A regular fullscreen application (such as MPlayer, VLC, Chromium, …) does not grab the keyboard and/or pointer.
Regular fullscreen games do, however. If there were some way to distinguish between a lock screen and a game, it would be worthwhile to keep the feature around.
Regular fullscreen games do, however. If there were some way to distinguish between a lock screen and a game, it would be worthwhile to keep the feature around.