Bug:
I was softlocked as my opponent conceded. I was about to activate an ability (sacrifice a treasure) when my opponent conceded. My opponent left but the option to leave the game never popped up for me. Had to restart the application. I could highlight cards, see other zones (my sideboard, opponents exile etc) but I could not progress the game.

I did have another Arena application running in the background, as startup was really slow so I started again.