When I use menuPortalTarget it breaks the styling on my dropdown. Here is a codesandbox showing the problem. In the codesandbox I have a button to toggle menuPortalTarget. When menuPortalTarget is active and you open the dropdown you can see the messed up styling vs when its not active.
I tried playing around with the parent styling and the menuPortal styling but I couldn't get it working.
This example is based off this from the react-select docs.