fix(ui-kit-chat): user input detaches thread auto-follow so touch scrolling wins over streaming re-pin

[omridevk]

On iOS WKWebView the thread pane was un-scrollable during streaming: the auto-follow re-pins to bottom on every mutation, and release depended on a scroll event observed away-from-bottom - a race WKWebView loses because its scroll events arrive late/coalesced while mutations apply instantly. Desktop never showed it because Blink delivers scroll synchronously. The only gesture listener (pointerdown) never gated the re-pin.

Fix: an explicit userDetached flag set by real scroll intent (touchmove, wheel, scroll keys) gates the re-pin; it clears when the user lands back at bottom or explicitly follows again (the Latest pill / run start). The snap-back suppressor no longer applies during an active touch, so rubber-band bounces cannot mask release. Once a finger moves, no stream batch can drag the view back - the race is deleted, not narrowed.

Tests: new real-Chromium vitest browser project for the hook (6 behavior tests: touch/wheel/key detach, at-bottom + pill re-attach, tap does not detach), proven non-vacuous (gate reverted: 5/6 fail). Existing storybook play coverage for the hook still 4/4. Adds two test-only devDeps at pins already used by sibling packages.

Verified by orchestrator: full diff review + hard-rule greps (flagged hits are pre-existing lines), package suite rerun 57/57 incl. the chromium project, fallow pass 0 introduced.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds gesture-aware thread auto-follow detachment to prevent streaming content from overriding user scrolling.

Changes:

• Tracks touch, wheel, and keyboard scroll intent.
• Adds six Chromium browser tests.
• Adds browser-test dependencies and release changeset.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
.changeset/thread-scroll-user-detach.md	Documents the patch.
packages/ui-kit-chat/package.json	Adds browser-test dependencies.
packages/ui-kit-chat/src/behaviors/use-thread-auto-scroll.ts	Implements gesture detachment and reattachment.
packages/ui-kit-chat/test/thread-auto-scroll.browser.test.tsx	Tests auto-follow behavior in Chromium.
packages/ui-kit-chat/vitest.config.ts	Adds a Playwright browser project.
pnpm-lock.yaml	Locks the new dependencies.

Files not reviewed (1)

• pnpm-lock.yaml: Generated file

Comments suppressed due to low confidence (1)

packages/ui-kit-chat/src/behaviors/use-thread-auto-scroll.ts:129

• Downward keys (ArrowDown, PageDown, and End) also detach while already at bottom. If a stream mutation arrives before keyup, the viewport becomes geometrically away from bottom, so the keyup recomputation cannot reattach and a key intended to stay at/latest breaks auto-follow. Only upward keys should detach from the bottom; downward keys should detach only when the viewport is already away from it.

    const handleKeyDown = (event: KeyboardEvent) => {
      if (SCROLL_KEYS.has(event.key)) detach()
    }

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 6 changed files in this pull request and generated no new comments.

Files not reviewed (1)

• pnpm-lock.yaml: Generated file

Comments suppressed due to low confidence (1)

packages/ui-kit-chat/src/behaviors/use-thread-auto-scroll.ts:168

• Downward keys are treated as detach intent even when the viewport is already at the bottom. If a streaming mutation lands between keydown and keyup, userDetached blocks the re-pin; keyup then observes the newly increased distance from the bottom and cannot reattach, so ArrowDown/PageDown/End can permanently stop following despite never moving the reader away. Apply the same direction-aware handling as wheel input, and add a regression case that streams between a downward keydown/keyup at the bottom.

    const handleKeyDown = (event: KeyboardEvent) => {
      if (SCROLL_KEYS.has(event.key)) detach()
    }