feat(Section): add logic that triggers a callback when sections title moves out of the viewport

src/shared/ui/Section/Section.svelte

@@ -29,15 +29,53 @@ interface Props extends Omit<HTMLAttributes<HTMLElement>, 'title'> {
      * Index of the section
      */
     index?: number;
+    /**
+     * Callback function to notify when the title visibility status changes
+     *
+     * @param index - Index of the section
+     * @param isPast - Whether the section is past the current scroll position
+     * @param title - Snippet for a title itself
+     * @returns Cleanup callback
+     */
+    onTitleStatusChange?: (index: number, isPast: boolean, title?: Snippet<[{ className?: string }]>) => () => void;
     /**
      * Snippet for the section content
      */
     children?: Snippet;
 }
 
-const { class: className, title, icon, index, children }: Props = $props();
+const { class: className, title, icon, index = 0, onTitleStatusChange, children }: Props = $props();
 
+let titleContainer = $state<HTMLElement>();
 const flyParams: FlyParams = { y: 0, x: -50, duration: 300, easing: cubicOut, opacity: 0.2 };
+
+// Track if the user has actually scrolled away from view
+let isScrolledPast = $state(false);
+
+$effect(() => {
+    if (!titleContainer) {
+        return;
+    }
+    let cleanup: ((index: number) => void) | undefined;
+    const observer = new IntersectionObserver(entries => {
+        const entry = entries[0];
+        const isPast = !entry.isIntersecting && entry.boundingClientRect.top < 0;
+
+        if (isPast !== isScrolledPast) {
+            isScrolledPast = isPast;
+            cleanup = onTitleStatusChange?.(index, isPast, title);
+        }
+    }, {
+        // Set threshold to 0 to trigger exactly when the last pixel leaves
+        threshold: 0,
+    });
+
+    observer.observe(titleContainer);
+    return () => {
+        observer.disconnect();
+        cleanup?.(index);
+    };
+});
 </script>
 
 <section
@@ -48,7 +86,7 @@ const flyParams: FlyParams = { y: 0, x: -50, duration: 300, easing: cubicOut, op
     in:fly={flyParams}
     out:fly={flyParams}
 >
-    <div class="flex flex-col gap-2">
+    <div class="flex flex-col gap-2" bind:this={titleContainer}>
         <div class="flex items-center gap-3 opacity-60">
             {#if icon}
                 {@render icon({ className: 'size-4 stroke-gray-900 stroke-1' })}