Flickering black bar on the bottom of the screen

When I first got Minecraft java on my pc everything was running fine when I created a new world, then after I had closed my pc and reopened it a flickering black bar appears on the bottom of my screen whenever I'm in single player or multiplayer. even if I'm not in a world as long as I'm in the single or multiplayer menu the flicker black bar appears and stays there until I go back to the main menu.