When I’m browsing, reddit on my iphone on Safari, when the little pop-up shows up that tells you to use the app it completely locks up Safari (it doesn’t hang)

When I’m browsing, reddit on my iphone on Safari, when the little pop-up shows up that tells you to use the app it completely locks up Safari (it doesn’t hang) I can’t change tabs, bring up the back control buttons or do anything else except exit out of Safari how are they doing that?