This is on Catalina while running Mathematica 12 (64-bit version which is technically supported on Catalina). I can open notebooks but they seem unresponsive to typing or scrolling. However, if I drag or resize the window then I can see the changes I made. I am unsure if the issue is with Mathematica or with Catalina. I wanted to post here in case anyone has faced similar issues / knows a solution. I will be contacting Wolfram tomorrow.
Executing the following in the Terminal solves the issue:
defaults write com.wolfram.Mathematica NSRequiresAquaSystemAppearance -bool YES