ui-sortable does not work properly for activity post boxes on edit page

[oztaser]

• If you click move up button first post box from side (status box), the box disappears.
• We cannot move a postbox from bottom to side.
• The first box's move up button should be disable but it's clickable right now.

[imath]

Hi @oztaser

Good catch! Thanks a lot for your patch.

[imath]

In 13106:

Admin: Make sure the .postbox class is only used into sortable boxes

Otherwise editing the meta-boxes order into the Activity & Group WP Admin screens does not behave the expected way.

Props oztaser

Fixes #8566