If a custom game is being restarted and you happen to press the Leave Game button at any point close enough to the restart, (i.e. right after the message pops up in the chatbox), your game will be softlocked into a perpetually loading state where the only thing on the screen will be a black background with the overwatch logo rotating in the bottom-right. The only way to get out of this is to restart the game.
https://i.imgur.com/a/YBDCifi
My suggestion would be to add a button to return to the main menu if loading is taking longer than 5 seconds