Customize: Fix keyboard a11y on back button.

Following [59224], the back button for section navigation was no longer keyboard navigable. This was caused by an accidental removal of scripts that conditionally handled the `tabindex` value of this control to make it focusable only when visible.

Fixes the issue by restoring code removed in [59224].

Props wildworks, poojapadamad, abcd95, ankitmaru, iamshashank, sourabhjain, joedolson.
Fixes #63011.

git-svn-id: https://develop.svn.wordpress.org/trunk@60482 602fd350-edb4-49c9-b593-d223f7449a82

Author: joedolson

src/js/_enqueues/wp/customize/controls.js

@@ -1615,6 +1615,7 @@
 				} else {
 					expand = function() {
 						section._animateChangeExpanded( function() {
+							backBtn.attr( 'tabindex', '0' );
 							backBtn.trigger( 'focus' );
 							content.css( 'top', '' );
 							container.scrollTop( 0 );
@@ -1660,7 +1661,7 @@
 					}
 				}
 				section._animateChangeExpanded( function() {
-
+					backBtn.attr( 'tabindex', '-1' );
 					sectionTitle.trigger( 'focus' );
 					content.css( 'top', '' );
 
@@ -2714,6 +2715,7 @@
 				} else {
 					expand = function() {
 						section._animateChangeExpanded( function() {
+							backBtn.attr( 'tabindex', '0' );
 							backBtn.trigger( 'focus' );
 							content.css( 'top', '' );
 							container.scrollTop( 0 );
@@ -2744,7 +2746,7 @@
 					}
 				}
 				section._animateChangeExpanded( function() {
-
+					backBtn.attr( 'tabindex', '-1' );
 					sectionTitle.trigger( 'focus' );
 					content.css( 'top', '' );
 
@@ -2964,6 +2966,7 @@
 					} );
 				} else {
 					panel._animateChangeExpanded( function() {
+						backBtn.attr( 'tabindex', '0' );
 						backBtn.trigger( 'focus' );
 						accordionSection.css( 'top', '' );
 						container.scrollTop( 0 );