Add scroll lock when mobile menu is open

src/components/site-header/site-header.tsx

@@ -72,6 +72,13 @@ export class SiteHeader {
   @Listen('mobileMenuToggle') mobileMenuToggle() {
     this.isMenuOpen = !this.isMenuOpen;
     this.menuMobileToggled.emit();
+    let bodyElement = document.getElementsByTagName('body')[0] as HTMLElement;
+
+    if (this.isMenuOpen) {
+      bodyElement.style.overflow = 'hidden';
+    } else {
+      bodyElement.style.overflow = 'initial';
+    }
   }
 
   // Listener for toggling mobile menu top panel on or off.