fix(ComparisonSlider): change the way width is calculated to avoid transform:scale issues

src/entities/Font/model/store/appliedFontsStore/appliedFontsStore.svelte.ts

@@ -1,67 +1,109 @@
 import { SvelteMap } from 'svelte/reactivity';
 
+/** Loading state of a font. Failed loads may be retried up to MAX_RETRIES. */
 export type FontStatus = 'loading' | 'loaded' | 'error';
 
+/** Configuration for a font load request. */
 export interface FontConfigRequest {
     /**
-     * Font id
+     * Unique identifier for the font (e.g., "lato", "roboto").
      */
     id: string;
     /**
-     * Real font name (e.g. "Lato")
+     * Actual font family name recognized by the browser (e.g., "Lato", "Roboto").
      */
     name: string;
     /**
-     * The .ttf URL
+     * URL pointing to the font file (typically .ttf or .woff2).
      */
     url: string;
     /**
-     * Font weight
+     * Numeric weight (100-900). Variable fonts load once per ID regardless of weight.
      */
     weight: number;
     /**
-     * Flag of the variable weight
+     * Variable fonts load once per ID; static fonts load per weight.
      */
     isVariable?: boolean;
 }
 
 /**
- * Manager that handles loading of fonts.
- * Logic:
- * - Variable fonts: Loaded once per id (covers all weights).
- * - Static fonts: Loaded per id + weight combination.
+ * Manages web font loading with caching, adaptive concurrency, and automatic cleanup.
+ *
+ * **Two-Phase Loading Strategy:**
+ * 1. *Concurrent Fetching*: Font files fetched in parallel (network I/O is non-blocking)
+ * 2. *Sequential Parsing*: Buffers parsed into FontFace objects one at a time with periodic yields
+ *
+ * **Yielding Strategy:**
+ * - Chromium: Yields only when user input is pending (via `scheduler.yield()` + `isInputPending()`)
+ * - Others: Time-based fallback, yields every 8ms
+ *
+ * **Network Adaptation:**
+ * - 2G: 1 concurrent request, 3G: 2, 4G+: 4 (via Network Information API)
+ * - Respects `saveData` mode to defer non-critical weights
+ *
+ * **Cache Integration:** Cache API with best-effort fallback (handles private browsing, quota limits)
+ *
+ * **Cleanup:** LRU-style eviction after 5 minutes of inactivity; cleanup runs every 60 seconds
+ *
+ * **Font Identity:** Variable fonts use `{id}@vf`, static fonts use `{id}@{weight}`
+ *
+ * **Browser APIs Used:** `scheduler.yield()`, `isInputPending()`, `requestIdleCallback`, Cache API, Network Information API
  */
 export class AppliedFontsManager {
-    // Stores the actual FontFace objects for cleanup
+    // Loaded FontFace instances registered with document.fonts. Key: `{id}@{weight}` or `{id}@vf`
     #loadedFonts = new Map<string, FontFace>();
-    // Optimization: Map<batchId, Set<fontKeys>> to avoid O(N^2) scans
-    #batchToKeys = new Map<string, Set<string>>();
-    // Optimization: Map<fontKey, batchId> for reverse lookup
-    #keyToBatch = new Map<string, string>();
 
+    // Last-used timestamps for LRU cleanup. Key: `{id}@{weight}` or `{id}@vf`, Value: unix timestamp (ms)
     #usageTracker = new Map<string, number>();
+
+    // Fonts queued for loading by `touch()`, processed by `#processQueue()`
     #queue = new Map<string, FontConfigRequest>();
+
+    // Handle for scheduled queue processing (requestIdleCallback or setTimeout)
     #timeoutId: ReturnType<typeof setTimeout> | null = null;
 
-    readonly #PURGE_INTERVAL = 60000;
-    readonly #TTL = 5 * 60 * 1000;
-    readonly #CHUNK_SIZE = 5;
+    // Interval handle for periodic cleanup (runs every PURGE_INTERVAL)
+    #intervalId: ReturnType<typeof setInterval> | null = null;
 
+    // AbortController for canceling in-flight fetches on destroy
+    #abortController = new AbortController();
+
+    // Tracks which callback type is pending ('idle' | 'timeout' | null) for proper cancellation
+    #pendingType: 'idle' | 'timeout' | null = null;
+
+    // Retry counts for failed loads; fonts exceeding MAX_RETRIES are permanently skipped
+    #retryCounts = new Map<string, number>();
+
+    readonly #MAX_RETRIES = 3;
+    readonly #PURGE_INTERVAL = 60000; // 60 seconds
+    readonly #TTL = 5 * 60 * 1000; // 5 minutes
+    readonly #CACHE_NAME = 'font-cache-v1'; // Versioned for future invalidation
+
+    // Reactive status map for Svelte components to track font states
     statuses = new SvelteMap<string, FontStatus>();
 
+    // Starts periodic cleanup timer (browser-only).
     constructor() {
         if (typeof window !== 'undefined') {
-            // Using a weak reference style approach isn't possible for DOM,
-            // so we stick to the interval but make it highly efficient.
-            setInterval(() => this.#purgeUnused(), this.#PURGE_INTERVAL);
+            this.#intervalId = setInterval(() => this.#purgeUnused(), this.#PURGE_INTERVAL);
         }
     }
 
+    // Generates font key: `{id}@vf` for variable, `{id}@{weight}` for static.
     #getFontKey(id: string, weight: number, isVariable: boolean): string {
         return isVariable ? `${id.toLowerCase()}@vf` : `${id.toLowerCase()}@${weight}`;
     }
 
+    /**
+     * Requests fonts to be loaded. Updates usage tracking and queues new fonts.
+     *
+     * Retry behavior: 'loaded' and 'loading' fonts are skipped; 'error' fonts retry if count < MAX_RETRIES.
+     * Scheduling: Prefers requestIdleCallback (150ms timeout), falls back to setTimeout(16ms).
+     */
     touch(configs: FontConfigRequest[]) {
+        if (this.#abortController.signal.aborted) return;
+
         const now = Date.now();
         let hasNewItems = false;
 
@@ -69,105 +111,244 @@ export class AppliedFontsManager {
             const key = this.#getFontKey(config.id, config.weight, !!config.isVariable);
             this.#usageTracker.set(key, now);
 
-            if (this.statuses.get(key) === 'loaded' || this.statuses.get(key) === 'loading' || this.#queue.has(key)) {
-                continue;
-            }
+            const status = this.statuses.get(key);
+            if (status === 'loaded' || status === 'loading' || this.#queue.has(key)) continue;
+            if (status === 'error' && (this.#retryCounts.get(key) ?? 0) >= this.#MAX_RETRIES) continue;
 
             this.#queue.set(key, config);
             hasNewItems = true;
         }
 
-        // IMPROVEMENT: Only trigger timer if not already pending
         if (hasNewItems && !this.#timeoutId) {
-            this.#timeoutId = setTimeout(() => this.#processQueue(), 16); // ~1 frame delay
+            if (typeof requestIdleCallback !== 'undefined') {
+                this.#timeoutId = requestIdleCallback(
+                    () => this.#processQueue(),
+                    { timeout: 150 },
+                ) as unknown as ReturnType<typeof setTimeout>;
+                this.#pendingType = 'idle';
+            } else {
+                this.#timeoutId = setTimeout(() => this.#processQueue(), 16);
+                this.#pendingType = 'timeout';
+            }
         }
     }
 
-    #processQueue() {
+    /** Yields to main thread during CPU-intensive parsing. Uses scheduler.yield() (Chrome/Edge) or MessageChannel fallback. */
+    async #yieldToMain(): Promise<void> {
+        // @ts-expect-error - scheduler not in TypeScript lib yet
+        if (typeof scheduler !== 'undefined' && 'yield' in scheduler) {
+            // @ts-expect-error - scheduler.yield not in TypeScript lib yet
+            await scheduler.yield();
+        } else {
+            await new Promise<void>(resolve => {
+                const ch = new MessageChannel();
+                ch.port1.onmessage = () => resolve();
+                ch.port2.postMessage(null);
+            });
+        }
+    }
+
+    /** Returns optimal concurrent fetches based on Network Information API: 1 for 2G, 2 for 3G, 4 for 4G/default. */
+    #getEffectiveConcurrency(): number {
+        const nav = navigator as any;
+        const conn = nav.connection;
+        if (!conn) return 4;
+
+        switch (conn.effectiveType) {
+            case 'slow-2g':
+            case '2g':
+                return 1;
+            case '3g':
+                return 2;
+            default:
+                return 4;
+        }
+    }
+
+    /** Returns true if data-saver mode is enabled (defers non-critical weights). */
+    #shouldDeferNonCritical(): boolean {
+        const nav = navigator as any;
+        return nav.connection?.saveData === true;
+    }
+
+    /**
+     * Processes queued fonts in two phases:
+     * 1. Concurrent fetching (network I/O, non-blocking)
+     * 2. Sequential parsing with periodic yields (CPU-intensive, can block UI)
+     *
+     * Yielding: Chromium uses `isInputPending()` for optimal responsiveness; others yield every 8ms.
+     */
+    async #processQueue() {
         this.#timeoutId = null;
-        const entries = Array.from(this.#queue.entries());
-        if (entries.length === 0) return;
+        this.#pendingType = null;
 
+        let entries = Array.from(this.#queue.entries());
+        if (!entries.length) return;
         this.#queue.clear();
 
-        // Process in chunks to keep the UI responsive
-        for (let i = 0; i < entries.length; i += this.#CHUNK_SIZE) {
-            this.#applyBatch(entries.slice(i, i + this.#CHUNK_SIZE));
-        }
+        if (this.#shouldDeferNonCritical()) {
+            entries = entries.filter(([, c]) => c.isVariable || [400, 700].includes(c.weight));
         }
 
-    async #applyBatch(batchEntries: [string, FontConfigRequest][]) {
-        if (typeof document === 'undefined') return;
+        // Phase 1: Concurrent fetching (I/O bound, non-blocking)
+        const concurrency = this.#getEffectiveConcurrency();
+        const buffers = new Map<string, ArrayBuffer>();
 
-        const batchId = crypto.randomUUID();
-        const keysInBatch = new Set<string>();
-
-        const loadPromises = batchEntries.map(([key, config]) => {
+        for (let i = 0; i < entries.length; i += concurrency) {
+            const chunk = entries.slice(i, i + concurrency);
+            const results = await Promise.allSettled(
+                chunk.map(async ([key, config]) => {
                     this.statuses.set(key, 'loading');
-            this.#keyToBatch.set(key, batchId);
-            keysInBatch.add(key);
+                    const buffer = await this.#fetchFontBuffer(
+                        config.url,
+                        this.#abortController.signal,
+                    );
+                    buffers.set(key, buffer);
+                }),
+            );
 
-            // Use a unique internal family name to prevent collisions
-            // while keeping the "real" name for the browser to resolve weight/style.
-            const internalName = `f_${config.id}`;
+            for (let j = 0; j < results.length; j++) {
+                if (results[j].status === 'rejected') {
+                    const [key, config] = chunk[j];
+                    console.error(`Font fetch failed: ${config.name}`, (results[j] as PromiseRejectedResult).reason);
+                    this.statuses.set(key, 'error');
+                    this.#retryCounts.set(key, (this.#retryCounts.get(key) ?? 0) + 1);
+                }
+            }
+        }
+
+        // Phase 2: Sequential parsing (CPU-intensive, yields periodically)
+        const hasInputPending = !!(navigator as any).scheduling?.isInputPending;
+        let lastYield = performance.now();
+        const YIELD_INTERVAL = 8; // ms
+
+        for (const [key, config] of entries) {
+            const buffer = buffers.get(key);
+            if (!buffer) continue;
+
+            try {
                 const weightRange = config.isVariable ? '100 900' : `${config.weight}`;
-
-            const font = new FontFace(config.name, `url(${config.url}) format('woff2')`, {
+                const font = new FontFace(config.name, buffer, {
                     weight: weightRange,
                     style: 'normal',
                     display: 'swap',
                 });
-
+                await font.load();
+                document.fonts.add(font);
                 this.#loadedFonts.set(key, font);
-
-            return font.load()
-                .then(loadedFace => {
-                    document.fonts.add(loadedFace);
                 this.statuses.set(key, 'loaded');
-                })
-                .catch(e => {
-                    console.error(`Font load failed: ${config.name}`, e);
+            } catch (e) {
+                if (e instanceof Error && e.name === 'AbortError') continue;
+                console.error(`Font parse failed: ${config.name}`, e);
                 this.statuses.set(key, 'error');
-                });
-        });
-
-        this.#batchToKeys.set(batchId, keysInBatch);
-        await Promise.allSettled(loadPromises);
+                this.#retryCounts.set(key, (this.#retryCounts.get(key) ?? 0) + 1);
             }
 
+            const shouldYield = hasInputPending
+                ? (navigator as any).scheduling.isInputPending({ includeContinuous: true })
+                : (performance.now() - lastYield > YIELD_INTERVAL);
+
+            if (shouldYield) {
+                await this.#yieldToMain();
+                lastYield = performance.now();
+            }
+        }
+    }
+
+    /**
+     * Fetches font with cache-aside pattern: checks Cache API first, falls back to network.
+     * Cache failures (private browsing, quota limits) are silently ignored.
+     */
+    async #fetchFontBuffer(url: string, signal?: AbortSignal): Promise<ArrayBuffer> {
+        try {
+            if (typeof caches !== 'undefined') {
+                const cache = await caches.open(this.#CACHE_NAME);
+                const cached = await cache.match(url);
+                if (cached) return cached.arrayBuffer();
+            }
+        } catch {
+            // Cache unavailable (private browsing, security restrictions) — fall through to network
+        }
+
+        const response = await fetch(url, { signal });
+        if (!response.ok) throw new Error(`HTTP ${response.status}`);
+
+        try {
+            if (typeof caches !== 'undefined') {
+                const cache = await caches.open(this.#CACHE_NAME);
+                await cache.put(url, response.clone());
+            }
+        } catch {
+            // Cache write failed (quota, storage pressure) — return font anyway
+        }
+
+        return response.arrayBuffer();
+    }
+
+    /** Removes fonts unused within TTL (LRU-style cleanup). Runs every PURGE_INTERVAL. */
     #purgeUnused() {
         const now = Date.now();
+        for (const [key, lastUsed] of this.#usageTracker) {
+            if (now - lastUsed < this.#TTL) continue;
 
-        // We iterate over batches, not individual fonts, to reduce loops
-        for (const [batchId, keys] of this.#batchToKeys.entries()) {
-            let canPurgeBatch = true;
-
-            for (const key of keys) {
-                const lastUsed = this.#usageTracker.get(key) || 0;
-                if (now - lastUsed < this.#TTL) {
-                    canPurgeBatch = false;
-                    break;
-                }
-            }
-
-            if (canPurgeBatch) {
-                keys.forEach(key => {
             const font = this.#loadedFonts.get(key);
             if (font) document.fonts.delete(font);
 
             this.#loadedFonts.delete(key);
-                    this.#keyToBatch.delete(key);
             this.#usageTracker.delete(key);
             this.statuses.delete(key);
-                });
-                this.#batchToKeys.delete(batchId);
-            }
+            this.#retryCounts.delete(key);
         }
     }
 
+    /** Returns current loading status for a font, or undefined if never requested. */
     getFontStatus(id: string, weight: number, isVariable = false) {
         return this.statuses.get(this.#getFontKey(id, weight, isVariable));
     }
+
+    /** Waits for all fonts to finish loading using document.fonts.ready. */
+    async ready(): Promise<void> {
+        if (typeof document === 'undefined') return;
+        try {
+            await document.fonts.ready;
+        } catch {
+            // document.fonts.ready can reject in some edge cases
+            // (e.g., document unloaded). Silently resolve.
+        }
+    }
+
+    /** Aborts all operations, removes fonts from document, and clears state. Manager cannot be reused after. */
+    destroy() {
+        this.#abortController.abort();
+
+        if (this.#timeoutId !== null) {
+            if (this.#pendingType === 'idle' && typeof cancelIdleCallback !== 'undefined') {
+                cancelIdleCallback(this.#timeoutId as unknown as number);
+            } else {
+                clearTimeout(this.#timeoutId);
+            }
+            this.#timeoutId = null;
+            this.#pendingType = null;
+        }
+
+        if (this.#intervalId) {
+            clearInterval(this.#intervalId);
+            this.#intervalId = null;
+        }
+
+        if (typeof document !== 'undefined') {
+            for (const font of this.#loadedFonts.values()) {
+                document.fonts.delete(font);
+            }
+        }
+
+        this.#loadedFonts.clear();
+        this.#usageTracker.clear();
+        this.#retryCounts.clear();
+        this.statuses.clear();
+        this.#queue.clear();
+    }
 }
 
+/** Singleton instance — use throughout the application for unified font loading state. */
 export const appliedFontsManager = new AppliedFontsManager();

src/entities/Font/model/store/unifiedFontStore.svelte.ts

@@ -215,7 +215,6 @@ export class UnifiedFontStore extends BaseFontStore<ProxyFontsParams> {
         // Note: For offset === 0, we rely on the $effect above to handle the reset/init
         // This prevents race conditions and double-setting.
         if (params.offset !== 0) {
-            // Append new fonts to existing ones only for pagination
             this.#accumulatedFonts = [...this.#accumulatedFonts, ...response.fonts];
         }
 

src/entities/Font/ui/FontVirtualList/FontVirtualList.svelte

@@ -8,7 +8,10 @@ import {
     Skeleton,
     VirtualList,
 } from '$shared/ui';
-import type { ComponentProps } from 'svelte';
+import type {
+    ComponentProps,
+    Snippet,
+} from 'svelte';
 import { fade } from 'svelte/transition';
 import { getFontUrl } from '../../lib';
 import {
@@ -28,19 +31,21 @@ interface Props extends
      * Callback for when visible items change
      */
     onVisibleItemsChange?: (items: UnifiedFont[]) => void;
-    /**
-     * Weight of the font
-     */
     /**
      * Weight of the font
      */
     weight: number;
+    /**
+     * Skeleton snippet
+     */
+    skeleton?: Snippet;
 }
 
 let {
     children,
     onVisibleItemsChange,
     weight,
+    skeleton,
     ...rest
 }: Props = $props();
 
@@ -100,24 +105,18 @@ function handleNearBottom(_lastVisibleIndex: number) {
 }
 </script>
 
-{#key isLoading}
-    <div class="relative w-full h-full" transition:fade={{ duration: 300 }}>
-        {#if isLoading}
-            <div class="flex flex-col gap-3 sm:gap-4 p-3 sm:p-4">
-                {#each Array(5) as _, i}
-                    <div class="flex flex-col gap-1.5 sm:gap-2 p-3 sm:p-4 border rounded-lg sm:rounded-xl border-border-subtle bg-background-40">
-                        <div class="flex items-center justify-between mb-3 sm:mb-4">
-                            <Skeleton class="h-6 sm:h-8 w-1/3" />
-                            <Skeleton class="h-6 sm:h-8 w-6 sm:w-8 rounded-full" />
-                        </div>
-                        <Skeleton class="h-24 sm:h-32 w-full" />
-                    </div>
-                {/each}
+<div class="relative w-full h-full">
+    {#if skeleton && isLoading && unifiedFontStore.fonts.length === 0}
+        <!-- Show skeleton only on initial load when no fonts are loaded yet -->
+        <div transition:fade={{ duration: 300 }}>
+            {@render skeleton()}
         </div>
     {:else}
+        <!-- VirtualList persists during pagination - no destruction/recreation -->
         <VirtualList
             items={unifiedFontStore.fonts}
             total={unifiedFontStore.pagination.total}
+            isLoading={isLoading}
             onVisibleItemsChange={handleInternalVisibleChange}
             onNearBottom={handleNearBottom}
             {...rest}
@@ -127,5 +126,4 @@ function handleNearBottom(_lastVisibleIndex: number) {
             {/snippet}
         </VirtualList>
     {/if}
-    </div>
-{/key}
+</div>

src/shared/lib/helpers/createCharacterComparison/createCharacterComparison.svelte.ts

@@ -2,7 +2,13 @@
  * Interface representing a line of text with its measured width.
  */
 export interface LineData {
+    /**
+     * Line's text
+     */
     text: string;
+    /**
+     * It's width
+     */
     width: number;
 }
 
@@ -80,16 +86,23 @@ export function createCharacterComparison<
         container: HTMLElement | undefined,
         measureCanvas: HTMLCanvasElement | undefined,
     ) {
-        if (!container || !measureCanvas || !fontA() || !fontB()) return;
+        if (!container || !measureCanvas || !fontA() || !fontB()) {
+            return;
+        }
 
-        const rect = container.getBoundingClientRect();
-        containerWidth = rect.width;
+        // Use offsetWidth instead of getBoundingClientRect() to avoid CSS transform scaling issues
+        // getBoundingClientRect() returns transformed dimensions, which causes incorrect line breaking
+        // when PerspectivePlan applies scale() transforms (e.g., scale(0.5) in settings mode)
+        const width = container.offsetWidth;
+        containerWidth = width;
 
         // Padding considerations - matches the container padding
         const padding = window.innerWidth < 640 ? 48 : 96;
-        const availableWidth = rect.width - padding;
+        const availableWidth = width - padding;
         const ctx = measureCanvas.getContext('2d');
-        if (!ctx) return;
+        if (!ctx) {
+            return;
+        }
 
         const controlledFontSize = size();
         const fontSize = getFontSize();

src/shared/lib/helpers/createVirtualizer/createVirtualizer.svelte.ts

@@ -3,6 +3,7 @@
  *
  * Used to render visible items with absolute positioning based on computed offsets.
  */
+
 export interface VirtualItem {
     /**
      * Index of the item in the data array
@@ -132,6 +133,7 @@ export function createVirtualizer<T>(
             // Accessing measuredSizes here creates the subscription
             accumulated += measuredSizes[i] ?? options.estimateSize(i);
         }
+
         return result;
     });
 
@@ -189,10 +191,13 @@ export function createVirtualizer<T>(
             const isFullyVisible = itemStart >= scrollOffset && itemEnd <= viewportEnd;
 
             // Proximity calculation: 1.0 at center, 0.0 at edges
+            // Guard against division by zero (containerHeight can be 0 on initial render)
             const itemCenter = itemStart + (itemSize / 2);
             const distanceToCenter = Math.abs(viewportCenter - itemCenter);
             const maxDistance = containerHeight / 2;
-            const proximity = Math.max(0, 1 - (distanceToCenter / maxDistance));
+            const proximity = maxDistance > 0
+                ? Math.max(0, 1 - (distanceToCenter / maxDistance))
+                : 0;
 
             result.push({
                 index: i,
@@ -206,16 +211,6 @@ export function createVirtualizer<T>(
             });
         }
 
-        // console.log('🎯 Virtual Items Calculation:', {
-        //     scrollOffset,
-        //     containerHeight,
-        //     viewportEnd,
-        //     startIdx,
-        //     endIdx,
-        //     withOverscan: { start, end },
-        //     itemCount: end - start,
-        // });
-
         return result;
     });
     // Svelte Actions (The DOM Interface)
@@ -256,25 +251,19 @@ export function createVirtualizer<T>(
                     scrollOffset = scrolledPastTop;
                     rafId = null;
                 });
-
-                // 🔍 DIAGNOSTIC
-                // console.log('📜 Scroll Event:', {
-                //     windowScrollY: window.scrollY,
-                //     elementRectTop: rect.top,
-                //     scrolledPastTop,
-                //     containerHeight
-                // });
             };
 
             const handleResize = () => {
                 containerHeight = window.innerHeight;
-                cachedOffsetTop = getElementOffset();
+                elementOffsetTop = getElementOffset();
+                cachedOffsetTop = elementOffsetTop;
                 handleScroll();
             };
 
             // Initial setup
             requestAnimationFrame(() => {
-                cachedOffsetTop = getElementOffset();
+                elementOffsetTop = getElementOffset();
+                cachedOffsetTop = elementOffsetTop;
                 handleScroll();
             });
 
@@ -293,6 +282,11 @@ export function createVirtualizer<T>(
                         cancelAnimationFrame(rafId);
                         rafId = null;
                     }
+                    // Disconnect shared ResizeObserver
+                    if (sharedResizeObserver) {
+                        sharedResizeObserver.disconnect();
+                        sharedResizeObserver = null;
+                    }
                     elementRef = null;
                 },
             };
@@ -314,6 +308,11 @@ export function createVirtualizer<T>(
                 destroy() {
                     node.removeEventListener('scroll', handleScroll);
                     resizeObserver.disconnect();
+                    // Disconnect shared ResizeObserver
+                    if (sharedResizeObserver) {
+                        sharedResizeObserver.disconnect();
+                        sharedResizeObserver = null;
+                    }
                     elementRef = null;
                 },
             };
@@ -325,33 +324,40 @@ export function createVirtualizer<T>(
     // Signal to trigger updates when mutating measuredSizes in place
     let _version = $state(0);
 
+    // Single shared ResizeObserver for all items (performance optimization)
+    let sharedResizeObserver: ResizeObserver | null = null;
+
     /**
      * Svelte action to measure individual item elements for dynamic height support.
      *
-     * Attaches a ResizeObserver to track actual element height and updates
-     * measured sizes when dimensions change. Requires `data-index` attribute on the element.
+     * Uses a single shared ResizeObserver for all items to track actual element heights.
+     * Requires `data-index` attribute on the element.
      *
      * @param node - The DOM element to measure (should have `data-index` attribute)
      * @returns Object with destroy method for cleanup
      */
     function measureElement(node: HTMLElement) {
-        const resizeObserver = new ResizeObserver(([entry]) => {
-            if (!entry) return;
-            const index = parseInt(node.dataset.index || '', 10);
-            const height = entry.borderBoxSize[0]?.blockSize ?? node.offsetHeight;
+        // Initialize shared observer on first use
+        if (!sharedResizeObserver) {
+            sharedResizeObserver = new ResizeObserver(entries => {
+                // Process all entries in a single batch
+                for (const entry of entries) {
+                    const target = entry.target as HTMLElement;
+                    const index = parseInt(target.dataset.index || '', 10);
+                    const height = entry.borderBoxSize[0]?.blockSize ?? target.offsetHeight;
 
                     if (!isNaN(index)) {
-                // Accessing the version ensures we have the latest state if needed,
-                // though here we just read the raw object.
                         const oldHeight = measuredSizes[index];
 
                         // Only update if the height difference is significant (> 0.5px)
                         if (oldHeight === undefined || Math.abs(oldHeight - height) > 0.5) {
-                    // Stuff the measurement into a temporary buffer to batch updates
                             measurementBuffer[index] = height;
+                        }
+                    }
+                }
 
                 // Schedule a single update for the next animation frame
-                    if (frameId === null) {
+                if (frameId === null && Object.keys(measurementBuffer).length > 0) {
                     frameId = requestAnimationFrame(() => {
                         // Mutation in place for performance
                         Object.assign(measuredSizes, measurementBuffer);
@@ -364,12 +370,18 @@ export function createVirtualizer<T>(
                         frameId = null;
                     });
                 }
-                }
-            }
             });
+        }
 
-        resizeObserver.observe(node);
-        return { destroy: () => resizeObserver.disconnect() };
+        // Observe this element with the shared observer
+        sharedResizeObserver.observe(node);
+
+        // Return cleanup that only unobserves this specific element
+        return {
+            destroy: () => {
+                sharedResizeObserver?.unobserve(node);
+            },
+        };
     }
 
     // Programmatic Scroll
@@ -409,6 +421,28 @@ export function createVirtualizer<T>(
         }
     }
 
+    /**
+     * Scrolls the container to a specific pixel offset.
+     * Used for preserving scroll position during data updates.
+     *
+     * @param offset - The scroll offset in pixels
+     * @param behavior - Scroll behavior: 'auto' for instant, 'smooth' for animated
+     *
+     * @example
+     * ```ts
+     * virtualizer.scrollToOffset(1000, 'auto'); // Instant scroll to 1000px
+     * ```
+     */
+    function scrollToOffset(offset: number, behavior: ScrollBehavior = 'auto') {
+        const { useWindowScroll } = optionsGetter();
+
+        if (useWindowScroll) {
+            window.scrollTo({ top: offset + elementOffsetTop, behavior });
+        } else if (elementRef) {
+            elementRef.scrollTo({ top: offset, behavior });
+        }
+    }
+
     return {
         get scrollOffset() {
             return scrollOffset;
@@ -430,6 +464,8 @@ export function createVirtualizer<T>(
         measureElement,
         /** Programmatic scroll method to scroll to a specific item */
         scrollToIndex,
+        /** Programmatic scroll method to scroll to a specific pixel offset */
+        scrollToOffset,
     };
 }
 

src/shared/ui/VirtualList/VirtualList.svelte

@@ -6,16 +6,13 @@
     - 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';
-import { quintOut } from 'svelte/easing';
 
 interface Props {
     /**
@@ -136,10 +133,13 @@ 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 +147,34 @@ 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) {
@@ -157,7 +185,7 @@ $effect(() => {
 
 const throttledVisibleChange = throttle((visibleItems: T[]) => {
     onVisibleItemsChange?.(visibleItems);
-}, 150); // 150ms debounce
+}, 150); // 150ms throttle
 
 const throttledNearBottom = throttle((lastVisibleIndex: number) => {
     onNearBottom?.(lastVisibleIndex);
@@ -170,7 +198,8 @@ $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 +213,7 @@ $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 +224,7 @@ $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 +237,16 @@ $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 +256,7 @@ $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 +267,5 @@ $effect(() => {
                 </div>
             {/each}
         </div>
-    </ScrollArea>
+    </div>
 {/if}

src/widgets/ComparisonSlider/ui/ComparisonSlider/components/Controls.svelte

@@ -8,10 +8,13 @@
 <script lang="ts">
 import { appliedFontsManager } from '$entities/Font';
 import { getFontUrl } from '$entities/Font/lib';
+import type { ResponsiveManager } from '$shared/lib';
 import { cn } from '$shared/shadcn/utils/shadcn-utils';
 import { SidebarMenu } from '$shared/ui';
+import Drawer from '$shared/ui/Drawer/Drawer.svelte';
 import { comparisonStore } from '$widgets/ComparisonSlider/model';
-import ComparisonList from './FontList.svelte';
+import { getContext } from 'svelte';
+import FontList from './FontList.svelte';
 import ToggleMenuButton from './ToggleMenuButton.svelte';
 import TypographyControls from './TypographyControls.svelte';
 
@@ -33,6 +36,7 @@ const fontA = $derived(comparisonStore.fontA);
 const fontB = $derived(comparisonStore.fontB);
 const typography = $derived(comparisonStore.typography);
 let menuWrapper = $state<HTMLElement | null>(null);
+const responsive = getContext<ResponsiveManager>('responsive');
 
 $effect(() => {
     if (!fontA || !fontB) {
@@ -64,7 +68,29 @@ $effect(() => {
 });
 </script>
 
-<SidebarMenu
+{#if responsive.isMobile}
+    <Drawer>
+        {#snippet trigger({ onClick })}
+            <div class={cn('absolute bottom-2 inset-x-0 z-50')}>
+                <ToggleMenuButton bind:isActive={visible} {onClick} />
+            </div>
+        {/snippet}
+        {#snippet content({ className })}
+            <div class={cn(className, 'flex flex-col gap-2 h-[60vh]')}>
+                <div class="h-full overflow-hidden">
+                    <FontList />
+                </div>
+
+                <div class="relative flex w-auto border-b border-gray-400/50 px-2 ml-4 mr-8 lg:mr-10 flex-shrink-0">
+                </div>
+                <div class="mr-4 sm:mr-6 flex-shrink-0">
+                    <TypographyControls />
+                </div>
+            </div>
+        {/snippet}
+    </Drawer>
+{:else}
+    <SidebarMenu
         class={cn(
             'w-96 flex flex-col h-full pl-4 lg:pl-6 py-4 sm:py-6 sm:pt-12 gap-4 sm:gap-6 pointer-events-auto overflow-hidden',
             'relative h-full transition-all duration-700 ease-out',
@@ -73,7 +99,7 @@ $effect(() => {
         bind:visible
         bind:wrapper={menuWrapper}
         onClickOutside={handleToggle}
->
+    >
         {#snippet action()}
             <!-- Always-visible mode switch -->
             <div class={cn('absolute top-4 left-0 z-50', visible && 'w-full')}>
@@ -81,11 +107,12 @@ $effect(() => {
             </div>
         {/snippet}
         <div class="h-2/3 overflow-hidden">
-        <ComparisonList />
+            <FontList />
         </div>
 
         <div class="relative flex w-auto border-b border-gray-400/50 px-2 ml-4 mr-8 lg:mr-10"></div>
         <div class="mr-4 sm:mr-6">
             <TypographyControls />
         </div>
-</SidebarMenu>
+    </SidebarMenu>
+{/if}

src/widgets/ComparisonSlider/ui/ComparisonSlider/components/FontList.svelte

@@ -146,7 +146,7 @@ function isFontB(font: UnifiedFont): boolean {
         <FontVirtualList
             weight={typography.weight}
             itemHeight={36}
-            class="bg-transparent"
+            class="bg-transparent h-full"
         >
             {#snippet children({ item: font })}
                 {@const isSelectedA = isFontA(font)}

src/widgets/SampleList/ui/SampleList/SampleList.svelte

@@ -16,6 +16,7 @@ import {
     controlManager,
 } from '$features/SetupFont';
 import { throttle } from '$shared/lib/utils';
+import { Skeleton } from '$shared/ui';
 
 let text = $state('The quick brown fox jumps over the lazy dog...');
 let wrapper = $state<HTMLDivElement | null>(null);
@@ -43,6 +44,20 @@ const checkPosition = throttle(() => {
 }, 100);
 </script>
 
+{#snippet skeleton()}
+    <div class="flex flex-col gap-3 sm:gap-4 p-3 sm:p-4">
+        {#each Array(5) as _, i}
+            <div class="flex flex-col gap-1.5 sm:gap-2 p-3 sm:p-4 border rounded-lg sm:rounded-xl border-border-subtle bg-background-40">
+                <div class="flex items-center justify-between mb-3 sm:mb-4">
+                    <Skeleton class="h-6 sm:h-8 w-1/3" />
+                    <Skeleton class="h-6 sm:h-8 w-6 sm:w-8 rounded-full" />
+                </div>
+                <Skeleton class="h-24 sm:h-32 w-full" />
+            </div>
+        {/each}
+    </div>
+{/snippet}
+
 <svelte:window
     bind:innerHeight
     onscroll={checkPosition}
@@ -54,6 +69,7 @@ const checkPosition = throttle(() => {
         itemHeight={220}
         useWindowScroll={true}
         weight={controlManager.weight}
+        {skeleton}
     >
         {#snippet children({
     item: font,