Usually the site's header will only cut off the order summary and progress bar when the original theme's header is being used, by default Fluid Checkout will replace the theme's header with a simplified version.
If you are experiencing this issue on your website, you are probably using your theme's header on the checkout page.
Although there are some cases where using the theme's header at the checkout can be beneficial, we recommend using the default header provided by Fluid Checkout as is optimized for conversion and is also required to display the mobile cart button that gives the users access to the total order amount and the order summary on mobile devices.
Why does the order summary and progress bar get cut off?
The order summary and progress bar rely on the existence of the checkout header element provided by Fluid Checkout to define the position where it should be positioned fixed, following the user while scrolling, and when the element should scroll with the page out of the user's immediate view.
Since each theme might use a different ID for the site header, and they differ a lot, it is not possible for us to define a default value that will work for all themes or even most of them.
We need to add compatibility for each theme separately as we discover which ID is used for them. That is one reason why the feature to use the site's header is marked as "Experimental".
We can't possibly test all existing themes free and paid and add compatibility for all of them in advance.