Make mobile scroll transitions much smoother

src/components/organisms/mobileHeader.module.scss

@@ -25,12 +25,20 @@
 	align-items: center;
 	justify-content: space-between;
 	z-index: 1;
+	padding-top: 24px;
+	padding-bottom: 14px;
+	transition: $transition-quick padding-top, $transition-quick padding-bottom;
 
 	> :first-child {
 		margin-right: 16px;
 	}
 }
 
+.down .title {
+	padding-top: 16px;
+	padding-bottom: 8px;
+}
+
 .subnav {
 	display: flex;
 	align-items: flex-end;
@@ -39,6 +47,12 @@
 	position: relative;
 	overflow: hidden;
 	background: $ts-cream;
+	height: 24px;
+	transition: $transition-quick height;
+}
+
+.down .subnav {
+	height: 0;
 }
 
 .subnavItems {

src/components/organisms/mobileHeader.tsx

@@ -16,24 +16,11 @@ import IconExitSmall from '~public/img/icons/icon-exit-small.svg';
 import MoreIcon from '~public/img/icons/icon-more.svg';
 
 import styles from './mobileHeader.module.scss';
-import { useTransitionProgress } from './mobileHeader.useTransitionProgress';
+import useScrollDirection, {
+	SCROLL_DIRECTIONS,
+} from './mobileHeader.useScrollDirection';
 import { EntityFilterId } from './searchResults.filters';
 
-type Transition = [number, number];
-
-const SUBNAV_HEIGHT: Transition = [24, 0];
-const TITLE_PAD_TOP: Transition = [24, 16];
-const TITLE_PAD_BOTTOM: Transition = [14, 8];
-
-const COLLAPSING_HEIGHT = [
-	SUBNAV_HEIGHT,
-	TITLE_PAD_TOP,
-	TITLE_PAD_BOTTOM,
-].reduce((acc, [s, e]) => acc + (s - e), 0);
-
-const px = (progress: number, [s, e]: Transition) =>
-	`${s + (e - s) * progress}px`;
-
 export default function MobileHeader({
 	setShowingMenu,
 	scrollRef,
@@ -53,18 +40,17 @@ export default function MobileHeader({
 	const lang = useLanguageRoute();
 	const navItems = useNavigationItems();
 	const { getRecording } = useContext(PlaybackContext);
-	const progress = useTransitionProgress(scrollRef, COLLAPSING_HEIGHT);
+	const scrollDirection = useScrollDirection(scrollRef);
 
 	return (
-		<div className={styles.base}>
+		<div
+			className={clsx(styles.base, {
+				[styles.up]: scrollDirection === SCROLL_DIRECTIONS.UP,
+				[styles.down]: scrollDirection === SCROLL_DIRECTIONS.DOWN,
+			})}
+		>
 			<div className={styles.wrapper}>
-				<div
-					className={styles.title}
-					style={{
-						paddingTop: px(progress, TITLE_PAD_TOP),
-						paddingBottom: px(progress, TITLE_PAD_BOTTOM),
-					}}
-				>
+				<div className={styles.title}>
 					<Header />
 					<Button
 						type="super"
@@ -78,10 +64,7 @@ export default function MobileHeader({
 					/>
 					{getRecording() && <ButtonPlayback />}
 				</div>
-				<div
-					className={styles.subnav}
-					style={{ height: px(progress, SUBNAV_HEIGHT) }}
-				>
+				<div className={styles.subnav}>
 					<Mininav
 						items={navItems.slice(0, -2).map((item) => {
 							if (!item.href) {

src/components/organisms/mobileHeader.useScrollDirection.ts

@@ -0,0 +1,37 @@
+import { useEffect, useState } from 'react';
+
+import useScrollTop from '~src/lib/hooks/useScrollTop';
+
+export const SCROLL_DIRECTIONS = {
+	UP: 'up',
+	DOWN: 'down',
+};
+
+export default function useScrollDirection(
+	scrollRef: React.RefObject<HTMLDivElement>
+) {
+	const scrollTop = useScrollTop(scrollRef);
+	const [lastScrollTop, setLastScrollTop] = useState<number>(0);
+	const [direction, setDirection] = useState<string>(SCROLL_DIRECTIONS.UP);
+
+	useEffect(() => {
+		setLastScrollTop(Math.max(scrollTop, 0));
+	}, [scrollTop]);
+
+	useEffect(() => {
+		const scrollHeight = scrollRef.current?.scrollHeight || 0;
+		const clientHeight = scrollRef.current?.clientHeight || 0;
+		const clientTop = scrollHeight - scrollTop;
+		const buffer = clientTop - clientHeight;
+		const maxHeaderHeight = 101;
+		if (buffer < maxHeaderHeight) return;
+		if (scrollTop > lastScrollTop) {
+			setDirection(SCROLL_DIRECTIONS.DOWN);
+		}
+		if (scrollTop < lastScrollTop) {
+			setDirection(SCROLL_DIRECTIONS.UP);
+		}
+	}, [scrollTop, lastScrollTop, scrollRef]);
+
+	return direction;
+}

src/lib/hooks/useScrollTop.ts

@@ -0,0 +1,23 @@
+import { useCallback, useEffect, useState } from 'react';
+
+import isServerSide from '../isServerSide';
+
+export default function useScrollTop(
+	scrollRef: React.RefObject<HTMLDivElement>
+) {
+	const [scrollTop, setScrollTop] = useState<number>(0);
+
+	const listener = useCallback(() => {
+		if (isServerSide() || !scrollRef.current) return;
+		setScrollTop(scrollRef.current.scrollTop);
+	}, [scrollRef]);
+
+	useEffect(() => {
+		if (isServerSide() || !scrollRef.current) return;
+		const el = scrollRef.current;
+		el.addEventListener('scroll', listener);
+		return () => el.removeEventListener('scroll', listener);
+	}, [listener, scrollRef]);
+
+	return scrollTop;
+}