Run unity8 with QDBUS_DEBUG=1, when I hit power button twice fast (screen on at start) I get these messages:

QDBusConnectionPrivate(0x47513e28) got message (signal): QDBusMessage(type=MethodCall, service=":1.3", path="/com/canonical/Unity/Screen", interface="com.canonical.Unity.Screen", member="setScreenPowerMode", signature="s", contents=("off") ) 
DBusScreen::setScreenPowerMode "off" 
QDBusConnectionPrivate(0x47513e28) Automatically sending reply: (QVariant(bool, true) )  
QDBusConnectionPrivate(0x47513e28) sending message (no reply): QDBusMessage(type=MethodReturn, service="", signature="", contents=(true) ) 
QDBusConnectionPrivate(0x47513e28) got message (signal): QDBusMessage(type=Signal, service=":1.3", path="/com/canonical/powerd", interface="com.canonical.powerd", member="DisplayPowerStateChange", signature="iu", contents=(0, 0) ) 
QDBusConnectionPrivate(0x47513e28) got message (signal): QDBusMessage(type=MethodCall, service=":1.3", path="/com/canonical/Unity/Screen", interface="com.canonical.Unity.Screen", member="setScreenPowerMode", signature="s", contents=("on") ) 
DBusScreen::setScreenPowerMode "on" 
QDBusConnectionPrivate(0x47513e28) Automatically sending reply: (QVariant(bool, true) )  
QDBusConnectionPrivate(0x47513e28) sending message (no reply): QDBusMessage(type=MethodReturn, service="", signature="", contents=(true) ) 
QDBusConnectionPrivate(0x47513e28) got message (signal): QDBusMessage(type=Signal, service=":1.3", path="/com/canonical/powerd", interface="com.canonical.powerd", member="DisplayPowerStateChange", signature="iu", contents=(1, 4) ) 
QDBusConnectionPrivate(0x47513e28) got message (signal): QDBusMessage(type=MethodCall, service=":1.3", path="/com/canonical/Unity/Screen", interface="com.canonical.Unity.Screen", member="setScreenPowerMode", signature="s", contents=("on") ) 
DBusScreen::setScreenPowerMode "on" 
terminate called after throwing an instance of 'boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<std::runtime_error> >'
  what():  could not activate surface with eglMakeCurrent

Aborted (core dumped)