diff --git a/resources/js/ext.uls.dialog.js b/resources/js/ext.uls.dialog.js
index 1b52102c..013a99c2 100644
--- a/resources/js/ext.uls.dialog.js
+++ b/resources/js/ext.uls.dialog.js
@@ -1,10 +1,141 @@
+/*!
+ * A simple dialog to be used inside ULS.
+ *
+ * @private
+ * @since 2020.01
+ *
+ * Copyright (C) 2019-2020 Alolita Sharma, Amir Aharoni, Arun Ganesh, Brandon Harris,
+ * Niklas Laxström, Pau Giner, Santhosh Thottingal, Siebrand Mazeland and other
+ * contributors. See CREDITS for a list.
+ *
+ * UniversalLanguageSelector is dual licensed GPLv2 or later and MIT. You don't
+ * have to do anything special to choose one license or the other and you don't
+ * have to notify anyone which license you are using. You are free to use
+ * UniversalLanguageSelector in commercial projects as long as the copyright
+ * header is left intact. See files GPL-LICENSE and MIT-LICENSE for details.
+ *
+ * @file
+ * @ingroup Extensions
+ * @licence GNU General Public Licence 2.0 or later
+ * @licence MIT License
+ */
+
 ( function () {
 	'use strict';
 
 	var ULSDialog = function ( options ) {
 		var $dialog = options.container,
 			hasOverlay = options.hasOverlay,
-			$overlay;
+			$overlay,
+			// Source: https://github.com/ghosh/Micromodal/blob/master/lib/src/index.js#L4
+			FOCUSABLE_NODES = [
+				'a[href]',
+				'area[href]',
+				'input:not([disabled]):not([type="hidden"]):not([aria-hidden])',
+				'select:not([disabled]):not([aria-hidden])',
+				'textarea:not([disabled]):not([aria-hidden])',
+				'button:not([disabled]):not([aria-hidden])',
+				'iframe',
+				'object',
+				'embed',
+				'[contenteditable]',
+				'[tabindex]:not([tabindex^="-"])'
+			];
+
+		function getFocusableNodes() {
+			return $dialog.find( FOCUSABLE_NODES.join( ', ' ) );
+		}
+
+		function isElementInDialog( targetElement ) {
+			return $dialog.get( 0 ).contains( targetElement );
+		}
+
+		function focusOverlay() {
+			if ( $overlay ) {
+				$overlay.get( 0 ).focus();
+			}
+		}
+
+		function focusFirstNodeOrOverlay( $focusableNodes ) {
+			if ( $focusableNodes === undefined ) {
+				$focusableNodes = getFocusableNodes();
+			}
+
+			if ( $focusableNodes.length ) {
+				$focusableNodes.get( 0 ).focus();
+			} else {
+				focusOverlay();
+			}
+		}
+
+		function maintainFocus( event ) {
+			var $focusableNodes = getFocusableNodes(),
+				focusedItemIndex;
+
+			if ( !hasOverlay ) {
+				// overlay is not present, so let tabbing flow as normal.
+				return;
+			}
+
+			if ( !$focusableNodes.length ) {
+				// no focusable node in the dialog, focus on the overlay.
+				focusOverlay();
+				return;
+			}
+
+			if ( !isElementInDialog( document.activeElement ) ) {
+				focusFirstNodeOrOverlay( $focusableNodes );
+			} else {
+				focusedItemIndex = $focusableNodes.index( document.activeElement );
+
+				if ( event.shiftKey && focusedItemIndex === 0 ) {
+					$focusableNodes.get( -1 ).focus();
+					event.preventDefault();
+				} else if ( !event.shiftKey && focusedItemIndex === $focusableNodes.length - 1 ) {
+					focusFirstNodeOrOverlay( $focusableNodes );
+					event.preventDefault();
+				}
+			}
+		}
+
+		function handleFirstFocus( event ) {
+			if ( !hasOverlay ) {
+				// Overlay is not present, so let tabbing flow as normal.
+				return;
+			}
+
+			if ( isElementInDialog( event.target ) ) {
+				return;
+			}
+
+			focusFirstNodeOrOverlay();
+		}
+
+		function onKeydown( event ) {
+			switch ( event.key ) {
+				case 'Esc':
+				case 'Escape':
+					// eslint-disable-next-line no-use-before-define
+					close();
+					event.preventDefault();
+					break;
+				case 'Tab':
+					maintainFocus( event );
+					break;
+			}
+		}
+
+		function addEvents() {
+			$( document )
+				.on( 'keydown', onKeydown )
+				.on( 'focusin', handleFirstFocus );
+		}
+
+		function removeEvents() {
+			$( document )
+				.off( 'keydown', onKeydown )
+				.off( 'focusin', handleFirstFocus );
+		}
 
 		function showOverlay() {
 			if ( $overlay ) {
@@ -22,11 +153,14 @@
 
 		function open() {
 			$dialog.show();
+			addEvents();
 			showOverlay();
+			focusFirstNodeOrOverlay();
 		}
 
 		function close() {
 			$dialog.hide();
+			removeEvents();
 			hideOverlay();
 		}
 
@@ -39,6 +173,7 @@
 			if ( !$overlay ) {
 				$overlay = $( '<div>' )
 					.addClass( 'uls-overlay' )
+					.prop( 'tabindex', '-1' )
 					.on( 'click', close )
 					.appendTo( document.body );
 			}
diff --git a/resources/js/ext.uls.setlang.js b/resources/js/ext.uls.setlang.js
index 5f887718..82f54873 100644
--- a/resources/js/ext.uls.setlang.js
+++ b/resources/js/ext.uls.setlang.js
@@ -1,3 +1,25 @@
+/*!
+ * Loaded when setlang query paramter is set on the page.
+ *
+ * @private
+ * @since 2020.01
+ *
+ * Copyright (C) 2019-2020 Alolita Sharma, Amir Aharoni, Arun Ganesh, Brandon Harris,
+ * Niklas Laxström, Pau Giner, Santhosh Thottingal, Siebrand Mazeland and other
+ * contributors. See CREDITS for a list.
+ *
+ * UniversalLanguageSelector is dual licensed GPLv2 or later and MIT. You don't
+ * have to do anything special to choose one license or the other and you don't
+ * have to notify anyone which license you are using. You are free to use
+ * UniversalLanguageSelector in commercial projects as long as the copyright
+ * header is left intact. See files GPL-LICENSE and MIT-LICENSE for details.
+ *
+ * @file
+ * @ingroup Extensions
+ * @licence GNU General Public Licence 2.0 or later
+ * @licence MIT License
+ */
+
 ( function () {
 	'use strict';
 	var $cancelBtn, $acceptBtn;