This might be unrelated, but I faced a similar problem recently when designing a screen for a reactive web app. I copy-pasted an auto-generated login screen that came with code already, in order to create a registration screen, and this screen had this scroll down problem. I had to go the 'Interface' tab, select the respective screen, and in the 'Style Sheet' field (under 'Advanced'), clear the html code that was auto-inserted there. That allowed me to scroll down again in the App designer.
Pretty annoying feature with an unintuitive solution process.