i started to watch a replay but minimized after i clicked the "watch replay" button (because it usually takes a little while for them to load for me). i brought hon back up when i heard the "all pick" announcer and had this giant white box covering the screen. luckily, it was just a replay so i didn't have to scramble to find a hero but i have seen it happen to other people who actually join a game and can't see the heroes. the only way they can pick their hero is to hover over the general area where they think the hero is until they find it (the information on the hero still pops up at the bottom when you hover over them).
this only seems to happen occasionally when you minimize while the game is loading but it's quite frustrating having to blindly pick your hero when you can't even see what your allies have right clicked.
and before you ask, i don't have any mods installed.
:edit: i thought maybe it could be the message of the day box but the white box is too big