animation strip

src/lib/utils/url-state.ts

@@ -6,6 +6,11 @@
  * Writes use `replaceState` rather than `goto`, so mirroring state never adds a
  * history entry or re-runs a load - the back button still means "the page
  * before", not "the previous day I clicked".
+ *
+ * Callers must pass the URL *untracked* (`get(page).url`, not `$page.url`).
+ * Reading it reactively inside the same effect that writes it creates a loop:
+ * replaceState publishes a new URL, the effect re-runs, writes again - which
+ * Svelte eventually kills with `effect_update_depth_exceeded`, hanging the page.
  */
 import { browser } from '$app/environment';
 import { replaceState } from '$app/navigation';

src/routes/weather/14-day/[location]/+page.svelte

@@ -94,7 +94,7 @@
 	$effect(() => {
 		const model = params.models?.[0];
 		if (!mounted || !model) return;
-		syncSearchParams($page.url, { model });
+		syncSearchParams(get(page).url, { model });
 	});
 
 	// components persist across refetches; entries are null while unmounted

src/routes/weather/compare/[location]/+page.svelte

@@ -123,7 +123,7 @@
 		const models = params.models;
 		const vars = params.hourly;
 		if (!mounted) return;
-		syncSearchParams($page.url, {
+		syncSearchParams(get(page).url, {
 			models: models?.length ? models.join(',') : null,
 			vars: vars?.length ? vars.join(',') : null
 		});

src/routes/weather/week/[location]/+page.svelte

@@ -185,9 +185,9 @@
 
 		const target = Date.parse(`${param}T12:00:00Z`);
 		if (Number.isNaN(target)) return;
-		const today = new Date();
-		today.setUTCHours(12, 0, 0, 0);
-		const offset = Math.round((target - today.getTime()) / 86_400_000);
+		const now = new Date();
+		const todayNoon = Date.UTC(now.getFullYear(), now.getMonth(), now.getDate(), 12);
+		const offset = Math.round((target - todayNoon) / 86_400_000);
 
 		if (offset < -3 || offset > 15) return; // out of reach: stay on today
 		wantedDay = param;
@@ -215,7 +215,7 @@
 		const isToday = fetchedDaily
 			? dayKey === formatZoned(new Date(), fetchedDaily.timezone, 'yyyy-MM-dd')
 			: false;
-		syncSearchParams($page.url, {
+		syncSearchParams(get(page).url, {
 			day: isToday ? null : dayKey,
 			model: model && model !== 'best_match' ? model : null
 		});

src/routes/weather/week/[location]/DailyStripSticky.svelte

@@ -62,6 +62,7 @@
 	//    with a short CSS transition instead: an IntersectionObserver on the same
 	//    sentinel toggles `.compact`. No per-frame scroll handler anywhere.
 	let sentinelEl = $state<HTMLDivElement>();
+	let stripEl = $state<HTMLDivElement>();
 	let stripScrollEl = $state<HTMLDivElement>();
 	let daysWrapEl = $state<HTMLDivElement>();
 	let needsSnapFallback = $state(false);
@@ -71,21 +72,40 @@
 	onMount(() => {
 		const scrubSupported =
 			CSS.supports('animation-timeline: view()') && CSS.supports('timeline-scope: none');
-		if (scrubSupported || !sentinelEl) return;
-		needsSnapFallback = true;
-		// `stuck` fires the moment the strip pins (turns the bar opaque before any
-		// content can slide under it); `compact` snaps the cells once more than
-		// half of the sentinel band has scrolled past. The transitions smooth both.
-		const io = new IntersectionObserver(
-			(entries) => {
-				const e = entries[entries.length - 1];
-				stuck = e.intersectionRatio < 0.97;
-				compact = e.intersectionRatio < 0.5;
-			},
-			{ threshold: [0.25, 0.5, 0.75, 0.97] }
-		);
-		io.observe(sentinelEl);
-		return () => io.disconnect();
+		if (scrubSupported || !sentinelEl || !stripEl) return;
+
+		// Without scroll-driven animations (Firefox, older Safari) the collapse used
+		// to snap between the two states at a threshold, which reads as a jump next
+		// to the smooth scrub everywhere else. Drive `--strip-p` from the scroll
+		// position instead, so the cells shrink with the scroll exactly as they do
+		// natively. One rAF-coalesced write per frame, no layout thrash.
+		const sentinel = sentinelEl;
+		const strip = stripEl;
+		const scroller = sentinel.closest('main');
+
+		let frame = 0;
+		const update = () => {
+			frame = 0;
+			const band = sentinel.getBoundingClientRect();
+			const top = scroller ? scroller.getBoundingClientRect().top : 0;
+			const height = band.height || 1;
+			const progress = Math.min(1, Math.max(0, (top - band.top) / height));
+			strip.style.setProperty('--strip-p', String(progress));
+			strip.style.setProperty('--stuck', progress > 0.02 ? '1' : '0');
+		};
+		const schedule = () => {
+			if (!frame) frame = requestAnimationFrame(update);
+		};
+
+		update();
+		const target: EventTarget = scroller ?? window;
+		target.addEventListener('scroll', schedule, { passive: true });
+		window.addEventListener('resize', schedule);
+		return () => {
+			if (frame) cancelAnimationFrame(frame);
+			target.removeEventListener('scroll', schedule);
+			window.removeEventListener('resize', schedule);
+		};
 	});
 
 	// Start scrolled so the "Past" button sits just off the left edge (revealed by
@@ -129,6 +149,7 @@
 <div bind:this={sentinelEl} class="sentinel" aria-hidden="true"></div>
 
 <div
+	bind:this={stripEl}
 	class="daystrip sticky -top-3 z-30 -mx-3 lg:-top-6 lg:-mx-8 mt-1"
 	class:js-snap={needsSnapFallback}
 	class:compact