Index: src/wp-admin/css/customize-controls.css
===================================================================
--- src/wp-admin/css/customize-controls.css	(revision 40475)
+++ src/wp-admin/css/customize-controls.css	(working copy)
@@ -1189,20 +1189,18 @@
 .customize-section-description-container + #customize-control-custom_css:last-child {
 	margin-left: -12px;
 	width: 299px;
+	width: -webkit-calc( 100% + 24px );
+	width: calc( 100% + 24px );
 	margin-bottom: -12px;
 }
 
 @media screen and ( max-width: 640px ) {
 	.customize-section-description-container + #customize-control-custom_css:last-child {
-		margin-left: 0;
 		margin-right: 0;
-		width: 100%;
 	}
 	.customize-section-description-container + #customize-control-custom_css:last-child textarea {
 		height: -webkit-calc( 100vh - 140px );
 		height: calc( 100vh - 140px );
-		width: 100%;
-		border: solid 1px #ddd;
 	}
 }
 
Index: src/wp-admin/css/themes.css
===================================================================
--- src/wp-admin/css/themes.css	(revision 40475)
+++ src/wp-admin/css/themes.css	(working copy)
@@ -1332,7 +1332,9 @@
 	-moz-box-sizing: border-box;
 	box-sizing: border-box;
 	position: fixed;
-	width: 300px;
+	min-width: 300px;
+	max-width: 600px;
+	width: 18%;
 	height: 100%;
 	top: 0;
 	bottom: 0;
@@ -1361,6 +1363,26 @@
 	margin-left: -300px;
 }
 
+@media screen and (min-width: 1667px) {
+	.wp-full-overlay.expanded {
+		margin-left: 18%;
+	}
+
+	.wp-full-overlay.collapsed .wp-full-overlay-sidebar {
+		margin-left: -18%;
+	}
+}
+
+@media screen and (min-width: 3333px) {
+	.wp-full-overlay.expanded {
+		margin-left: 600px;
+	}
+
+	.wp-full-overlay.collapsed .wp-full-overlay-sidebar {
+		margin-left: -600px;
+	}
+}
+
 .wp-full-overlay-sidebar:after {
 	content: "";
 	display: block;
@@ -1520,7 +1542,6 @@
 	box-shadow: none !important;
 	-webkit-border-radius: 0 !important;
 	border-radius: 0 !important;
-	z-index: -1; /* Below device buttons */
 }
 
 .wp-core-ui .wp-full-overlay .collapse-sidebar:hover,
@@ -1616,14 +1637,20 @@
 	position: fixed;
 	bottom: 0;
 	left: 0;
-	width: 299px;
+	min-width: 299px;
+	max-width: 599px;
+	width: 18%;
 	height: 45px;
 	border-top: 1px solid #ddd;
 	background: #eee;
 }
 
-.wp-full-overlay-footer .devices {
+.wp-full-overlay-footer .devices-wrapper {
 	float: right;
+}
+
+.wp-full-overlay-footer .devices {
+	position: relative;
 	background: #eee;
 	-webkit-box-shadow: -20px 0 10px -5px #eee;
 	box-shadow: -20px 0 10px -5px #eee;
Index: src/wp-admin/customize.php
===================================================================
--- src/wp-admin/customize.php	(revision 40475)
+++ src/wp-admin/customize.php	(working copy)
@@ -171,30 +171,32 @@
 		</div>
 
 		<div id="customize-footer-actions" class="wp-full-overlay-footer">
-			<?php $previewable_devices = $wp_customize->get_previewable_devices(); ?>
-			<?php if ( ! empty( $previewable_devices ) ) : ?>
-			<div class="devices">
-				<?php foreach ( (array) $previewable_devices as $device => $settings ) : ?>
-					<?php
-					if ( empty( $settings['label'] ) ) {
-						continue;
-					}
-					$active = ! empty( $settings['default'] );
-					$class = 'preview-' . $device;
-					if ( $active ) {
-						$class .= ' active';
-					}
-					?>
-					<button type="button" class="<?php echo esc_attr( $class ); ?>" aria-pressed="<?php echo esc_attr( $active ) ?>" data-device="<?php echo esc_attr( $device ); ?>">
-						<span class="screen-reader-text"><?php echo esc_html( $settings['label'] ); ?></span>
-					</button>
-				<?php endforeach; ?>
-			</div>
-			<?php endif; ?>
 			<button type="button" class="collapse-sidebar button" aria-expanded="true" aria-label="<?php echo esc_attr( _x( 'Hide Controls', 'label for hide controls button without length constraints' ) ); ?>">
 				<span class="collapse-sidebar-arrow"></span>
 				<span class="collapse-sidebar-label"><?php _ex( 'Hide Controls', 'short (~12 characters) label for hide controls button' ); ?></span>
 			</button>
+			<?php $previewable_devices = $wp_customize->get_previewable_devices(); ?>
+			<?php if ( ! empty( $previewable_devices ) ) : ?>
+			<div class="devices-wrapper">
+				<div class="devices">
+					<?php foreach ( (array) $previewable_devices as $device => $settings ) : ?>
+						<?php
+						if ( empty( $settings['label'] ) ) {
+							continue;
+						}
+						$active = ! empty( $settings['default'] );
+						$class = 'preview-' . $device;
+						if ( $active ) {
+							$class .= ' active';
+						}
+						?>
+						<button type="button" class="<?php echo esc_attr( $class ); ?>" aria-pressed="<?php echo esc_attr( $active ) ?>" data-device="<?php echo esc_attr( $device ); ?>">
+							<span class="screen-reader-text"><?php echo esc_html( $settings['label'] ); ?></span>
+						</button>
+					<?php endforeach; ?>
+				</div>
+			</div>
+			<?php endif; ?>
 		</div>
 	</form>
 	<div id="customize-preview" class="wp-full-overlay-main"></div>