feat(ComparisonView): replace window resize listener with ResiseObserver on the container to catch the container size change on sidebar open/close

src/widgets/ComparisonView/ui/SliderArea/SliderArea.svelte

@@ -61,6 +61,26 @@ const comparisonEngine = new CharacterComparisonEngine();
 
 let layoutResult = $state<ReturnType<typeof comparisonEngine.layout>>({ lines: [], totalHeight: 0 });
 
+// Track container width changes (window resize, sidebar toggle, etc.)
+$effect(() => {
+    if (!container) {
+        return;
+    }
+
+    const observer = new ResizeObserver(entries => {
+        for (const entry of entries) {
+            // Use borderBoxSize if available, fallback to contentRect
+            const width = entry.borderBoxSize?.[0]?.inlineSize ?? entry.contentRect.width;
+            if (width > 0) {
+                containerWidth = width;
+            }
+        }
+    });
+
+    observer.observe(container);
+    return () => observer.disconnect();
+});
+
 const sliderSpring = new Spring(50, {
     stiffness: 0.2,
     damping: 0.7,
@@ -124,25 +144,25 @@ $effect(() => {
     }
 });
 
+// Layout effect — depends on content, settings AND containerWidth
 $effect(() => {
     const _text = comparisonStore.text;
     const _weight = typography.weight;
     const _size = typography.renderedSize;
     const _height = typography.height;
     const _spacing = typography.spacing;
+    const _width = containerWidth;
+    const _isMobile = isMobile;
 
-    if (container && fontA && fontB) {
+    if (container && fontA && fontB && _width > 0) {
         // PRETEXT API strings: "weight sizepx family"
         const fontAStr = getPretextFontString(_weight, _size, fontA.name);
         const fontBStr = getPretextFontString(_weight, _size, fontB.name);
 
-        // Use offsetWidth to avoid transform scaling issues
-        const width = container.offsetWidth;
-        const padding = isMobile ? 48 : 96;
-        const availableWidth = width - padding;
+        const padding = _isMobile ? 48 : 96;
+        const availableWidth = Math.max(0, _width - padding);
         const lineHeight = _size * _height;
 
-        containerWidth = width;
         layoutResult = comparisonEngine.layout(
             _text,
             fontAStr,
@@ -155,30 +175,6 @@ $effect(() => {
     }
 });
 
-$effect(() => {
-    if (typeof window === 'undefined') {
-        return;
-    }
-    const handleResize = () => {
-        if (container && fontA && fontB) {
-            const width = container.offsetWidth;
-            const padding = isMobile ? 48 : 96;
-            containerWidth = width;
-            layoutResult = comparisonEngine.layout(
-                comparisonStore.text,
-                getPretextFontString(typography.weight, typography.renderedSize, fontA.name),
-                getPretextFontString(typography.weight, typography.renderedSize, fontB.name),
-                width - padding,
-                typography.renderedSize * typography.height,
-                typography.spacing,
-                typography.renderedSize,
-            );
-        }
-    };
-    window.addEventListener('resize', handleResize);
-    return () => window.removeEventListener('resize', handleResize);
-});
-
 // Dynamic backgroundSize based on isMobile — can't express this in Tailwind.
 // Color is set to currentColor so it respects dark mode via text color.
 const gridStyle = $derived(
@@ -273,8 +269,8 @@ const scaleClass = $derived(
         class={cn(
             'absolute z-10',
             responsive.isMobileOrTablet
-                ? 'bottom-4 right-4 -translate-1/2'
-                : 'bottom-5 left-1/2 right-[unset] -translate-x-1/2',
+                ? 'bottom-0 right-0 -translate-1/2'
+                : 'bottom-2.5 left-1/2 -translate-x-1/2',
         )}
     />
 </div>