Fixed a series of z-index related issues, that made other elements ap…

…pear in front of the sidebar on mobile. (#5165)

edit-post/assets/stylesheets/_z-index.scss

@@ -25,6 +25,7 @@ $z-layers: (
 	'.editor-block-mover': 1,
 	'.blocks-gallery-item__inline-menu': 20,
 	'.editor-block-settings-menu__popover': 20, // Below the header
+	'.blocks-url-input__suggestions': 30,
 	'.edit-post-header': 30,
 	'.wp-block-image__resize-handlers': 1, // Resize handlers above sibling inserter
 
@@ -37,6 +38,10 @@ $z-layers: (
 	'.edit-post-sidebar': 100000,
 	'.edit-post-layout .edit-post-post-publish-panel': 100001,
 
+	// Show sidebar in greater than small viewports above editor related elements
+	// but bellow #adminmenuback { z-index: 100 }
+	'.edit-post-sidebar {greater than small}': 50,
+
 	// Show notices below expanded wp-admin submenus:
 	// #adminmenuwrap { z-index: 9990 }
 	'.components-notice-list': 9989,
@@ -45,7 +50,6 @@ $z-layers: (
 	// #adminmenuwrap { z-index: 9990 }
 	'.components-popover': 1000000,
 	'.components-autocomplete__results': 1000000,
-	'.blocks-url-input__suggestions': 9999,
 );
 
 @function z-index( $key ) {

edit-post/components/sidebar/style.scss

@@ -13,7 +13,7 @@
 
 	@include break-small() {
 		top: $admin-bar-height-big + $header-height;
-		z-index: auto;
+		z-index: z-index( '.edit-post-sidebar {greater than small}' );
 		height: auto;
 		overflow: auto;
 		-webkit-overflow-scrolling: touch;