Folders collapsing when going to different session tab

When I click on a different session tab, all the folders in my other collapse, bug or feature?

Neither. It's just the way it is now. It can be improved. You can vote for it here:
Issue 1057 – Remember panel scroll position and selection and directory tree state when switching sessions

Damn no, need an account to vote :( How can it be neither, thanks for putting up the vote anyway!