fix(frontend): make infinite scroll viewport-relative, stop eager-loading

Lazy load fetched the entire list at once: every list's loader had a
"fill the viewport" recursion gated on
scrollContainer.scrollHeight <= clientHeight, but <main> is not the
scroller (the window/body is), so that condition is always true and it
recursed through every page (with a 10-item window, ~all pages fired at
once).

Move the filling logic into InfiniteScroll and base it on the sentinel's
viewport rect instead: load while the sentinel is within 300px of the
viewport bottom, re-checked synchronously after each load. This works
regardless of which element scrolls and loads only enough pages to reach
past the viewport.

Drop the per-page recursion (and now-unused scrollContainer refs / tick
imports) from the files, trash, tags, categories and pools lists.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/components/common/InfiniteScroll.svelte

@@ -7,23 +7,44 @@
 
 	let { loading = false, hasMore = true, onLoadMore }: Props = $props();
 
+	// Lookahead distance below the viewport at which we start loading.
+	const MARGIN = 300;
+
 	let sentinel = $state<HTMLDivElement | undefined>();
 
-	$effect(() => {
-		if (!sentinel) return;
-
-		const observer = new IntersectionObserver(
-			(entries) => {
-				if (entries[0].isIntersecting && !loading && hasMore) {
+	// Fire onLoadMore while the sentinel is within MARGIN px of the viewport
+	// bottom. Measuring the sentinel's viewport rect (rather than a scroll
+	// container's scrollHeight/clientHeight) makes this correct whether the page
+	// scrolls on <main> or on the window — and it loads exactly enough pages to
+	// reach past the viewport, instead of eagerly loading everything.
+	function maybeLoad() {
+		if (loading || !hasMore || !sentinel) return;
+		const rect = sentinel.getBoundingClientRect();
+		if (rect.top <= window.innerHeight + MARGIN) {
 			onLoadMore();
 		}
-			},
-			{ rootMargin: '300px' },
-		);
+	}
 
+	// Load on scroll: the observer notifies us when the sentinel nears the viewport.
+	$effect(() => {
+		if (!sentinel) return;
+		const observer = new IntersectionObserver(
+			(entries) => {
+				if (entries[0].isIntersecting) maybeLoad();
+			},
+			{ rootMargin: `${MARGIN}px` },
+		);
 		observer.observe(sentinel);
 		return () => observer.disconnect();
 	});
+
+	// After each load settles (loading → false), re-check synchronously: if the
+	// freshly appended content still didn't push the sentinel past the viewport,
+	// load again. This fills short pages without the throttled observer lagging
+	// and over-fetching.
+	$effect(() => {
+		if (!loading) maybeLoad();
+	});
 </script>
 
 <div bind:this={sentinel} class="sentinel" aria-hidden="true"></div>

frontend/src/routes/categories/+page.svelte

@@ -3,7 +3,6 @@
 	import { api, ApiError } from '$lib/api/client';
 	import { categorySorting, type CategorySortField } from '$lib/stores/sorting';
 	import InfiniteScroll from '$lib/components/common/InfiniteScroll.svelte';
-	import { tick } from 'svelte';
 	import type { Category, CategoryOffsetPage } from '$lib/api/types';
 
 	const LIMIT = 100;
@@ -15,7 +14,6 @@
 	];
 
 	let categories = $state<Category[]>([]);
-	let scrollContainer = $state<HTMLElement | undefined>();
 	let total = $state(0);
 	let offset = $state(0);
 	let loading = $state(false);
@@ -64,12 +62,6 @@
 			loading = false;
 			initialLoaded = true;
 		}
-		// Keep loading until the content fills the viewport so the infinite-scroll
-		// sentinel ends up below the fold; then stop.
-		await tick();
-		if (hasMore && scrollContainer && scrollContainer.scrollHeight <= scrollContainer.clientHeight) {
-			void load();
-		}
 	}
 
 	let hasMore = $derived(categories.length < total);
@@ -134,7 +126,7 @@
 		</div>
 	</div>
 
-	<main bind:this={scrollContainer}>
+	<main>
 		{#if error}
 			<p class="error" role="alert">{error}</p>
 		{/if}

frontend/src/routes/files/+page.svelte

@@ -203,12 +203,9 @@
 		} finally {
 			loading = false;
 		}
-		// If the loaded content doesn't fill the viewport yet (no scrollbar),
-		// keep loading until it does or there's nothing left.
-		await tick();
-		if (hasMore && scrollContainer && scrollContainer.scrollHeight <= scrollContainer.clientHeight) {
-			void loadMore();
-		}
+		// Viewport filling is handled by InfiniteScroll, which re-checks after each
+		// load — no manual recursion (which over-fetched here because <main> isn't
+		// the scroller, so its scrollHeight never exceeds its clientHeight).
 	}
 
 	function applyFilter(filter: string | null) {

frontend/src/routes/files/trash/+page.svelte

@@ -1,7 +1,6 @@
 <script lang="ts">
 	import { goto } from '$app/navigation';
 	import { api, ApiError } from '$lib/api/client';
-	import { tick } from 'svelte';
 	import FileCard from '$lib/components/file/FileCard.svelte';
 	import InfiniteScroll from '$lib/components/common/InfiniteScroll.svelte';
 	import ConfirmDialog from '$lib/components/common/ConfirmDialog.svelte';
@@ -9,8 +8,6 @@
 	import { appSettings } from '$lib/stores/appSettings';
 	import type { File, FileCursorPage } from '$lib/api/types';
 
-	let scrollContainer = $state<HTMLElement | undefined>();
-
 	let LIMIT = $derived($appSettings.fileLoadLimit);
 
 	let files = $state<File[]>([]);
@@ -47,10 +44,6 @@
 			loading = false;
 			initialLoaded = true;
 		}
-		await tick();
-		if (hasMore && scrollContainer && scrollContainer.scrollHeight <= scrollContainer.clientHeight) {
-			void loadMore();
-		}
 	}
 
 	// ---- Selection ----
@@ -165,7 +158,7 @@
 		</button>
 	</header>
 
-	<main bind:this={scrollContainer}>
+	<main>
 		{#if error}
 			<p class="error" role="alert">{error}</p>
 		{/if}

frontend/src/routes/pools/+page.svelte

@@ -3,7 +3,6 @@
 	import { api, ApiError } from '$lib/api/client';
 	import { poolSorting, type PoolSortField } from '$lib/stores/sorting';
 	import InfiniteScroll from '$lib/components/common/InfiniteScroll.svelte';
-	import { tick } from 'svelte';
 	import type { Pool, PoolOffsetPage } from '$lib/api/types';
 
 	const LIMIT = 50;
@@ -14,7 +13,6 @@
 	];
 
 	let pools = $state<Pool[]>([]);
-	let scrollContainer = $state<HTMLElement | undefined>();
 	let total = $state(0);
 	let offset = $state(0);
 	let loading = $state(false);
@@ -63,12 +61,6 @@
 			loading = false;
 			initialLoaded = true;
 		}
-		// Keep loading until the content fills the viewport so the infinite-scroll
-		// sentinel ends up below the fold; then stop.
-		await tick();
-		if (hasMore && scrollContainer && scrollContainer.scrollHeight <= scrollContainer.clientHeight) {
-			void load();
-		}
 	}
 
 	let hasMore = $derived(pools.length < total);
@@ -137,7 +129,7 @@
 		</div>
 	</div>
 
-	<main bind:this={scrollContainer}>
+	<main>
 		{#if error}
 			<p class="error" role="alert">{error}</p>
 		{/if}

frontend/src/routes/tags/+page.svelte

@@ -4,7 +4,6 @@
 	import { tagSorting, type TagSortField } from '$lib/stores/sorting';
 	import TagBadge from '$lib/components/tag/TagBadge.svelte';
 	import InfiniteScroll from '$lib/components/common/InfiniteScroll.svelte';
-	import { tick } from 'svelte';
 	import type { Tag, TagOffsetPage } from '$lib/api/types';
 
 	const LIMIT = 100;
@@ -17,7 +16,6 @@
 	];
 
 	let tags = $state<Tag[]>([]);
-	let scrollContainer = $state<HTMLElement | undefined>();
 	let total = $state(0);
 	let offset = $state(0);
 	let loading = $state(false);
@@ -69,12 +67,6 @@
 			loading = false;
 			initialLoaded = true;
 		}
-		// Keep loading until the content fills the viewport so the infinite-scroll
-		// sentinel ends up below the fold; then stop.
-		await tick();
-		if (hasMore && scrollContainer && scrollContainer.scrollHeight <= scrollContainer.clientHeight) {
-			void load();
-		}
 	}
 
 	function onSearch(e: Event) {
@@ -145,7 +137,7 @@
 		</div>
 	</div>
 
-	<main bind:this={scrollContainer}>
+	<main>
 		{#if error}
 			<p class="error" role="alert">{error}</p>
 		{/if}