Hi. The issue is that the pusher is being translate 3D in the X direction, more than what is necessary.
You can look at the sidebar.js file to see how that works.
This issue is mentioned in the Semantic-UI documentation:
So you can either use the thin class to make it calculate the distance correctly: https://jsfiddle.net/dgte4p59/
But this would not look the best due to lots of empty space.
Alternatively, you can override the semantic transform property for the pusher and write your own in your main.css