Consider the case where both players (Warlocks) have Mal'Gannis who has been reduced to one power by damage plus Crazed Alchemist or Reversing Switch. Each player also has a Stoneskin Gargoyle with taunt from Rusty Horn (to make the board even more unchangeable we can give each Stoneskin Gargoyle +1 health from Armor Plating). Both players have nothing else on the board and no cards in their deck or hand.
Each turn, neither player will be able to kill the opposing Stoneskin Gargoyle and neither player can take fatigue damage due to their characters being immune. Will the game go on until one player surrenders, or is there some mechanism for detecting nothing changing/loops for umpteen turns and declaring the game a draw?