I have "Enable Collapsible Sections" checked in the User Interface settings. However it currently only works for Users, not Partner Users. Is there a way to enable this for Partner Users too?
Unfortunately it is not possible to enable this for Partner Users. For some obscure reason this only works for internal users.
You could vote for this Idea and hope it will be picked up by Product Management. https://success.salesforce.com/ideaView?id=08730000000cGVbAAM