fix(FontList): address the bug with selected font transition animations

src/entities/Font/model/const/const.ts

@@ -86,3 +86,9 @@ export const DEFAULT_TYPOGRAPHY_CONTROLS_DATA: ControlModel<ControlId>[] = [
 export const MULTIPLIER_S = 0.5;
 export const MULTIPLIER_M = 0.75;
 export const MULTIPLIER_L = 1;
+
+/**
+ * Index value for items not yet loaded in a virtualized list.
+ * Treated as being at the very bottom of the infinite scroll.
+ */
+export const VIRTUAL_INDEX_NOT_LOADED = Infinity;

src/widgets/ComparisonView/lib/index.ts

@@ -1 +1,2 @@
+export * from './utils/dotTransition';
 export * from './utils/getPretextFontString';

src/widgets/ComparisonView/lib/utils/dotTransition.ts

@@ -0,0 +1,99 @@
+import { VIRTUAL_INDEX_NOT_LOADED } from '$entities/Font';
+import { cubicOut } from 'svelte/easing';
+import {
+    type CrossfadeParams,
+    type TransitionConfig,
+    crossfade,
+} from 'svelte/transition';
+
+/**
+ * Custom parameters for dot transitions in virtualized lists.
+ */
+export interface DotTransitionParams extends CrossfadeParams {
+    /**
+     * Unique key for crossfade pairing
+     */
+    key: any;
+    /**
+     * Current index of the item in the list
+     */
+    index: number;
+    /**
+     * Target index to move towards (e.g. counterpart side index)
+     */
+    otherIndex: number;
+}
+
+/**
+ * Type-safe helper to create dot transition parameters.
+ */
+export function getDotTransitionParams(
+    key: 'active-dot' | 'inactive-dot',
+    index: number,
+    otherIndex: number,
+): DotTransitionParams {
+    return { key, index, otherIndex };
+}
+
+/**
+ * Type guard for DotTransitionParams.
+ */
+function isDotTransitionParams(p: CrossfadeParams): p is DotTransitionParams {
+    return (
+        p !== null
+        && typeof p === 'object'
+        && 'index' in p
+        && 'otherIndex' in p
+    );
+}
+
+/**
+ * Creates a crossfade transition pair optimized for virtualized font lists.
+ *
+ * It uses the 'index' and 'otherIndex' params to calculate the direction
+ * of the slide animation when a matching pair cannot be found in the DOM
+ * (e.g. because it was virtualized out).
+ */
+export function createDotCrossfade() {
+    return crossfade({
+        duration: 300,
+        easing: cubicOut,
+        fallback(node: Element, params: CrossfadeParams, _intro: boolean): TransitionConfig {
+            if (!isDotTransitionParams(params)) {
+                return {
+                    duration: 300,
+                    easing: cubicOut,
+                    css: t => `opacity: ${t};`,
+                };
+            }
+
+            const { index, otherIndex } = params;
+
+            // If the other target is unknown, just fade in place
+            if (otherIndex === undefined || otherIndex === -1) {
+                return {
+                    duration: 300,
+                    easing: cubicOut,
+                    css: t => `opacity: ${t};`,
+                };
+            }
+
+            const diff = otherIndex - index;
+            const sign = diff > 0 ? 1 : (diff < 0 ? -1 : 0);
+
+            // Use container height for a full-height slide
+            const listEl = node.closest('[data-font-list]');
+            const h = listEl?.clientHeight ?? 300;
+            const fromY = sign * h;
+
+            return {
+                duration: 300,
+                easing: cubicOut,
+                css: (t, u) => `
+                    transform: translateY(${fromY * u}px);
+                    opacity: ${t};
+                `,
+            };
+        },
+    });
+}

src/widgets/ComparisonView/ui/FontList/FontList.svelte

@@ -8,65 +8,67 @@ import {
     FontApplicator,
     FontVirtualList,
     type UnifiedFont,
+    VIRTUAL_INDEX_NOT_LOADED,
+    fontStore,
 } from '$entities/Font';
 import {
     Button,
     Label,
 } from '$shared/ui';
 import DotIcon from '@lucide/svelte/icons/dot';
-import { cubicOut } from 'svelte/easing';
-import { crossfade } from 'svelte/transition';
-import { comparisonStore } from '../../model';
+import {
+    createDotCrossfade,
+    getDotTransitionParams,
+} from '../../lib';
+import {
+    type Side,
+    comparisonStore,
+} from '../../model';
 
 const side = $derived(comparisonStore.side);
-let prevIndexA: number | null = null;
-let prevIndexB: number | null = null;
-let selectedIndexA: number | null = null;
-let selectedIndexB: number | null = null;
-let pendingDirection: 1 | -1 = 1;
 
-const [send, receive] = crossfade({
-    duration: 300,
-    easing: cubicOut,
-    fallback(node) {
-        // Read pendingDirection synchronously — no reactive timing issues
-        const fromY = pendingDirection * (node.closest('[data-font-list]')?.clientHeight ?? 300);
-        return {
-            duration: 300,
-            easing: cubicOut,
-            css: t => `transform: translateY(${fromY * (1 - t)}px);`,
-        };
-    },
+// Treat -1 (not loaded) as being at the very bottom of the infinite list
+function getVirtualIndex(fontId: string | undefined): number {
+    if (!fontId) {
+        return -1;
+    }
+    const idx = fontStore.fonts.findIndex(f => f.id === fontId);
+    if (idx === -1) {
+        return VIRTUAL_INDEX_NOT_LOADED;
+    }
+    return idx;
+}
+
+// Reactive indices of the currently selected fonts in the full list
+const indexA = $derived(getVirtualIndex(comparisonStore.fontA?.id));
+const indexB = $derived(getVirtualIndex(comparisonStore.fontB?.id));
+
+// Track previous state for directional fallback transitions.
+// We use plain variables here. In Svelte 5, updates to these in an $effect
+// happen AFTER the render/DOM update, so transitions starting as a result
+// of that update will see the "old" values of these variables.
+let prevIndexA = indexA;
+let prevIndexB = indexB;
+let prevSide: Side = side;
+
+const [send, receive] = createDotCrossfade();
+
+$effect(() => {
+    // This effect runs after every change to indexA, indexB, or side.
+    // It captures the "current" values which will serve as "previous" values
+    // for the NEXT transition.
+    prevIndexA = indexA;
+    prevIndexB = indexB;
+    prevSide = side;
 });
 
-function handleSelect(font: UnifiedFont, index: number) {
+function handleSelect(font: UnifiedFont) {
     if (side === 'A') {
-        if (prevIndexA !== null) {
-            pendingDirection = index > prevIndexA ? -1 : 1;
-        }
-        prevIndexA = index;
-        selectedIndexA = index;
         comparisonStore.fontA = font;
-    } else if (side === 'B') {
-        if (prevIndexB !== null) {
-            pendingDirection = index > prevIndexB ? -1 : 1;
-        }
-        prevIndexB = index;
-        selectedIndexB = index;
+    } else {
         comparisonStore.fontB = font;
     }
 }
-
-// When side switches, compute direction from relative positions of A vs B
-$effect(() => {
-    const _ = side; // track side
-    if (selectedIndexA !== null && selectedIndexB !== null) {
-        // Switching TO B means dot moves toward B's position relative to A
-        pendingDirection = side === 'B'
-            ? (selectedIndexB > selectedIndexA ? 1 : -1)
-            : (selectedIndexA > selectedIndexB ? 1 : -1);
-    }
-});
 </script>
 
 <div class="flex-1 min-h-0 h-full">
@@ -90,7 +92,7 @@ $effect(() => {
                 <Button
                     variant="tertiary"
                     {active}
-                    onclick={() => handleSelect(font, index)}
+                    onclick={() => handleSelect(font)}
                     class="w-full px-3 md:px-4 py-2.5 md:py-3 flex !justify-between text-left text-sm"
                     iconPosition="right"
                 >
@@ -99,15 +101,32 @@ $effect(() => {
                     {#snippet icon()}
                         {#if active}
                             <div
-                                in:receive={{ key: 'active-dot' }}
-                                out:send={{ key: 'active-dot' }}
+                                in:receive={getDotTransitionParams(
+                                    'active-dot',
+                                    index,
+                                    prevSide === 'A' ? prevIndexA : prevIndexB,
+                                )}
+                                out:send={getDotTransitionParams(
+                                    'active-dot',
+                                    index,
+                                    side === 'A' ? indexA : indexB,
+                                )}
                             >
                                 <DotIcon class="size-8 stroke-brand" />
                             </div>
                         {:else if isSelectedA || isSelectedB}
+                            {@const isA = isSelectedA}
                             <div
-                                in:receive={{ key: 'inactive-dot' }}
-                                out:send={{ key: 'inactive-dot' }}
+                                in:receive={getDotTransitionParams(
+                                    'inactive-dot',
+                                    index,
+                                    isA ? prevIndexB : prevIndexA,
+                                )}
+                                out:send={getDotTransitionParams(
+                                    'inactive-dot',
+                                    index,
+                                    isA ? indexB : indexA,
+                                )}
                             >
                                 <DotIcon class="size-8 stroke-neutral-300 dark:stroke-neutral-600" />
                             </div>