Rewritten in 2019. Previous name is Agar.io auto respawn (evergreen)
That's just how browsers work. Inactive tabs are 'ignored' like that. To work around this, you must make agar.io tab active.
Here's how: Open two Chrome windows. One with only agar.io tab in it, while the other one is for your work and other things. You can work on things on window #2 while having window #1 with active agar.io tab still collecting coins.
Problem with collecting coins
It stucs when working on backgraund and there are not yet coins to collect, to reproduce: set collectig coins to 1 min, with agario on active tab it works fine, when I switch to other tab then collectins coins windows does not close, script should check when clicking to collect coin if previes window is not closed.