Blackening of the beta client

Hay fellow summoners,

does anyone have following problem with the beta client:

sometimes when minimizing the beta client or even alt+tabbing to any other window the beta client is covered in a black screen when going over the clients some black squares lite up and return the the actual picture but others just remain black. In order to fix this bug I have to retart the client. Any suggenstion how to avoid this?

Maybe I'm the only one having this problem :P

greetings,

song of prayer