If you visit my website at this link, try resizing the window until the responsive design kicks in. Then, click on the "right" button at the top of the page. You'll notice that the right sidebar disappears and reappears, but when it disappears, the bar seems to shift to the left by -300px, allowing you to scroll to the right. Why do you think this is happening? Everything works fine with the left sidebar. P.S. Use the middle mouse button to scroll.