Pretty much what it says on the title: Events lists can get pretty long depending on how complex the object is, and it seems the default presentation is every event and event group expanded
It could use the same “event blocks” icon with a couple arrows to show the effect, something like the (horrendous) mockup below
This is already a feature. Right click any event in the event editor, it will say Collapse All and beneath it Expand to level. The latter opens things within it sequentially based on the level you specify, so 1 might open the main group but leave all sub groups as closed.