fix(chat): follow the assistant response to the bottom unless the user scrolls away - #23
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…r scrolls away Follow mode was a boolean flipped by every scroll event, including the app's own programmatic scrolls and passive layout shifts, so a mid-growth event could pin the viewport at the user prompt for the rest of the turn. It is now an explicit mode: growth of the transcript (any height change, not just message count or final text length, via a ResizeObserver on the content) keeps the viewport pinned to the bottom while following, and following stops only on genuine upward scroll intent (wheel, touch drag, ArrowUp/PageUp/Home) and resumes when the user returns to the bottom. Closes #19
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Closes #19.
What broke
After submitting at the bottom, the viewport could stay anchored near the user prompt while the assistant response grew below the fold. Two compounding causes in
useChatSessionState:handleScrollransetIsUserScrolledUp(!isNearBottom())on every scroll event, including events fired by the app's own programmatic scrolls and passive layout shifts. One mid-growth event could flip follow mode off for the rest of the turn.chatMessages.lengthand the final message'scontent.length, missing tool cards, reasoning blocks, images/KaTeX, and any async reflow.Fix
New
useChatFollowScrollhook (extracted, DOM-tested):scrollToBottom()resumes following, so the existing "new messages below" button keeps working;isUserScrolledUp,hasNewMessagesBelow,scrollToBottomAndReset, and the load-older-at-top pagination path keep their exact public behavior (isUserScrolledUpis now!isFollowinginternally).Coverage
useChatFollowScroll.dom.bun.test.tsx(5 tests): growth pins to bottom; wheel-up stops follow; passive scroll events do not stop follow (the regression); returning to the bottom resumes;scrollToBottomresumes.useChatSessionState.test.tsstays green; both tsconfigs and eslint clean.Known limitation: dragging the scrollbar upward during active streaming doesn't emit wheel/touch/key intent, so follow resumes on the next content growth — wheel/touch/keyboard gestures, the common case, are all covered.