feat(comparisonStore): add checkFontsLoading method to improve isLoading flag

src/widgets/ComparisonSlider/model/stores/comparisonStore.svelte.ts

@@ -34,6 +34,7 @@ class ComparisonStore {
     #fontB = $state<UnifiedFont | undefined>();
     #sampleText = $state('The quick brown fox jumps over the lazy dog');
     #isRestoring = $state(true);
+    #fontsReady = $state(false);
     #typography = createTypographyControlManager(DEFAULT_TYPOGRAPHY_CONTROLS_DATA, 'glyphdiff:comparison:typography');
 
     constructor() {
@@ -49,6 +50,7 @@ class ComparisonStore {
 
                 // If we already have a selection, do nothing
                 if (this.#fontA && this.#fontB) {
+                    this.#checkFontsLoaded();
                     return;
                 }
 
@@ -66,6 +68,48 @@ class ComparisonStore {
         });
     }
 
+    /**
+     * Checks if fonts are actually loaded in the browser at current weight.
+     * Uses CSS Font Loading API to prevent FOUT.
+     */
+    async #checkFontsLoaded() {
+        if (!('fonts' in document)) {
+            this.#fontsReady = true;
+            return;
+        }
+
+        this.#fontsReady = false;
+
+        const weight = this.#typography.weight;
+        const size = this.#typography.renderedSize;
+        const fontAName = this.#fontA?.name;
+        const fontBName = this.#fontB?.name;
+
+        if (!fontAName || !fontBName) return;
+
+        try {
+            // Step 1: Load fonts into memory
+            await Promise.all([
+                document.fonts.load(`${weight} ${size}px "${fontAName}"`),
+                document.fonts.load(`${weight} ${size}px "${fontBName}"`),
+            ]);
+
+            // Step 2: Wait for browser to be ready to render
+            await document.fonts.ready;
+
+            // Step 3: Force a layout/paint cycle (critical!)
+            await new Promise(resolve => {
+                requestAnimationFrame(() => {
+                    requestAnimationFrame(resolve); // Double rAF ensures paint completes
+                });
+            });
+
+            this.#fontsReady = true;
+        } catch (error) {
+            console.warn('[ComparisonStore] Font loading failed:', error);
+            setTimeout(() => this.#fontsReady = true, 1000);
+        }
+    }
     /**
      * Restore state from persistent storage
      */
@@ -141,13 +185,12 @@ class ComparisonStore {
      * Check if both fonts are selected
      */
     get isReady() {
-        return !!this.#fontA && !!this.#fontB;
+        return !!this.#fontA && !!this.#fontB && this.#fontsReady;
     }
 
     get isLoading() {
-        return this.#isRestoring;
+        return this.#isRestoring || !this.#fontsReady;
     }
-
     /**
      * Public initializer (optional, as constructor starts it)
      * Kept for compatibility if manual re-init is needed