How does tick tick web show it's message box?

I recently started using the tick tick app for web.

After some days, I was navigating with chrome, with multiple tabs, and I was looking for some answers in stackoverflow, when suddenly, at the botton right part of the navigator (while I was at stackoverflow) a messagebox of tick tick apeared!

I liked it, I think it's a little "agressive" or "obtrusive" (don't know the propper word), but it's what I'm looking for in some web apps.

How do they show a message box even if you are not in their tab?