The recent development of sample environments dedicated to in situ and /or operando XRR studies opened new opportunities such as e.g. the investigations of 2D materials during their growth on liquid metal catalysts (LMCats) . These systems are promising for the growth of high-quality materials but, at the same time, can be very demanding experimentally. They must be adapted to high operation temperatures, high material evaporation, and exposure to a mixture of reactive gases at atmospheric pressure. In addition, they are confined to a limited-size sample holder resulting in a significant wetting angle with the support, and they demand extreme surface sensitivity. Despite all these difficulties, we have recently shown that in situ and real-time investigations of 2D materials on LMCats are now possible. Using synchrotron XRR, we resolved the structure of the liquid copper surface fully covered by floating single-layer graphene, which is a very weak X-ray scatterer. However, the growth kinetics of graphene on liquid copper is very complex, and different strategies for its growth result in different morphology evolution during growth. That brings us to another technical challenge, which is the investigation of a patchy surface using XRR.