Answer
In some cases that you only have the Fluid widget on the page and you do not have any other objects on the page, Muse is not smart enough to understand that the widget may want to expand in height so Muse removes the scrollbar, please use the following code to fix this.
<style>body{overflow-y:auto !important;} #page .verticalspacer { height: 0 !important;}</style>
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Update/Additional comment:
All you need to do is to copy the code, then paste it in the page where you have the widget, you just need to be on that page when you paste (Cmd + v), and Muse automatically creates an HTML box for you, then move the HTML box to the side (preferably top-left), outside of the design area, in the gray area, and pin it there so it will not get on your way when designing or reviewing.
