Widgets panel will not wait to display until the preview loads.
Also fixes problems with `margin-top` in panels where other panels' `active` states change, as well as ensuring sections of deactivated panel collapse before panel is hidden to prevent the pane from becoming empty of controls.
Fixes#33052.
Fixes#33567.
Built from https://develop.svn.wordpress.org/trunk@35231
git-svn-id: http://core.svn.wordpress.org/trunk@35197 1a063a9b-81f0-0310-95a4-ce76da25c4cd