When you watch a video in VLC, full screen, the controller bar is the one with the play and stop buttons on it. It is usually shown at the bottom of the video screen. It only becomes visible if you move your mouse during video playback.
Did you know you could click on that bar and drag it to anywhere on that screen? And if you have a dual monitor setup, you can play your video in full-screen mode on one monitor and drag the control bar to your second monitor. It doesn’t stay there, once it fades it returns to the video.
But you can also lock it into place at the bottom of the screen. VLC refers this to Docking and Undocking.
I believe its undocked by default. To dock it or lock it into place at the bottom of the screen. The only place you can dock it. Just click on the icon usually down by the volume control. The two-headed arrow.