoring enabled while browsing history, and disable it only while intentionally following the bottom or restoring a prepended history page.
• Treat wheel, touch, and scrollbar-pointer input as immediate user intent. Upward input cancels queued bottom-follow work, smooth scrolling, and stale pin state before the browser emits scroll; input that cannot move a short conversation keeps following.
• Cancel old scroll writers before switching sessions or navigating through the conversation outline, so a follow lock from the previous target cannot pull the new viewport to the bottom.
• Restore prepended history from nearby turn or tool-call anchors plus the current scroll position. Stable tool ids cover a single long assistant turn that crosses a page boundary while new output is still arriving.
• Keep load state and deferred restoration per session, so switching sessions during a request cannot adjust the wrong conversation.
• Stop treating arbitrary DOM mutations as proof that a new message arrived; actual turn changes still control the new-message indicator.

The change stays inside the existing chat scroll state machine and adds no dependency or new module.

Verification:

• pnpm --filter @moonshot-ai/kimi-web typecheck
• pnpm --filter @moonshot-ai/kimi-web test (390 tests)
• pnpm --filter @moonshot-ai/kimi-web build
• pnpm --filter @moonshot-ai/kimi-web check:style
• Real Chromium checks for delayed layout shifts, upward input racing bottom-follow, history prepend stability, and no console errors

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works. (kimi-web has no component layout harness; the existing logic suite and real Chromium exercise cover the observable behavior.)
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update. (This restores expected behavior without changing the documented interface.)

[changeset-bot]

🦋 Changeset detected

Latest commit: 7e6dc54

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Patch

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[pkg-pr-new]

pnpm dlx https://pkg.pr.new/@moonshot-ai/kimi-code@7e6dc54

npx https://pkg.pr.new/@moonshot-ai/kimi-code@7e6dc54

commit: 7e6dc54

[wbxl2000]

@codex review

[chatgpt-codex-connector]

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[wbxl2000]

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Codex Review: Didn't find any major issues. Keep them coming!

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[wbxl2000]

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Codex Review: Didn't find any major issues. 👍

Reviewed commit: 046fd7d43e

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Codex Review: Didn't find any major issues. 🎉

Reviewed commit: 7e6dc5419c

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