From 4d57f2084c461b34ce9cc0bb354db9e0742615c3 Mon Sep 17 00:00:00 2001
From: Ilia Mashkov <hikkikomori7@gmail.com>
Date: Mon, 16 Feb 2026 14:15:19 +0300
Subject: [PATCH] feat(VirtualList): add estimated total size calculation

---
 src/shared/ui/VirtualList/VirtualList.svelte | 59 +++++++++++++++-----
 1 file changed, 45 insertions(+), 14 deletions(-)

diff --git a/src/shared/ui/VirtualList/VirtualList.svelte b/src/shared/ui/VirtualList/VirtualList.svelte
index 71d066a..0c42845 100644
--- a/src/shared/ui/VirtualList/VirtualList.svelte
+++ b/src/shared/ui/VirtualList/VirtualList.svelte
@@ -6,12 +6,11 @@
     - Keyboard navigation (ArrowUp/Down, Home, End)
     - Fixed or dynamic item heights
     - ARIA listbox/option pattern with single tab stop
-    - Custom shadcn ScrollArea scrollbar
+    - Native browser scroll
 -->
 <script lang="ts" generics="T">
 import { createVirtualizer } from '$shared/lib';
 import { throttle } from '$shared/lib/utils';
-import { ScrollArea } from '$shared/shadcn/ui/scroll-area';
 import { cn } from '$shared/shadcn/utils/shadcn-utils';
 import type { Snippet } from 'svelte';
 import { flip } from 'svelte/animate';
@@ -136,10 +135,13 @@ let {
     isLoading = false,
 }: Props = $props();
 
-// Reference to the ScrollArea viewport element for attaching the virtualizer
+// Reference to the scroll container element for attaching the virtualizer
 let viewportRef = $state<HTMLElement | null>(null);
 
+// 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,
     data: items,
     estimateSize: typeof itemHeight === 'function' ? itemHeight : () => itemHeight,
@@ -147,6 +149,34 @@ const virtualizer = createVirtualizer(() => ({
     useWindowScroll,
 }));
 
+// 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) {
+        // No unloaded items, use virtualizer's totalSize
+        return virtualizer.totalSize;
+    }
+
+    // Start with the virtualized (loaded) items size
+    const loadedSize = virtualizer.totalSize;
+
+    // Add estimated size for unloaded items
+    const unloadedCount = total - items.length;
+    if (unloadedCount <= 0) return loadedSize;
+
+    // Estimate the size of unloaded items
+    // Get the average size of loaded items, or use the estimateSize function
+    const estimateFn = typeof itemHeight === 'function' ? itemHeight : () => itemHeight;
+
+    // Use estimateSize for unloaded items (index from items.length to total - 1)
+    let unloadedSize = 0;
+    for (let i = items.length; i < total; i++) {
+        unloadedSize += estimateFn(i);
+    }
+
+    return loadedSize + unloadedSize;
+});
+
 // Attach virtualizer.container action to the viewport when it becomes available
 $effect(() => {
     if (viewportRef) {
@@ -170,7 +200,8 @@ $effect(() => {
 
 $effect(() => {
     // Trigger onNearBottom when user scrolls near the end of loaded items (within 5 items)
-    if (virtualizer.items.length > 0 && onNearBottom) {
+    // 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];
         // Compare against loaded items length, not total
         const itemsRemaining = items.length - lastVisibleItem.index;
@@ -184,7 +215,7 @@ $effect(() => {
 
 {#if useWindowScroll}
     <div class={cn('relative w-full', className)} bind:this={viewportRef}>
-        <div style:height="{virtualizer.totalSize}px" class="relative w-full">
+        <div style:height="{estimatedTotalSize}px" class="relative w-full">
             {#each virtualizer.items as item (item.key)}
                 <div
                     use:virtualizer.measureElement
@@ -195,7 +226,7 @@ $effect(() => {
                 >
                     {#if item.index < items.length}
                         {@render children({
-    // TODO: Fix indenation rule for this case
+    // TODO: Fix indentation rule for this case
     item: items[item.index],
     index: item.index,
     isFullyVisible: item.isFullyVisible,
@@ -208,16 +239,16 @@ $effect(() => {
         </div>
     </div>
 {:else}
-    <ScrollArea
-        bind:viewportRef
+    <div
+        bind:this={viewportRef}
         class={cn(
-            'relative rounded-md bg-background',
-            'h-150 w-full',
+            'relative overflow-y-auto overflow-x-hidden',
+            'rounded-md bg-background',
+            'w-full min-h-[200px]',
             className,
         )}
-        orientation="vertical"
     >
-        <div style:height="{virtualizer.totalSize}px" class="relative w-full">
+        <div style:height="{estimatedTotalSize}px" class="relative w-full">
             {#each virtualizer.items as item (item.key)}
                 <div
                     use:virtualizer.measureElement
@@ -227,7 +258,7 @@ $effect(() => {
                 >
                     {#if item.index < items.length}
                         {@render children({
-    // TODO: Fix indenation rule for this case
+    // TODO: Fix indentation rule for this case
     item: items[item.index],
     index: item.index,
     isFullyVisible: item.isFullyVisible,
@@ -238,5 +269,5 @@ $effect(() => {
                 </div>
             {/each}
         </div>
-    </ScrollArea>
+    </div>
 {/if}