I upgraded my dev environment to 4.9, and the customizer chokes on my _underscores-based theme. The customizer works fine on TwentySixteen, so I think there is a problem with my theme.
The tell was that the Customizer's Save & Publish button is missing entirely from the UI. Some other strange things happen, like the labels for my settings under the Colors panel are not visible until you click on the color picker.
Firefox dev console says
TypeError: a.elements is undefined [Learn More] load-scripts.php:6:12121
And when I click [Learn more] it shows
<anonymous> http://localhost/test-site/wp-admin/load-scripts.php:6:12121 i http://localhost/test-site/wp-admin/load-scripts.php:2:27444 fireWith http://localhost/test-site/wp-admin/load-scripts.php:2:28213 when/< http://localhost/test-site/wp-admin/load-scripts.php:2:4062 i http://localhost/test-site/wp-admin/load-scripts.php:2:27444 add http://localhost/test-site/wp-admin/load-scripts.php:2:27748 when http://localhost/test-site/wp-admin/load-scripts.php:2:3810 instance http://localhost/test-site/wp-admin/load-scripts.php:2:2847 f http://localhost/test-site/wp-admin/load-scripts.php:2:526 <anonymous> http://localhost/test-site/wp-admin/load-scripts.php:6:12068 i http://localhost/test-site/wp-admin/load-scripts.php:2:27444 fireWith http://localhost/test-site/wp-admin/load-scripts.php:2:28213 ready http://localhost/test-site/wp-admin/load-scripts.php:2:30004 K http://localhost/test-site/wp-admin/load-scripts.php:2:30366
The front end seems to be working fine. Is there guidance for theme developers about the changes in the Customizer that 4.9 has delivered? I don't know what I should be looking for.
I tested this theme on my SiteGround account to make sure the problem wasn't something related to my local machine or server instance. The same issue exists on one of my live sites on SiteGround only after I updated it to 4.9, so I believe this is definitely a problem between my theme and the latest version of WP.