Youtube Full Screen is broken when using Theater Mode
This morning I noticed that screen would turn blank with audio still playing in background as soon as YouTube player were switched to full screen, I thought my pc was broken, almost was ready to reboot it on the spot. Then I found that the theater mode would be the cause. It seems to be a yet another bad youtube updade, will have to wait till they fix it and avoid the theater mode wherever possible.
Update: It appears that this YouTube bug has just recently been fixed, I don't encounter any new issues while using YouTube in fullscreen when the player is in the theater mode.