The Safari activity window can be useful for finding resources in a web page. With a lot of pages open, it shows too much information by default, as each web page’s disclosure triangle is expanded. To make the Activity window open with all sites collapsed by default, close Safari and type:
defaults write com.apple.Safari ActivitiesStartExpanded -boolean false
Quit and relaunch Safari for this to take effect.
To reverse the setting to default:
defaults write com.apple.Safari ActivitiesStartExpanded -boolean true