Site Editor Sidebar: Hide horizontal scrollbar when navigating (#63194)

* Site Editor Sidebar: Hide horizontal scrollbar when navigating

* Add `contain: content` and comment

Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: jsnajdr <jsnajdr@git.wordpress.org>

packages/edit-site/src/components/sidebar/style.scss

@@ -1,6 +1,12 @@
 .edit-site-sidebar__content {
 	flex-grow: 1;
 	overflow-y: auto;
+	// Prevents horizontal overflow while animating screen transitions
+	overflow-x: hidden;
+	// Mark this section of the DOM as isolated, providing performance benefits
+	// by limiting calculations of layout, style and paint to a DOM subtree rather
+	// than the entire page.
+	contain: content;
 }
 
 @keyframes local--slide-from-right {