Media: cleanup recent changes to `media.view.FocusManager`.

* Adjusts the focus method to target the menu for the current modal
* Refactor the FocusManager so that the method names and the functionality matches more closely with how it currently works. The changes in 4.0 to the FocusManager change the purpose of the focus manager entirely — instead of being a way to save and restore focus state, the new FocusManager is about constraining keyboard navigation via the tab key to inside the modal. 

Props gcorne.
See #23560.

Built from https://develop.svn.wordpress.org/trunk@29602


git-svn-id: http://core.svn.wordpress.org/trunk@29376 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/js/media-views.js

@@ -2498,9 +2498,7 @@
 
 						// Keep focus inside media modal
 						// after canceling a gallery
-						new media.view.FocusManager({
-							el: this.el
-						}).focus();
+						this.controller.modal.focusManager.focus();
 					}
 				},
 				separateCancel: new media.View({
@@ -2689,9 +2687,7 @@
 
 					// Keep focus inside media modal
 					// after jumping to gallery view
-					new media.view.FocusManager({
-						el: this.el
-					}).focus();
+					this.controller.modal.focusManager.focus();
 				}
 			});
 		},
@@ -2721,9 +2717,7 @@
 
 					// Keep focus inside media modal
 					// after jumping to playlist view
-					new media.view.FocusManager({
-						el: this.el
-					}).focus();
+					this.controller.modal.focusManager.focus();
 				}
 			});
 		},
@@ -2753,9 +2747,7 @@
 
 					// Keep focus inside media modal
 					// after jumping to video playlist view
-					new media.view.FocusManager({
-						el: this.el
-					}).focus();
+					this.controller.modal.focusManager.focus();
 				}
 			});
 		},
@@ -3253,7 +3245,7 @@
 				}
 			}
 
-			this.$( '.media-modal-close' ).focus();
+			this.$el.focus();
 
 			return this.propagate('open');
 		},
@@ -3351,42 +3343,37 @@
 	 * @augments Backbone.View
 	 */
 	media.view.FocusManager = media.View.extend({
+
 		events: {
-			keydown: 'recordTab',
-			focusin: 'updateIndex'
+			'keydown': 'constrainTabbing'
 		},
 
-		focus: function() {
-			// Reset focus on first left menu item
-			$('.media-menu-item').first().focus();
+		focus: function() { // Reset focus on first left menu item
+			this.$('.media-menu-item').first().focus();
 		},
 		/**
 		 * @param {Object} event
 		 */
-		recordTab: function( event ) {
+		constrainTabbing: function( event ) {
+			var tabbables;
+
 			// Look for the tab key.
 			if ( 9 !== event.keyCode ) {
 				return;
 			}
 
+			tabbables = this.$( ':tabbable' );
+
 			// Keep tab focus within media modal while it's open
-			if ( event.target === this.tabbableLast[0] && !event.shiftKey ) {
-				this.tabbableFirst.focus();
+			if ( tabbables.last()[0] === event.target && ! event.shiftKey ) {
+				tabbables.first().focus();
 				return false;
-			} else if ( event.target === this.tabbableFirst[0] && event.shiftKey ) {
-				this.tabbableLast.focus();
+			} else if ( tabbables.first()[0] === event.target && event.shiftKey ) {
+				tabbables.last().focus();
 				return false;
 			}
-		},
-		/**
-		 * @param {Object} event
-		 */
-		updateIndex: function() {
-			// Resets tabbable elements
-			this.tabbables = $( ':tabbable', this.$el );
-			this.tabbableFirst = this.tabbables.filter( ':first' );
-			this.tabbableLast = this.tabbables.filter( ':last' );
 		}
+
 	});
 
 	/**
@@ -6165,9 +6152,7 @@
 
 			// Keep focus inside media modal
 			// after clear link is selected
-			new media.view.FocusManager({
-				el: this.el
-			}).focus();
+			this.controller.modal.focusManager.focus();
 		}
 	});
 
@@ -6520,9 +6505,7 @@
 				this.model.destroy();
 				// Keep focus inside media modal
 				// after image is deleted
-				new media.view.FocusManager({
-					el: this.el
-				}).focus();
+				this.controller.modal.focusManager.focus();
 			}
 		},
 		/**