Currently, pressing the delete key (with default keybindings) allows you to remove all of the UI apart from the map and the only way to come out of this, seemingly, is by pressing the delete key again or alt-f4ing the game. Recently I accidentally pressed the delete key at the end of an encounter when replying to someone in chat and, wrongly, assumed that the game had a bugged cinematic and I made a forum post in the bug report section, which I have since deleted.
My suggestion would be to allow for the usage of the escape key to toggle the UI back on after it has been removed. To try and avoid any future confusion for other players.