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

@@ -1,5 +1,8 @@
 import { SvelteMap } from 'svelte/reactivity';
-import { getEffectiveConcurrency } from './utils/getEffectiveConcurrency/getEffectiveConcurrency';
+import {
+    getEffectiveConcurrency,
+    yieldToMainThread,
+} from './utils';
 
 /** Loading state of a font. Failed loads may be retried up to MAX_RETRIES. */
 export type FontStatus = 'loading' | 'loaded' | 'error';
@@ -143,37 +146,7 @@ export class AppliedFontsManager {
         }
     }
 
-    /** 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 {
@@ -261,7 +234,7 @@ export class AppliedFontsManager {
                 : (performance.now() - lastYield > YIELD_INTERVAL);
 
             if (shouldYield) {
-                await this.#yieldToMain();
+                await yieldToMainThread();
                 lastYield = performance.now();
             }
         }

src/entities/Font/model/store/appliedFontsStore/utils/index.ts

@@ -0,0 +1,2 @@
+export { getEffectiveConcurrency } from './getEffectiveConcurrency/getEffectiveConcurrency';
+export { yieldToMainThread } from './yieldToMainThread/yieldToMainThread';

src/entities/Font/model/store/appliedFontsStore/utils/yieldToMainThread/yieldToMainThread.test.ts

@@ -0,0 +1,17 @@
+import { yieldToMainThread } from './yieldToMainThread';
+
+describe('yieldToMainThread', () => {
+    it('uses scheduler.yield when available', async () => {
+        const mockYield = vi.fn().mockResolvedValue(undefined);
+        vi.stubGlobal('scheduler', { yield: mockYield });
+
+        await yieldToMainThread();
+
+        expect(mockYield).toHaveBeenCalledOnce();
+        vi.unstubAllGlobals();
+    });
+    it('falls back to MessageChannel when scheduler is unavailable', async () => {
+        // scheduler is not defined in jsdom by default
+        await expect(yieldToMainThread()).resolves.toBeUndefined();
+    });
+});

src/entities/Font/model/store/appliedFontsStore/utils/yieldToMainThread/yieldToMainThread.ts

@@ -0,0 +1,16 @@
+/**
+ * Yields to main thread during CPU-intensive parsing. Uses scheduler.yield() where available or MessageChannel fallback.
+ */
+export async function yieldToMainThread(): 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);
+        });
+    }
+}