The external screen goes black and cannot be used until you exit the fullscreen mode on the internal display.
Connected via Air Play, the width of the whole Steam client user interface does not scroll correctly, the width is somehow screwed. Even fullscreen works until you move your mouse pointer and focus some window on the main screen: The fullscreen playback on the external monitor just goes black.
Have completely removed steam (was pre-installed when it started) removed directories and registry entries and re-downloaded, no luck.
--steveob42 (talk) , 18 December 2016 (UTC) I've got a 15" Retina Mac Book Pro with an external display.
This happens every now and then and was frustrating when trying to figure out what was causing the problem. I first observed the error in Half-Life after installing Paranormal Beta, which runs on SDK 2007 and launches through Steam. On my dual-boot, Half-Life launched normally in Windows XP, then ran normally again in Windows 7 afterwards. When using this feature, the mouse pointer gains a small, black arrow.