Every so often - I think it is whenever my ISP "glitches" (i.e., when there is a brief loss of Internet connectivity), my Ubuntu system screen gets covered with a glossy film (bluish-grey) that says something about not being able to connect to something or other.
I have determined that this is caused by a program (process) called "gnome-shell-portal-helper" and if I kill that process, the glossy film goes away.
How to turn this off permanently?
Gnome-shell-portal or the wifi/hotspot login portal shows up when the system detects that it is connected to a network but internet is not accessible. Hence this portal shows up and redirects to the connected network/wifi's login portal (captive portal) to enable user to login.
The settings that control this is under settings/privacy > turn off connectivity check (in Ubuntu). BUT remember that when you turn this off, this wifi login portal will not automatically show up.
E:g. for most public wifi, we need to accept terms and login. If you turn this off, you can trigger this login portal yourself after connecting to wifi by typing in any ip address (eg. 8.8.8.8) in web browser and press enter. Make sure the address is http and not https.