Like Dimitry, I have experienced the same (or a similar) bug on a fresh 20.04 install. I'm attaching a picture of it. Both "Authenticate" and "Cancel" buttons are clickable ("Authenticate" only once) but don't make the window vanish. I confirm that killing the gnome-shell caused the window to disappear.
** Attachment added: "image of the bug" https://bugs.launchpad.net/ubuntu/+source/gnome-shell/+bug/1824874/+attachment/5364530/+files/bug.png -- You received this bug notification because you are a member of Desktop Packages, which is subscribed to gnome-shell in Ubuntu. https://bugs.launchpad.net/bugs/1824874 Title: undismissable, unclickable authentication dialog left on screen (top- left corner) after policykit authentication [pushModal: invocation of begin_modal failed] Status in gnome-shell package in Ubuntu: Confirmed Bug description: In disco, when policykit prompted me for a password in order for update-manager to dispatch instructions to aptdaemon to perform package updates, the password dialog remained on the screen after I clicked 'authenticate'. The window is not clickable, it appears not to even be a window - mouse presses are received by the window underneath. The exception is that the 'cancel' button is active and receives mouse events - but clicking it does nothing. This may be a gnome-shell issue rather than policykit. ProblemType: Bug DistroRelease: Ubuntu 19.04 Package: policykit-1 0.105-25 ProcVersionSignature: Ubuntu 5.0.0-8.9-generic 5.0.1 Uname: Linux 5.0.0-8-generic x86_64 NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair ApportVersion: 2.20.10-0ubuntu27 Architecture: amd64 CurrentDesktop: ubuntu:GNOME Date: Mon Apr 15 13:14:33 2019 InstallationDate: Installed on 2010-09-24 (3125 days ago) InstallationMedia: Ubuntu 10.04.1 LTS "Lucid Lynx" - Release amd64 (20100816.1) SourcePackage: policykit-1 UpgradeStatus: Upgraded to disco on 2019-04-11 (3 days ago) To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/gnome-shell/+bug/1824874/+subscriptions -- Mailing list: https://launchpad.net/~desktop-packages Post to : desktop-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~desktop-packages More help : https://help.launchpad.net/ListHelp