Works for me in Chromium. It triggers when you move your cursor up to the top of the viewport.
If your cursor never enters the viewport it doesn't fire. If your cursor exits the viewport at the side or bottom and doesn't reenter it doesn't fire. If you close or switch tabs with a key combination it doesn't fire.
I don't think it's intended that the plugin prevents the tab from actually closing (that would be very annoying). It's just to present the user something (special offer, ad, ...)
If your cursor never enters the viewport it doesn't fire. If your cursor exits the viewport at the side or bottom and doesn't reenter it doesn't fire. If you close or switch tabs with a key combination it doesn't fire.
Also, I hate modal popups.