Is it possible to collapse, for example, the “Object Tools” panel or “Rigid Body Tools” panel via the API?
Basically, I’m looking for an API shortcut to the little collapse arrows in various panels.
no, re-ordering and collapsing panels can’t be done with python. You can edit the actual panel scripts however to let panels start collapsed or not (bl_options flag, i think it’s HIDE_DEFAULT or something like that)
Yes, I figured that might be the case. Addons could then abuse that ability and mess up the standard Blender experience.
What I really wanted was a visual cue for my addon users (who are not experienced Blender users), so they could quickly see where my custom panels appear. By default they are located deep underneath those other GUI panels, and you have to scroll down to find them.