It seems like the arrow keys stop working properly in some browsers at some image to screen ratios/sizes. I was using Vivaldi, a browser based on the Chromium engine, before and I have a 4K desktop. The up and down arrows do not work at all and the right arrow stops working when I go into fullscreen with the browser or I change the image to be larger than a certain size, either through the browser built-in zoom or through the reader. I'm using Blue Moon, a browser based on the Firefox engine, now and it works perfectly always.