Index: src/wp-admin/css/customize-controls.css
===================================================================
--- src/wp-admin/css/customize-controls.css	(revision 30348)
+++ src/wp-admin/css/customize-controls.css	(working copy)
@@ -228,8 +228,8 @@
 	transition: left .18s ease-in-out, color .1s ease-in-out, background .1s ease-in-out;
 }
 
-.collapsed .control-panel-back {
-	display: none;
+.collapsed .customize-controls-close {
+	padding-right: 0;
 }
 
 .customize-controls-close:focus,
@@ -469,19 +469,82 @@
 	box-shadow: none !important;
 	-webkit-border-radius: 0 !important;
 	border-radius: 0 !important;
+	position: fixed;
+	left: 0;
+	bottom: 0;
 }
 
-
 .collapse-sidebar:active,
 .collapse-sidebar:active .collapse-sidebar-label,
 .collapse-sidebar:active .collapse-sidebar-arrow:before {
 	text-shadow: none;
 }
 
-.collapsed .collapse-sidebar-arrow:before {
-	color: #888;
+
+/* Collapsed Mode */
+.collapsed .control-panel-back,
+.expanded .expand-sidebar-label {
+	display: none;
 }
 
+.collapsed .accordion-section-content {
+	display: none !important;
+}
+
+.collapsed.in-sub-panel #customize-info,
+.collapsed.in-sub-panel #customize-theme-controls > ul > .accordion-section,
+.collapsed #customize-theme-controls .control-section.current-panel > h3.accordion-section-title {
+	left: 0;
+}
+
+@media screen and (min-width:600px) {
+	.collapsed #customize-preview {
+		left: 46px;
+	}
+
+	.wp-full-overlay.collapsed #customize-controls {
+		margin-left: 0;
+		width: 46px;
+		white-space: nowrap;
+	}
+
+	.collapsed .wp-full-overlay-main {
+		right: 0;
+		width: auto;
+	}
+
+	.collapsed #customize-info,
+	.collapsed #customize-controls .accordion-section-title:after {
+		display: none;
+	}
+
+	.collapsed .control-section .accordion-section-title,
+	.collapsed .control-panel .accordion-section-title { 
+		white-space: nowrap;
+		overflow: hidden;
+		text-overflow: ellipsis;
+	}
+
+	.collapsed #customize-controls .wp-full-overlay-sidebar-content {
+		overflow-y: hidden;
+	}
+
+	/* No animation coming in, as it causes layout issues */
+	.expanded.wp-full-overlay,
+	.expanded #customize-preview,
+	.expanded #customize-controls {
+		transition: none;
+	}
+
+	/* No animation going out, as it causes layout issues */
+	.collapsed.wp-full-overlay,
+	.collapsed #customize-preview,
+	.collapsed #customize-controls {
+		transition: none;
+	}
+}
+
+
 /* Style for custom settings */
 
 /*
Index: src/wp-admin/css/customize-widgets.css
===================================================================
--- src/wp-admin/css/customize-widgets.css	(revision 30348)
+++ src/wp-admin/css/customize-widgets.css	(working copy)
@@ -394,7 +394,7 @@
 	color: #777;
 }
 
-#customize-preview {
+.in-sub-panel #customize-preview {
 	-webkit-transition: all 0.2s;
 	transition: all 0.2s;
 }
Index: src/wp-admin/css/themes.css
===================================================================
--- src/wp-admin/css/themes.css	(revision 30348)
+++ src/wp-admin/css/themes.css	(working copy)
@@ -1468,7 +1468,7 @@
 
 /* Collapse Button */
 .wp-full-overlay a.collapse-sidebar {
-	position: absolute;
+	position: fixed;
 	bottom: 12px;
 	left: 0;
 	z-index: 50;
@@ -1487,11 +1487,6 @@
 	color: #0074a2;
 }
 
-.wp-full-overlay.collapsed .collapse-sidebar {
-	position: absolute;
-	left: 100%;
-}
-
 .wp-full-overlay .collapse-sidebar-arrow {
 	position: static;
 	margin-top: 0;
Index: src/wp-admin/customize.php
===================================================================
--- src/wp-admin/customize.php	(revision 30348)
+++ src/wp-admin/customize.php	(working copy)
@@ -174,9 +174,10 @@
 		</div>
 
 		<div id="customize-footer-actions" class="wp-full-overlay-footer">
-			<a href="#" class="collapse-sidebar button-secondary" title="<?php esc_attr_e('Collapse Sidebar'); ?>">
+			<a href="#" class="collapse-sidebar button-secondary">
 				<span class="collapse-sidebar-arrow"></span>
-				<span class="collapse-sidebar-label"><?php _e('Collapse'); ?></span>
+				<span class="collapse-sidebar-label"><?php _e( 'Collapse controls' ); ?></span>
+				<span class="expand-sidebar-label"><span class="screen-reader-text"><?php _e( 'Expand controls' ); ?></span></span>
 			</a>
 		</div>
 	</form>
Index: src/wp-admin/js/customize-controls.js
===================================================================
--- src/wp-admin/js/customize-controls.js	(revision 30348)
+++ src/wp-admin/js/customize-controls.js	(working copy)
@@ -2112,6 +2112,41 @@
 			event.preventDefault();
 		});
 
+		// Expand Customizer controls on section icon click in collapsed mode.
+		$('.accordion-section-title').on( 'click keydown', function( event ) {
+			if ( isKeydownButNotEnterEvent( event ) ) { // enter
+				return;
+			} else if ( overlay.hasClass( 'collapsed' ) ) {
+				var section = $( this ).closest( '.control-section' );
+				if ( section.hasClass( 'control-panel' ) ) {
+					if ( section.hasClass( 'current-panel' ) ) {
+						// Prevent panel-expand from triggering.
+						event.stopImmediatePropagation();
+					} else {
+						if ( overlay.hasClass( 'in-sub-panel' ) ) {
+							$( '.control-panel-back' ).trigger( 'click' );
+						}
+					}
+				} else {
+					if ( section.hasClass( 'open' ) ) {
+						// Prevent the section from trying to close.
+						event.stopImmediatePropagation();
+					}
+					else {
+						// Close any open sections/panels.
+						if ( overlay.hasClass( 'in-sub-panel' ) ) {
+							$( '.control-panel-back' ).trigger( 'click' );
+						} else {
+							$( '.control-section.open .accordion-section-title' ).trigger( 'click' );
+						}
+					}
+				}
+				// Expand the controls.
+				$( '.collapse-sidebar' ).trigger( 'click' );
+			}
+		});
+
+
 		// Bind site title display to the corresponding field.
 		if ( title.length ) {
 			$( '#customize-control-blogname input' ).on( 'input', function() {