feat(FontComparer): create FontComparer component that loads fonts and displays ComparisonSlider

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

@@ -12,7 +12,15 @@ export class FontshareStore extends BaseFontStore<FontshareParams> {
     }
 
     protected getQueryKey(params: FontshareParams) {
-        return ['fontshare', params] as const;
+        // Normalize params to treat empty arrays/strings as undefined
+        const normalized = Object.entries(params).reduce((acc, [key, value]) => {
+            if (value === '' || (Array.isArray(value) && value.length === 0)) {
+                return acc;
+            }
+            return { ...acc, [key]: value };
+        }, {});
+
+        return ['fontshare', normalized] as const;
     }
 
     protected async fetchFn(params: FontshareParams): Promise<UnifiedFont[]> {

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

@@ -3,7 +3,7 @@
     - Renders a virtualized list of fonts
     - Handles font registration with the manager
 -->
-<script lang="ts" generics="T extends { id: string }">
+<script lang="ts" generics="T extends ({ id: string } | [{ id: string }, { id: string }])">
 import { VirtualList } from '$shared/ui';
 import type { ComponentProps } from 'svelte';
 import { appliedFontsManager } from '../../model';
@@ -16,7 +16,12 @@ let { items, children, onVisibleItemsChange, ...rest }: Props = $props();
 
 function handleInternalVisibleChange(visibleItems: T[]) {
     // Auto-register fonts with the manager
-    const slugs = visibleItems.map(item => item.id);
+    const slugs = visibleItems.map(item => {
+        if (Array.isArray(item)) {
+            return item.map(font => font.id);
+        }
+        return item.id;
+    }).flat();
     appliedFontsManager.registerFonts(slugs);
 
     // Forward the call to any external listener

src/features/DisplayFont/model/store/displayedFontsStore.svelte.ts

@@ -1,4 +1,7 @@
-import { selectedFontsStore } from '$entities/Font';
+import {
+    type UnifiedFont,
+    selectedFontsStore,
+} from '$entities/Font';
 
 /**
  * Store for displayed font samples
@@ -12,10 +15,35 @@ export class DisplayedFontsStore {
         return selectedFontsStore.all;
     });
 
+    #fontPairs = $derived.by(() => {
+        const fonts = this.#displayedFonts;
+        const pairs = fonts.flatMap((v1, i) =>
+            fonts.slice(i + 1).map<[UnifiedFont, UnifiedFont]>(v2 => [v1, v2])
+        );
+        if (pairs.length && this.isSelectedPairEmpty()) {
+            this.selectedPair = pairs[0];
+        }
+        return pairs;
+    });
+
+    #selectedPair = $state<Partial<[UnifiedFont, UnifiedFont]>>([]);
+
     get fonts() {
         return this.#displayedFonts;
     }
 
+    get pairs() {
+        return this.#fontPairs;
+    }
+
+    get selectedPair() {
+        return this.#selectedPair;
+    }
+
+    set selectedPair(pair: Partial<[UnifiedFont, UnifiedFont]>) {
+        this.#selectedPair = pair;
+    }
+
     get text() {
         return this.#sampleText;
     }
@@ -23,6 +51,11 @@ export class DisplayedFontsStore {
     set text(text: string) {
         this.#sampleText = text;
     }
+
+    isSelectedPairEmpty(): boolean {
+        const [font1, font2] = this.#selectedPair;
+        return !font1 || !font2;
+    }
 }
 
 export const displayedFontsStore = new DisplayedFontsStore();

src/features/DisplayFont/ui/FontComparer/FontComparer.svelte

@@ -0,0 +1,28 @@
+<script lang="ts">
+import { appliedFontsManager } from '$entities/Font';
+import { Input } from '$shared/shadcn/ui/input';
+import { ComparisonSlider } from '$shared/ui';
+import { displayedFontsStore } from '../../model';
+import PairSelector from '../PairSelector/PairSelector.svelte';
+
+let displayedText = $state('The quick brown fox jumps over the lazy dog...');
+const [fontA, fontB] = $derived(displayedFontsStore.selectedPair);
+const hasAnyPairs = $derived(displayedFontsStore.fonts.length > 0);
+
+$effect(() => {
+    appliedFontsManager.touch(displayedFontsStore.fonts.map(font => font.id));
+});
+</script>
+
+{#if hasAnyPairs}
+    <div class="flex flex-col gap-4">
+        <div class="flex flex-row gap-4">
+            <Input bind:value={displayedText} />
+            <PairSelector />
+        </div>
+
+        {#if fontA && fontB}
+            <ComparisonSlider fontA={fontA} fontB={fontB} text={displayedText} />
+        {/if}
+    </div>
+{/if}

src/features/DisplayFont/ui/FontDisplay/FontDisplay.svelte

@@ -2,11 +2,14 @@
     Component: FontDisplay
     Displays a grid of FontSampler components for each displayed font.
 -->
-<script>
+<script lang="ts">
 import { displayedFontsStore } from '../../model';
+import FontComparer from '../FontComparer/FontComparer.svelte';
 import FontSampler from '../FontSampler/FontSampler.svelte';
 </script>
 
+<FontComparer />
+
 <div class="grid gap-2 grid-cols-[repeat(auto-fit,minmax(500px,1fr))]">
     {#each displayedFontsStore.fonts as font (font.id)}
         <FontSampler font={font} bind:text={displayedFontsStore.text} />

src/features/DisplayFont/ui/FontPair/FontPair.svelte

@@ -0,0 +1,29 @@
+<script lang="ts">
+import {
+    FontApplicator,
+    type UnifiedFont,
+} from '$entities/Font';
+
+interface Props {
+    pair: [UnifiedFont, UnifiedFont];
+    selectedPair: Partial<[UnifiedFont, UnifiedFont]>;
+}
+
+let { pair, selectedPair = $bindable() }: Props = $props();
+
+const [font1, font2] = $derived(pair);
+
+function handleClick() {
+    selectedPair = pair;
+}
+</script>
+
+<div class="flex flex-row justify-between w-full" onclick={handleClick}>
+    <FontApplicator id={font1.id} name={font1.name}>
+        {font1.name}
+    </FontApplicator>
+    vs
+    <FontApplicator id={font2.id} name={font2.name}>
+        {font2.name}
+    </FontApplicator>
+</div>

src/features/DisplayFont/ui/PairSelector/PairSelector.svelte

@@ -0,0 +1,34 @@
+<script lang="ts">
+import { FontVirtualList } from '$entities/Font';
+import { buttonVariants } from '$shared/shadcn/ui/button';
+import {
+    Content as PopoverContent,
+    Root as PopoverRoot,
+    Trigger as PopoverTrigger,
+} from '$shared/shadcn/ui/popover';
+import { displayedFontsStore } from '../../model';
+import FontPair from '../FontPair/FontPair.svelte';
+
+let open = $state(false);
+
+const triggerContent = $derived.by(() => {
+    const [beforeFont, afterFont] = displayedFontsStore.selectedPair ?? [];
+    if (!beforeFont || !afterFont) return 'Select a pair';
+    return `${beforeFont.name} vs ${afterFont.name}`;
+});
+
+const triggerDisabled = $derived(displayedFontsStore.pairs.length === 0);
+</script>
+
+<PopoverRoot bind:open>
+    <PopoverTrigger class={buttonVariants({ variant: 'outline' })} disabled={triggerDisabled}>
+        {triggerContent}
+    </PopoverTrigger>
+    <PopoverContent>
+        <FontVirtualList items={displayedFontsStore.pairs}>
+            {#snippet children({ item: pair })}
+                <FontPair {pair} bind:selectedPair={displayedFontsStore.selectedPair} />
+            {/snippet}
+        </FontVirtualList>
+    </PopoverContent>
+</PopoverRoot>

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

@@ -0,0 +1,170 @@
+/**
+ * Interface representing a line of text with its measured width.
+ */
+export interface LineData {
+    text: string;
+    width: number;
+}
+
+/**
+ * Creates a helper for splitting text into lines and calculating character proximity.
+ * This is used by the ComparisonSlider (TestTen) to render morphing text.
+ *
+ * @param text - The text to split and measure
+ * @param fontA - The first font definition
+ * @param fontB - The second font definition
+ * @returns Object with reactive state (lines, containerWidth) and methods (breakIntoLines, getCharState)
+ */
+export function createCharacterComparison(
+    text: () => string,
+    fontA: () => { name: string; id: string },
+    fontB: () => { name: string; id: string },
+) {
+    let lines = $state<LineData[]>([]);
+    let containerWidth = $state(0);
+
+    /**
+     * Measures text width using a canvas context.
+     * @param ctx - Canvas rendering context
+     * @param text - Text string to measure
+     * @param fontFamily - Font family name
+     * @param fontSize - Font size in pixels
+     */
+    function measureText(
+        ctx: CanvasRenderingContext2D,
+        text: string,
+        fontFamily: string,
+        fontSize: number,
+    ): number {
+        ctx.font = `bold ${fontSize}px ${fontFamily}`;
+        return ctx.measureText(text).width;
+    }
+
+    /**
+     * Determines the appropriate font size based on window width.
+     * Matches the Tailwind breakpoints used in the component.
+     */
+    function getFontSize() {
+        if (typeof window === 'undefined') return 64;
+        return window.innerWidth >= 1024
+            ? 112
+            : window.innerWidth >= 768
+            ? 96
+            : window.innerWidth >= 640
+            ? 80
+            : 64;
+    }
+
+    /**
+     * Breaks the text into lines based on the container width and measure canvas.
+     * Populates the `lines` state.
+     *
+     * @param container - The container element to measure width from
+     * @param measureCanvas - The canvas element used for text measurement
+     */
+    function breakIntoLines(
+        container: HTMLElement | undefined,
+        measureCanvas: HTMLCanvasElement | undefined,
+    ) {
+        if (!container || !measureCanvas) return;
+
+        const rect = container.getBoundingClientRect();
+        containerWidth = rect.width;
+
+        // Padding considerations - matches the container padding
+        const padding = window.innerWidth < 640 ? 48 : 96;
+        const availableWidth = rect.width - padding;
+
+        const ctx = measureCanvas.getContext('2d');
+        if (!ctx) return;
+
+        const fontSize = getFontSize();
+        const words = text().split(' ');
+        const newLines: LineData[] = [];
+        let currentLineWords: string[] = [];
+
+        function pushLine(words: string[]) {
+            if (words.length === 0) return;
+            const lineText = words.join(' ');
+            // Measure width to ensure we know exactly how wide this line renders
+            const widthA = measureText(ctx!, lineText, fontA().name, fontSize);
+            const widthB = measureText(ctx!, lineText, fontB().name, fontSize);
+            const maxWidth = Math.max(widthA, widthB);
+            newLines.push({ text: lineText, width: maxWidth });
+        }
+
+        for (const word of words) {
+            const testLine = currentLineWords.length > 0
+                ? currentLineWords.join(' ') + ' ' + word
+                : word;
+
+            // Measure with both fonts and use the wider one to prevent layout shifts
+            const widthA = measureText(ctx, testLine, fontA().name, fontSize);
+            const widthB = measureText(ctx, testLine, fontB().name, fontSize);
+            const maxWidth = Math.max(widthA, widthB);
+
+            if (maxWidth > availableWidth && currentLineWords.length > 0) {
+                pushLine(currentLineWords);
+                currentLineWords = [word];
+            } else {
+                currentLineWords.push(word);
+            }
+        }
+
+        if (currentLineWords.length > 0) {
+            pushLine(currentLineWords);
+        }
+
+        lines = newLines;
+    }
+
+    /**
+     * precise calculation of character state based on global slider position.
+     *
+     * @param lineIndex - Index of the line
+     * @param charIndex - Index of the character in the line
+     * @param lineData - The line data object
+     * @param sliderPos - Current slider position (0-100)
+     * @returns Object containing proximity (0-1) and isPast (boolean)
+     */
+    function getCharState(
+        lineIndex: number,
+        charIndex: number,
+        lineData: LineData,
+        sliderPos: number,
+    ) {
+        if (!containerWidth) return { proximity: 0, isPast: false };
+
+        // Calculate the pixel position of the character relative to the CONTAINER
+        // 1. Find the left edge of the centered line
+        const lineStartOffset = (containerWidth - lineData.width) / 2;
+
+        // 2. Find the character's center relative to the line
+        const charRelativePercent = (charIndex + 0.5) / lineData.text.length;
+        const charPixelPos = lineStartOffset + (charRelativePercent * lineData.width);
+
+        // 3. Convert back to global percentage (0-100)
+        const charGlobalPercent = (charPixelPos / containerWidth) * 100;
+
+        const distance = Math.abs(sliderPos - charGlobalPercent);
+
+        // Proximity range: +/- 15% around the slider
+        const range = 15;
+        const proximity = Math.max(0, 1 - distance / range);
+
+        const isPast = sliderPos > charGlobalPercent;
+
+        return { proximity, isPast };
+    }
+
+    return {
+        get lines() {
+            return lines;
+        },
+        get containerWidth() {
+            return containerWidth;
+        },
+        breakIntoLines,
+        getCharState,
+    };
+}

src/shared/lib/helpers/index.ts

@@ -26,3 +26,7 @@ export {
     type Entity,
     type EntityStore,
 } from './createEntityStore/createEntityStore.svelte';
+
+export {
+    createCharacterComparison,
+} from './createCharacterComparison/createCharacterComparison.svelte';

src/shared/lib/index.ts

@@ -1,6 +1,7 @@
 export {
     type ControlDataModel,
     type ControlModel,
+    createCharacterComparison,
     createDebouncedState,
     createEntityStore,
     createFilter,

src/shared/ui/ComparisonSlider/ComparisonSlider.svelte

@@ -0,0 +1,182 @@
+<!--
+    Component: ComparisonSlider (Ultimate Comparison Slider)
+
+    A multiline text comparison slider that morphs between two fonts.
+
+    Features:
+    - Multiline support with precise line breaking matching container width.
+    - Character-level morphing: Font changes exactly when the slider passes the character's global position.
+    - Responsive layout with Tailwind breakpoints for font sizing.
+    - Performance optimized using offscreen canvas for measurements and transform-based animations.
+-->
+<script lang="ts" generics="T extends { name: string; id: string }">
+import { createCharacterComparison } from '$shared/lib';
+import { Spring } from 'svelte/motion';
+import Labels from './components/Labels.svelte';
+import SliderLine from './components/SliderLine.svelte';
+
+interface Props<T extends { name: string; id: string }> {
+    /** First font definition ({name, id}) */
+    fontA: T;
+    /** Second font definition ({name, id}) */
+    fontB: T;
+    /** Text to display and compare */
+    text?: string;
+}
+
+let {
+    fontA,
+    fontB,
+    text = 'The quick brown fox jumps over the lazy dog',
+}: Props<T> = $props();
+
+let container: HTMLElement | undefined = $state();
+let measureCanvas: HTMLCanvasElement | undefined = $state();
+let isDragging = $state(false);
+
+/**
+ * Encapsulated helper for text splitting, measuring, and character proximity calculations.
+ * Manages line breaking and character state based on fonts and container dimensions.
+ */
+const charComparison = createCharacterComparison(
+    () => text,
+    () => fontA,
+    () => fontB,
+);
+
+/** Physics-based spring for smooth handle movement */
+const sliderSpring = new Spring(50, {
+    stiffness: 0.2, // Balanced for responsiveness
+    damping: 0.7, // No bounce, just smooth stop
+});
+const sliderPos = $derived(sliderSpring.current);
+
+/** Updates spring target based on pointer position */
+function handleMove(e: PointerEvent) {
+    if (!isDragging || !container) return;
+    const rect = container.getBoundingClientRect();
+    const x = Math.max(0, Math.min(e.clientX - rect.left, rect.width));
+    const percentage = (x / rect.width) * 100;
+    sliderSpring.target = percentage;
+}
+
+function startDragging(e: PointerEvent) {
+    e.preventDefault();
+    isDragging = true;
+    handleMove(e);
+}
+
+$effect(() => {
+    if (isDragging) {
+        window.addEventListener('pointermove', handleMove);
+        const stop = () => (isDragging = false);
+        window.addEventListener('pointerup', stop);
+        return () => {
+            window.removeEventListener('pointermove', handleMove);
+            window.removeEventListener('pointerup', stop);
+        };
+    }
+});
+
+// Re-run line breaking when container resizes or dependencies change
+$effect(() => {
+    if (container && measureCanvas) {
+        // Using rAF to ensure DOM is ready/stabilized
+        requestAnimationFrame(() => {
+            charComparison.breakIntoLines(container, measureCanvas);
+        });
+    }
+});
+
+$effect(() => {
+    if (typeof window === 'undefined') return;
+    const handleResize = () => {
+        if (container && measureCanvas) {
+            charComparison.breakIntoLines(container, measureCanvas);
+        }
+    };
+    window.addEventListener('resize', handleResize);
+    return () => window.removeEventListener('resize', handleResize);
+});
+</script>
+
+<!-- Hidden canvas used for text measurement by the helper -->
+<canvas bind:this={measureCanvas} class="hidden" width="1" height="1"></canvas>
+
+<div
+    bind:this={container}
+    role="slider"
+    tabindex="0"
+    aria-valuenow={Math.round(sliderPos)}
+    aria-label="Font comparison slider"
+    onpointerdown={startDragging}
+    class="group relative w-full py-16 px-6 sm:py-24 sm:px-12 overflow-hidden bg-white rounded-[2.5rem] border border-slate-100 shadow-2xl select-none touch-none cursor-ew-resize min-h-100 flex flex-col justify-center"
+>
+    <!-- Background Gradient Accent -->
+    <div
+        class="
+            absolute inset-0 bg-linear-to-br
+            from-slate-50/50 via-white to-slate-100/50
+            opacity-50 pointer-events-none
+        "
+    >
+    </div>
+
+    <!-- Text Rendering Container -->
+    <div
+        class="relative flex flex-col items-center gap-4 text-4xl sm:text-5xl md:text-6xl lg:text-7xl font-bold leading-[1.15] z-10 pointer-events-none text-center"
+        style:perspective="1000px"
+    >
+        {#each charComparison.lines as line, lineIndex}
+            <div class="relative w-full whitespace-nowrap">
+                {#each line.text.split('') as char, charIndex}
+                    {@const { proximity, isPast } = charComparison.getCharState(lineIndex, charIndex, line, sliderPos)}
+                    <!--
+                        Single Character Span
+                        - Font Family switches based on `isPast`
+                        - Transitions/Transforms provide the "morph" feel
+                    -->
+                    <span
+                        class="inline-block transition-all duration-300 ease-out will-change-transform"
+                        style:font-family={isPast ? fontB.name : fontA.name}
+                        style:color={isPast
+                        ? 'rgb(79, 70, 229)'
+                        : 'rgb(15, 23, 42)'}
+                        style:transform="
+                        scale({1 + proximity * 0.2}) translateY({-proximity *
+                            12}px) rotateY({proximity *
+                            25 *
+                            (isPast ? -1 : 1)}deg)
+                        "
+                        style:will-change={proximity > 0
+                        ? 'transform, font-family, color'
+                        : 'auto'}
+                    >
+                        {char === ' ' ? '\u00A0' : char}
+                    </span>
+                {/each}
+            </div>
+        {/each}
+    </div>
+
+    <!-- Visual Components -->
+    <SliderLine {sliderPos} />
+    <Labels {fontA} {fontB} {sliderPos} />
+</div>
+
+<style>
+span {
+    /*
+       Optimize for performance and smooth transitions.
+       step-end logic is effectively handled by binary font switching in JS.
+    */
+    transition:
+        font-family 0.15s ease-out,
+        color 0.2s ease-out,
+        transform 0.2s cubic-bezier(0.34, 1.56, 0.64, 1);
+    transform-style: preserve-3d;
+    backface-visibility: hidden;
+    -webkit-font-smoothing: antialiased;
+    -moz-osx-font-smoothing: grayscale;
+}
+</style>

src/shared/ui/ComparisonSlider/components/Labels.svelte

@@ -0,0 +1,37 @@
+<script lang="ts">
+import type { Snippet } from 'svelte';
+
+interface Props {
+    fontA: { name: string; id: string };
+    fontB: { name: string; id: string };
+    sliderPos: number;
+}
+let { fontA, fontB, sliderPos }: Props = $props();
+</script>
+
+<!-- Bottom Labels -->
+<div class="absolute bottom-6 inset-x-8 sm:inset-x-12 flex justify-between items-center pointer-events-none z-20">
+    <!-- Left Label (Font A) -->
+    <div
+        class="flex flex-col gap-1 transition-opacity duration-300"
+        style:opacity={sliderPos < 10 ? 0 : 1}
+    >
+        <span class="text-[0.5rem] font-mono uppercase tracking-widest text-indigo-400"
+        >Baseline</span>
+        <span class="text-xs sm:text-sm font-bold text-indigo-600">
+            {fontB.name}
+        </span>
+    </div>
+
+    <!-- Right Label (Font B) -->
+    <div
+        class="flex flex-col items-end text-right gap-1 transition-opacity duration-300"
+        style:opacity={sliderPos > 90 ? 0 : 1}
+    >
+        <span class="text-[0.5rem] font-mono uppercase tracking-widest text-slate-400"
+        >Comparison</span>
+        <span class="text-xs sm:text-sm font-bold text-slate-900">
+            {fontA.name}
+        </span>
+    </div>
+</div>

src/shared/ui/ComparisonSlider/components/SliderLine.svelte

@@ -0,0 +1,26 @@
+<script lang="ts">
+interface Props {
+    sliderPos: number;
+}
+let { sliderPos }: Props = $props();
+</script>
+
+<!-- Vertical Divider & Knobs -->
+<div
+    class="absolute top-0 bottom-0 z-30 pointer-events-none"
+    style:left="{sliderPos}%"
+>
+    <!-- Vertical Line -->
+    <div class="absolute inset-y-0 -left-px w-0.5 bg-indigo-500 shadow-[0_0_10px_rgba(99,102,241,0.5)]">
+    </div>
+
+    <!-- Top Knob -->
+    <div class="absolute top-6 left-0 -translate-x-1/2">
+        <div class="w-2.5 h-2.5 bg-indigo-500 rounded-full shadow ring-2 ring-white"></div>
+    </div>
+
+    <!-- Bottom Knob -->
+    <div class="absolute bottom-6 left-0 -translate-x-1/2">
+        <div class="w-2.5 h-2.5 bg-indigo-500 rounded-full shadow ring-2 ring-white"></div>
+    </div>
+</div>

src/shared/ui/index.ts

@@ -6,6 +6,7 @@
 
 import CheckboxFilter from './CheckboxFilter/CheckboxFilter.svelte';
 import ComboControl from './ComboControl/ComboControl.svelte';
+import ComparisonSlider from './ComparisonSlider/ComparisonSlider.svelte';
 import ContentEditable from './ContentEditable/ContentEditable.svelte';
 import SearchBar from './SearchBar/SearchBar.svelte';
 import VirtualList from './VirtualList/VirtualList.svelte';
@@ -13,6 +14,7 @@ import VirtualList from './VirtualList/VirtualList.svelte';
 export {
     CheckboxFilter,
     ComboControl,
+    ComparisonSlider,
     ContentEditable,
     SearchBar,
     VirtualList,