Sounds like something the website should fix, actually. Let me use an example I'm more familiar with ... look at this page: https://www.machinefinder.com/ww/en-US/machines/2014-john-deere-410k-backhoe-loader-4324132
Near the bottom of the visible area you see a set of 3 thumbnails and some arrows to scroll them left or right - depending on your screen size and how far you've scrolled the screen you will have trouble clicking the thumbnails or the arrows to scroll them. If you watch your cursor you'll see that it changes between the normal cursor and the one indicating there is something you can click at a region in there, like there is something on the page but invisible blocking it. And there is - they have a special "mobile toolbar" when you view the site on a smartphone which they hide on desktops, but the bar still covers up (i.e., prevents you from clicking on) stuff on the page even when you can't see it.
Without some idea where on exactly which page I can't say what it is on your chess site there, but I'm reasonably sure it is something similar.