Gnome-shell’s popup notifications and integrated chat are great, but sometimes I’m annoyed when the content of a chat is displayed on screen at the wrong moment (for instance if a colleague sends you a work-related message while you are sitting at a conference next to other people).
The Boss Mode extension allows you to quickly disable notifications, without any UI feedback, by just pressing Win+B. Press Win+N to enable notifications again.

The default keybindings can be modified by clicking the preferences button on the extension page (next to the switch to enable/disable the extension).