feat(VirtualList): add onJump callback for scroll-beyond-loaded detection

src/shared/ui/VirtualList/VirtualList.svelte

@@ -62,6 +62,10 @@ interface Props extends
      * Near bottom callback
      */
     onNearBottom?: (lastVisibleIndex: number) => void;
+    /**
+     * Fires when scroll position exceeds loaded content — user jumped beyond data
+     */
+    onJump?: (targetIndex: number) => void;
     /**
      * Item render snippet
      */
@@ -95,6 +99,7 @@ let {
     class: className,
     onVisibleItemsChange,
     onNearBottom,
+    onJump,
     children,
     useWindowScroll = false,
     isLoading = false,
@@ -170,6 +175,10 @@ const throttledNearBottom = throttle((lastVisibleIndex: number) => {
     onNearBottom?.(lastVisibleIndex);
 }, 200); // 200ms throttle
 
+const throttledOnJump = throttle((targetIndex: number) => {
+    onJump?.(targetIndex);
+}, 200);
+
 // Calculate top/bottom padding for spacer elements
 // In CSS Grid, gap creates space BETWEEN elements.
 // The top spacer should place the first row at its virtual offset.
@@ -227,6 +236,26 @@ $effect(() => {
         }
     }
 });
+
+$effect(() => {
+    // Fire onJump when scroll is beyond the loaded content boundary.
+    // Target index estimates which item the user scrolled to.
+    if (!onJump || !virtualizer.containerHeight || virtualizer.scrollOffset <= 0) {
+        return;
+    }
+
+    const isAhead = virtualizer.scrollOffset > virtualizer.totalSize;
+    if (!isAhead) {
+        return;
+    }
+
+    const estimatedItemHeight = typeof itemHeight === 'number' ? itemHeight : 80;
+    // Include visible rows + overscan so the bottom of the viewport is fully covered
+    const topItemIndex = Math.floor(virtualizer.scrollOffset / estimatedItemHeight) * columns;
+    const visibleRows = Math.ceil(virtualizer.containerHeight / estimatedItemHeight);
+    const targetIndex = topItemIndex + (visibleRows + overscan) * columns;
+    throttledOnJump(targetIndex);
+});
 </script>
 
 {#snippet content()}