perf: task-fix-list-virtualization changes

src/lib/components/VirtualItem.svelte

@@ -0,0 +1,56 @@
+<script lang="ts">
+  import type {Snippet} from "svelte"
+
+  let {
+    children,
+    root,
+    initiallyVisible = false,
+    estimatedHeight = 48,
+  }: {
+    children: Snippet
+    root?: HTMLElement
+    initiallyVisible?: boolean
+    estimatedHeight?: number
+  } = $props()
+
+  let visible = $state(initiallyVisible)
+  let height = $state(estimatedHeight)
+  let el: HTMLElement | undefined = $state()
+  let hasMeasured = false
+
+  $effect(() => {
+    if (!el) return
+
+    const observer = new IntersectionObserver(
+      ([entry]) => {
+        if (entry.isIntersecting) {
+          visible = true
+        } else {
+          // Measure actual height before hiding content
+          if (el) {
+            const h = el.offsetHeight
+            if (h > 0) {
+              height = h
+              hasMeasured = true
+            }
+          }
+          if (hasMeasured) {
+            visible = false
+          }
+        }
+      },
+      {root: root || null, rootMargin: "1000px 0px"},
+    )
+
+    observer.observe(el)
+    return () => observer.disconnect()
+  })
+</script>
+
+<div bind:this={el}>
+  {#if visible}
+    {@render children()}
+  {:else}
+    <div style:height="{height}px"></div>
+  {/if}
+</div>