That’s what it’s supposed to do: it hides everything that “overflows”.
You’ve given the .side-menu-right block a height of 100%, AND overflow:hidden;. Result is that the block will be as high as its parent (in this case, the viewport) and everything that won’t fit in there, will not be shown.
I’m sure you have your reasons for using overflow:hidden; this way, but not sure what else to say. It’s doing exactly what you’re telling it to do. You may want to change it to overflow:visible;?
If you comment out the overflow:hidden and have a look at what happens to my black footer and right grey border. You will see the problem i am having. I thought i may have fixed it with overflow:hidden, but this is not the case.