This Code inside UGameEngine::CreateGameWindow() :
// Do not set fullscreen mode here, since it doesn't take
// HMDDevice into account. The window mode will be set properly later
// from SwitchGameWindowToUseGameViewport() method (see ResizeWindow call).
if (WindowMode == EWindowMode::Fullscreen)
{
Window->SetWindowMode(EWindowMode::WindowedFullscreen);
}
else
{
Window->SetWindowMode(WindowMode);
}
Causes initialization loop of the engine to play loading movies in WindowedFullScreen Mode, and then switches to FullScreen after the movie has completed. Not sure what was originally intended w/regards to HMD devices, but it causes a lot of ugly flickering to occur once the movies stop playing when the game switches to FullScreen mode.