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import {
isNearAnchor,
computeScrollCompensation,
SCROLL_ANCHOR_THRESHOLD_PX,
} from "../utils/scrollUtils.js";
/** Options for the useSmartScroll hook. */
interface UseSmartScrollOptions {
/** Ref to the scrollable container element. */
// eslint-disable-next-line @rushstack/no-new-null
scrollRef: RefObject<HTMLDivElement | null>;
/** Length of the content list — triggers scroll checks on change. */
contentLength: number;
/** Whether newest-at-top mode is active (anchor is top instead of bottom). */
isReversed: boolean;
/** When true, auto-scroll is suppressed (e.g. during multi-select mode). */
paused?: boolean;
}
/** Return value from the useSmartScroll hook. */
interface UseSmartScrollReturn {
/** Whether the user is currently at the anchor position (bottom or top). */
isAtAnchor: boolean;
/** Smooth-scrolls to the anchor and re-enables auto-scroll. */
scrollToAnchor: () => void;
}
/**
* Smart auto-scroll hook that respects user reading position.
*
* - Auto-scrolls to anchor (bottom or top) when new content arrives and user is at anchor.
* - Detects when user scrolls away from anchor and disables auto-scroll.
* - Provides a callback to manually scroll back to anchor.
* - In reverse mode, compensates scrollTop to prevent viewport shift from prepended content.
*/
export function useSmartScroll({
scrollRef,
contentLength,
isReversed,
paused,
}: UseSmartScrollOptions): UseSmartScrollReturn {
const [isAtAnchor, setIsAtAnchor] = useState(true);
const prevScrollHeightRef = useRef<number>(0);
const mountedRef = useRef(false);
const rafIdRef = useRef<number>(0);
// Throttled scroll listener — uses rAF to avoid excessive React work during fast scrolling.
// Only calls setState when the boolean actually changes.
useEffect(() => {
const element = scrollRef.current;
Iif (!element) {
return;
}
let lastKnownValue: boolean = true;
const handleScroll = (): void => {
Iif (rafIdRef.current) {
return;
}
rafIdRef.current = requestAnimationFrame(() => {
rafIdRef.current = 0;
const near = isNearAnchor(
element.scrollTop,
element.scrollHeight,
element.clientHeight,
isReversed,
SCROLL_ANCHOR_THRESHOLD_PX,
);
Iif (near !== lastKnownValue) {
lastKnownValue = near;
setIsAtAnchor(near);
}
});
};
element.addEventListener("scroll", handleScroll, { passive: true });
return () => {
element.removeEventListener("scroll", handleScroll);
Iif (rafIdRef.current) {
cancelAnimationFrame(rafIdRef.current);
rafIdRef.current = 0;
}
};
}, [scrollRef, isReversed]);
// Initial scroll — useLayoutEffect to avoid flash before paint
useLayoutEffect(() => {
const element = scrollRef.current;
Iif (!element || mountedRef.current) {
return;
}
mountedRef.current = true;
Iif (isReversed) {
element.scrollTop = 0;
} else {
element.scrollTop = element.scrollHeight;
}
setIsAtAnchor(true);
}, [scrollRef, isReversed]);
// Auto-scroll on new content + reverse-mode compensation.
// prevScrollHeightRef is captured at the END of this effect so the NEXT
// invocation sees the prior commit's scrollHeight (not the current one).
useLayoutEffect(() => {
const element = scrollRef.current;
Iif (!element) {
return;
}
// Skip auto-scroll while paused (e.g. during multi-select mode) to avoid
// disrupting the user's selection. Still update prevScrollHeight so scroll
// compensation is correct when unpaused.
if (paused) {
prevScrollHeightRef.current = element.scrollHeight;
return;
}
Iif (isReversed && !isAtAnchor) {
// Compensate scrollTop to prevent viewport shift from prepended content
const compensation = computeScrollCompensation(
prevScrollHeightRef.current,
element.scrollHeight,
);
Iif (compensation > 0) {
element.scrollTop += compensation;
}
}
if (isAtAnchor) {
Iif (isReversed) {
element.scrollTo({ top: 0, behavior: "smooth" });
} else {
element.scrollTo({ top: element.scrollHeight, behavior: "smooth" });
}
}
// Update prevScrollHeight AFTER applying compensation/auto-scroll
// so the next render can compute the delta correctly.
prevScrollHeightRef.current = element.scrollHeight;
}, [contentLength, isAtAnchor, isReversed, paused, scrollRef]);
const scrollToAnchor = useCallback((): void => {
const element = scrollRef.current;
Iif (!element) {
return;
}
const target = isReversed ? 0 : element.scrollHeight;
element.scrollTo({ top: target, behavior: "smooth" });
setIsAtAnchor(true);
}, [scrollRef, isReversed]);
return { isAtAnchor, scrollToAnchor };
}
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