It seems to be linked to user profile somehow. If I open a map I created in new map viewer, it opens with the panel collapsed showing just the icons. If I open it in classic viewer, it opens to the contents panel.
If I open a map one of our other users created, it opens with the side panel expanded. Open that same map in classic viewer, and it opens to the legend panel, not the contents.
Not sure why/where this is set, but defiantly appear to be different depending on who created the map.
R_