On this webpage, I can't click the navigation items on the left.
There is no layer laid over the top. I can't see what is preventing a click.
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On a larger screen it works fine. I tried resizing my browser to make the page smaller and there is something overlapping the links:
With a screen resolution of 1024x768 this div overlaps the navigation links and prevents you from clicking them.
You can see what I mean in this image. I made the background colour of that DIV to red, so it's easy to see how much it overlaps.
So, even though the written content doesn't overlap the navigation links, the container does.