feat(VirtualList): add different layout support

src/shared/ui/VirtualList/VirtualList.svelte

@@ -51,6 +51,16 @@ interface Props {
      * (follows shadcn convention for className prop)
      */
     class?: string;
+    /**
+     * Number of columns for grid layout.
+     * @default 1
+     */
+    columns?: number;
+    /**
+     * Gap between items in pixels.
+     * @default 0
+     */
+    gap?: number;
     /**
      * An optional callback that will be called for each new set of loaded items
      * @param items - Loaded items
@@ -131,18 +141,26 @@ let {
     children,
     useWindowScroll = false,
     isLoading = false,
+    columns = 1,
+    gap = 0,
 }: Props = $props();
 
 // Reference to the scroll container element for attaching the virtualizer
 let viewportRef = $state<HTMLElement | null>(null);
 
+// Calculate row-based counts for grid layout
+const rowCount = $derived(Math.ceil(items.length / columns));
+const totalRows = $derived(Math.ceil(total / columns));
+
 // Use items.length for count to keep existing item positions stable
 // But calculate a separate totalSize for scrollbar that accounts for unloaded items
 const virtualizer = createVirtualizer(() => ({
     // Only virtualize loaded items - this keeps positions stable when new items load
-    count: items.length,
+    count: rowCount,
     data: items,
-    estimateSize: typeof itemHeight === 'function' ? itemHeight : () => itemHeight,
+    estimateSize: typeof itemHeight === 'function'
+        ? (index: number) => itemHeight(index) + gap
+        : () => itemHeight + gap,
     overscan,
     useWindowScroll,
 }));
@@ -150,25 +168,26 @@ const virtualizer = createVirtualizer(() => ({
 // Calculate total size including unloaded items for proper scrollbar sizing
 // Use estimateSize() for items that haven't been loaded yet
 const estimatedTotalSize = $derived.by(() => {
-    if (total === items.length) {
+    if (totalRows === rowCount) {
         // No unloaded items, use virtualizer's totalSize
         return virtualizer.totalSize;
     }
 
-    // Start with the virtualized (loaded) items size
+    // Start with the virtualized (loaded) rows size
     const loadedSize = virtualizer.totalSize;
 
-    // Add estimated size for unloaded items
+    // Add estimated size for unloaded rows
-    const unloadedCount = total - items.length;
+    const unloadedRows = totalRows - rowCount;
-    if (unloadedCount <= 0) return loadedSize;
+    if (unloadedRows <= 0) return loadedSize;
 
-    // Estimate the size of unloaded items
+    // Estimate the size of unloaded rows
-    // Get the average size of loaded items, or use the estimateSize function
+    const estimateFn = typeof itemHeight === 'function'
-    const estimateFn = typeof itemHeight === 'function' ? itemHeight : () => itemHeight;
+        ? (index: number) => itemHeight(index * columns) + gap
+        : () => itemHeight + gap;
 
-    // Use estimateSize for unloaded items (index from items.length to total - 1)
+    // Use estimateSize for unloaded rows (index from rowCount to totalRows - 1)
     let unloadedSize = 0;
-    for (let i = items.length; i < total; i++) {
+    for (let i = rowCount; i < totalRows; i++) {
         unloadedSize += estimateFn(i);
     }
 
@@ -191,8 +210,31 @@ const throttledNearBottom = throttle((lastVisibleIndex: number) => {
     onNearBottom?.(lastVisibleIndex);
 }, 200); // 200ms debounce
 
+// Calculate top/bottom padding for spacer elements
+const topPad = $derived(
+    virtualizer.items.length > 0 ? virtualizer.items[0].start - gap : 0,
+);
+const botPad = $derived(
+    virtualizer.items.length > 0
+        ? Math.max(
+            0,
+            estimatedTotalSize
+                - (virtualizer.items[virtualizer.items.length - 1].end + gap),
+        )
+        : estimatedTotalSize,
+);
+
 $effect(() => {
-    const visibleItems = virtualizer.items.map(item => items[item.index]);
+    // Expand row indices to item indices
+    const visibleItemIndices: number[] = [];
+    for (const row of virtualizer.items) {
+        const startItemIndex = row.index * columns;
+        const endItemIndex = Math.min(startItemIndex + columns, items.length);
+        for (let i = startItemIndex; i < endItemIndex; i++) {
+            visibleItemIndices.push(i);
+        }
+    }
+    const visibleItems = visibleItemIndices.map(index => items[index]);
     throttledVisibleChange(visibleItems);
 });
 
@@ -200,41 +242,87 @@ $effect(() => {
     // Trigger onNearBottom when user scrolls near the end of loaded items (within 5 items)
     // Only trigger if container has sufficient height to avoid false positives
     if (virtualizer.items.length > 0 && onNearBottom && virtualizer.containerHeight > 100) {
-        const lastVisibleItem = virtualizer.items[virtualizer.items.length - 1];
+        const lastVisibleRow = virtualizer.items[virtualizer.items.length - 1];
+        // Convert row index to last item index in that row
+        const lastVisibleItemIndex = Math.min(
+            (lastVisibleRow.index + 1) * columns - 1,
+            items.length - 1,
+        );
         // Compare against loaded items length, not total
-        const itemsRemaining = items.length - lastVisibleItem.index;
+        const itemsRemaining = items.length - lastVisibleItemIndex;
 
         if (itemsRemaining <= 5) {
-            throttledNearBottom(lastVisibleItem.index);
+            throttledNearBottom(lastVisibleItemIndex);
         }
     }
 });
 </script>
 
+{#snippet content()}
+    <div
+        style:grid-template-columns="repeat({columns}, minmax(0, 1fr))"
+        class="grid w-full"
+    >
+        <!-- Top spacer for padding-based virtualization -->
+        <div
+            style:grid-column="1 / -1"
+            style:height="{topPad}px"
+            class="shrink-0"
+        >
+        </div>
+
+        {#each virtualizer.items as row (row.key)}
+            {#if row.index < rowCount}
+                {@const startItemIndex = row.index * columns}
+                {@const endItemIndex = Math.min(startItemIndex + columns, items.length)}
+                {#each Array.from({ length: endItemIndex - startItemIndex }) as _, colIndex (startItemIndex + colIndex)}
+                    {@const itemIndex = startItemIndex + colIndex}
+                    {#if colIndex === 0}
+                        <div
+                            use:virtualizer.measureElement
+                            data-index={row.index}
+                            class="min-h-0"
+                        >
+                            {#if itemIndex < items.length}
+                                {@render children({
+    item: items[itemIndex],
+    index: itemIndex,
+    isFullyVisible: row.isFullyVisible,
+    isPartiallyVisible: row.isPartiallyVisible,
+    proximity: row.proximity,
+})}
+                            {/if}
+                        </div>
+                    {:else}
+                        <div class="min-h-0">
+                            {#if itemIndex < items.length}
+                                {@render children({
+    item: items[itemIndex],
+    index: itemIndex,
+    isFullyVisible: row.isFullyVisible,
+    isPartiallyVisible: row.isPartiallyVisible,
+    proximity: row.proximity,
+})}
+                            {/if}
+                        </div>
+                    {/if}
+                {/each}
+            {/if}
+        {/each}
+
+        <!-- Bottom spacer for padding-based virtualization -->
+        <div
+            style:grid-column="1 / -1"
+            style:height="{botPad}px"
+            class="shrink-0"
+        >
+        </div>
+    </div>
+{/snippet}
+
 {#if useWindowScroll}
     <div class={cn('relative w-full', className)} bind:this={viewportRef}>
-        <div style:height="{estimatedTotalSize}px" class="relative w-full">
+        {@render content()}
-            {#each virtualizer.items as item (item.key)}
-                <div
-                    use:virtualizer.measureElement
-                    data-index={item.index}
-                    class="absolute top-0 left-0 w-full will-change-transform"
-                    style:transform="translateY({item.start}px)"
-                    data-lenis-prevent
-                >
-                    {#if item.index < items.length}
-                        {@render children({
-    // TODO: Fix indentation rule for this case
-    item: items[item.index],
-    index: item.index,
-    isFullyVisible: item.isFullyVisible,
-    isPartiallyVisible: item.isPartiallyVisible,
-    proximity: item.proximity,
-})}
-                    {/if}
-                </div>
-            {/each}
-        </div>
     </div>
 {:else}
     <div
@@ -246,26 +334,6 @@ $effect(() => {
             className,
         )}
     >
-        <div style:height="{estimatedTotalSize}px" class="relative w-full">
+        {@render content()}
-            {#each virtualizer.items as item (item.key)}
-                <div
-                    use:virtualizer.measureElement
-                    data-index={item.index}
-                    class="absolute top-0 left-0 w-full will-change-transform"
-                    style:transform="translateY({item.start}px)"
-                >
-                    {#if item.index < items.length}
-                        {@render children({
-    // TODO: Fix indentation rule for this case
-    item: items[item.index],
-    index: item.index,
-    isFullyVisible: item.isFullyVisible,
-    isPartiallyVisible: item.isPartiallyVisible,
-    proximity: item.proximity,
-})}
-                    {/if}
-                </div>
-            {/each}
-        </div>
     </div>
 {/if}