1line love2d main.lua

function love.load()
    love.window.setMode(0, 0, {fullscreen = true, fullscreentype = "exclusive"})
    scr_width, scr_height = love.window.getMode()
    love.mouse.setVisible(false)

    thickness = 1
    where = love.mouse.getY()

    love.graphics.setColor(255,255,255)
end

function love.update()
    where = love.mouse.getY()
    if where + thickness > scr_height then
        where = scr_height - thickness
    end
end

function love.draw()
    love.graphics.rectangle("fill", 0, where, scr_width, thickness)
end

function love.wheelmoved(x, y)
    thickness = thickness + y
    if thickness < 0 then
        thickness = 0
    end
end

function love.mousepressed()
    love.event.quit()
end