Index: src/wp-admin/css/customize-controls.css
===================================================================
--- src/wp-admin/css/customize-controls.css	(revision 33611)
+++ src/wp-admin/css/customize-controls.css	(working copy)
@@ -1294,7 +1294,6 @@
 	#customize-preview {
 		top: 45px;
 		bottom: 0;
-		height: auto;
 	}
 
 	.wp-core-ui.wp-customizer .button {
Index: src/wp-admin/css/customize-nav-menus.css
===================================================================
--- src/wp-admin/css/customize-nav-menus.css	(revision 33611)
+++ src/wp-admin/css/customize-nav-menus.css	(working copy)
@@ -710,7 +710,11 @@
 	top: 60px; /* below title div / search input */
 	bottom: 0px; /* 100% height that still triggers lazy load */
 	max-height: none;
-	width: 270px;
+	width: 100%;
+	padding: 1px 15px 15px;
+	-webkit-box-sizing: border-box;
+	-moz-box-sizing: border-box;
+	box-sizing: border-box;
 }
 
 #available-menu-items .menu-item-tpl {
@@ -1036,6 +1040,12 @@
 	display: none;
 }
 
+@media screen and ( max-width: 782px ) {
+	#available-menu-items #available-menu-items-search .accordion-section-content {
+		top: 63px;
+	}
+}
+
 @media screen and ( max-width: 640px ) {
 	body.adding-menu-items div#available-menu-items {
 		top: 46px;
@@ -1044,6 +1054,10 @@
 		width: 100%;
 	}
 
+	#available-menu-items #available-menu-items-search .accordion-section-content {
+		top: 133px;
+	}
+
 	#available-menu-items .customize-section-title {
 		display: block;
 		margin: 0;
Index: src/wp-admin/js/customize-nav-menus.js
===================================================================
--- src/wp-admin/js/customize-nav-menus.js	(revision 33611)
+++ src/wp-admin/js/customize-nav-menus.js	(working copy)
@@ -360,18 +360,6 @@
 			});
 		},
 
-		// Adjust the height of each section of items to fit the screen.
-		itemSectionHeight: function() {
-			var sections, totalHeight, accordionHeight, diff;
-			totalHeight = window.innerHeight;
-			sections = this.$el.find( '.accordion-section-content' );
-			accordionHeight =  46 * ( 1 + sections.length ) - 16; // Magic numbers.
-			diff = totalHeight - accordionHeight;
-			if ( 120 < diff && 290 > diff ) {
-				sections.css( 'max-height', diff );
-			}
-		},
-
 		// Highlights a menu item.
 		select: function( menuitemTpl ) {
 			this.selected = $( menuitemTpl );
@@ -466,8 +454,6 @@
 		open: function( menuControl ) {
 			this.currentMenuControl = menuControl;
 
-			this.itemSectionHeight();
-
 			$( 'body' ).addClass( 'adding-menu-items' );
 
 			// Collapse all controls.
@@ -538,7 +524,12 @@
 				content = panelMeta.find( '.customize-panel-description' ),
 				options = $( '#screen-options-wrap' ),
 				button = panelMeta.find( '.customize-screen-options-toggle' );
-			button.on( 'click', function() {
+			button.on( 'click keydown', function( event ) {
+				if ( api.utils.isKeydownButNotEnterEvent( event ) ) {
+					return;
+				}
+				event.preventDefault();
+
 				// Hide description
 				if ( content.not( ':hidden' ) ) {
 					content.slideUp( 'fast' );
@@ -561,7 +552,12 @@
 			} );
 
 			// Help toggle
-			help.on( 'click', function() {
+			help.on( 'click keydown', function( event ) {
+				if ( api.utils.isKeydownButNotEnterEvent( event ) ) {
+					return;
+				}
+				event.preventDefault();
+
 				if ( 'true' === button.attr( 'aria-expanded' ) ) {
 					button.attr( 'aria-expanded', 'false' );
 					help.attr( 'aria-expanded', 'true' );