If you use Jqueryui then it will break the collapsible dock. Jqueryui tabs and accordians are good examples. I went through and removed Jqueryui from all our plugins. And just left it in 1 or 2 places. It was mentioned here: https://tracker.moodle.org/browse/MDL-74459
To confirm Justin's comment, JQuery UI can break a lot of things in Moodle 4.0. That is not a criticism Moodle 4.0 as devs have been encouraged away from it for quite a while.